Red Hat Enterprise Linux 7

Virtualization Deployment and Administration Guide

Installing, configuring, and managing virtual machines on a Red Hat Enterprise Linux physical machine

Laura Novich

Red Hat Customer Content Services

lnovich@redhat.com

Dayle Parker

Red Hat Customer Content Services

dayleparker@redhat.com

Scott Radvan

Red Hat Customer Content Services

sradvan@redhat.com

Tahlia Richardson

Red Hat Customer Content Services

trichard@redhat.com

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Abstract

This guide covers how to configure a Red Hat Enterprise Linux 7 host physical machine and how to install and configure guest virtual machines using the KVM hypervisor. Other topics include PCI device configuration, SR-IOV, networking, storage, device and guest virtual machine management, as well as troubleshooting, compatibility and restrictions.

Note

To expand your expertise, you might also be interested in the Red Hat Enterprise Virtualization (RH318) training course.

⁠Part I. Deployment

⁠Chapter 1. System requirements

Virtualization is available with the KVM hypervisor for Red Hat Enterprise Linux 7 on the Intel 64 and AMD64 architectures. This chapter lists system requirements for running virtual machines, also referred to as VMs.

For information on installing the virtualization packages, see Chapter 2, Installing the virtualization packages.

⁠1.1. Host system requirements

⁠

Minimum host system requirements

6 GB free disk space.

2 GB RAM.

⁠

Recommended system requirements

One core or thread for each virtualized CPU and one for the host.

2 GB of RAM, plus additional RAM for virtual machines.

6 GB disk space for the host, plus the required disk space for the virtual machine(s).

Most guest operating systems require at least 6 GB of disk space. Additional storage space for each guest depends on their workload.

Swap space

Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory. The size of your swap partition can be calculated from the physical RAM of the host. The Red Hat Customer Portal contains an article on safely and efficiently determining the size of the swap partition: https://access.redhat.com/site/solutions/15244.

When using raw image files, the total disk space required is equal to or greater than the sum of the space required by the image files, the 6 GB of space required by the host operating system, and the swap space for the guest.

⁠

Equation 1.1. Calculating required space for guest virtual machines using raw images

total for raw format = images + hostspace + swap

For qcow images, you must also calculate the expected maximum storage requirements of the guest (total for qcow format), as qcow and qcow2 images are able to grow as required. To allow for this expansion, first multiply the expected maximum storage requirements of the guest (expected maximum guest storage) by 1.01, and add to this the space required by the host (host), and the necessary swap space (swap).

⁠

Equation 1.2. Calculating required space for guest virtual machines using qcow images

total for qcow format = (expected maximum guest storage * 1.01) + host + swap

Guest virtual machine requirements are further outlined in Chapter 8, Overcommitting with KVM.

⁠1.2. KVM hypervisor requirements

The KVM hypervisor requires:

an Intel processor with the Intel VT-x and Intel 64 virtualization extensions for x86-based systems; or

an AMD processor with the AMD-V and the AMD64 virtualization extensions.

Virtualization extensions (Intel VT-x or AMD-V) are required for full virtualization. Run the following commands to determine whether your system has the hardware virtualization extensions, and that they are enabled.

⁠

Procedure 1.1. Verifying virtualization extensions

Verify the CPU virtualization extensions are available

Run the following command to verify the CPU virtualization extensions are available:

$ grep -E 'svm|vmx' /proc/cpuinfo

Analyze the output

The following example output contains a vmx entry, indicating an Intel processor with the Intel VT-x extension:

flags   : fpu tsc msr pae mce cx8 vmx apic mtrr mca cmov pat pse36 clflush
dts acpi mmx fxsr sse sse2 ss ht tm syscall lm constant_tsc pni monitor ds_cpl
vmx est tm2 cx16 xtpr lahf_lm

The following example output contains an svm entry, indicating an AMD processor with the AMD-V extensions:

flags   :  fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36 clflush
mmx fxsr sse sse2 ht syscall nx mmxext svm fxsr_opt lm 3dnowext 3dnow pni cx16
lahf_lm cmp_legacy svm cr8legacy ts fid vid ttp tm stc

If the grep -E 'svm|vmx' /proc/cpuinfo command returns any output, the processor contains the hardware virtualization extensions. In some circumstances, manufacturers disable the virtualization extensions in the BIOS. If the extensions do not appear, or full virtualization does not work, see Procedure A.4, “Enabling virtualization extensions in BIOS” for instructions on enabling the extensions in your BIOS configuration utility.

Ensure the KVM kernel modules are loaded

As an additional check, verify that the kvm modules are loaded in the kernel with the following command:

# lsmod | grep kvm

If the output includes kvm_intel or kvm_amd, the kvm hardware virtualization modules are loaded.

Note

The virsh utility (provided by the libvirt-client package) can output a full list of your system's virtualization capabilities with the following command:

# virsh capabilities

⁠1.3. KVM guest virtual machine compatibility

Red Hat Enterprise Linux 7 servers have certain support limits.

The following URLs explain the processor and memory amount limitations for Red Hat Enterprise Linux:

For host systems: https://access.redhat.com/articles/rhel-limits

For the KVM hypervisor: https://access.redhat.com/articles/rhel-kvm-limits

The following URL lists guest operating systems certified to run on a Red Hat Enterprise Linux KVM host:

https://access.redhat.com/articles/973133

Note

For additional information on the KVM hypervisor's restrictions and support limits, see Appendix B, Virtualization restrictions.

⁠1.4. Supported guest CPU models

Every hypervisor has its own policy for which CPU features the guest will see by default. The set of CPU features presented to the guest by the hypervisor depends on the CPU model chosen in the guest virtual machine configuration.

⁠1.4.1. Listing the guest CPU models

To view a full list of the CPU models supported for an architecture type, run the virsh cpu-models <arch> command. For example:

$ virsh cpu-models x86_64
486
pentium
pentium2
pentium3
pentiumpro
coreduo
n270
core2duo
qemu32
kvm32
cpu64-rhel5
cpu64-rhel6
kvm64
qemu64
Conroe
Penryn
Nehalem
Westmere
SandyBridge
Haswell
athlon
phenom
Opteron_G1
Opteron_G2
Opteron_G3
Opteron_G4
Opteron_G5

$ virsh cpu-models ppc64
POWER7
POWER7_v2.1
POWER7_v2.3
POWER7+_v2.1
POWER8_v1.0

The full list of supported CPU models and features is contained in the cpu_map.xml file, located in /usr/share/libvirt/:

# cat /usr/share/libvirt/cpu_map.xml

A guest's CPU model and features can be changed in the <cpu> section of the domain XML file. See Section 26.12, “CPU models and topology” for more information.

The host model can be configured to use a specified feature set as needed. For more information, see Section 26.12.1, “Changing the feature set for a specified CPU”.

⁠Chapter 2. Installing the virtualization packages

To use virtualization, the virtualization packages must be installed on your computer. Virtualization packages can be installed either during the host installation sequence or after host installation using the yum command and Subscription Manager.

The KVM hypervisor uses the default Red Hat Enterprise Linux kernel with the kvm kernel module.

⁠2.1. Configuring a virtualization host during a new Red Hat Enterprise Linux 7 installation

This section covers installing virtualization tools and virtualization packages as part of a fresh Red Hat Enterprise Linux installation.

Note

The Red Hat Enterprise Linux 7 Installation Guide covers installing Red Hat Enterprise Linux in detail.

⁠

Procedure 2.1. Installing the virtualization package group

Launch the Red Hat Enterprise Linux 7 installation program

Start an interactive Red Hat Enterprise Linux 7 installation from the Red Hat Enterprise Linux Installation CD-ROM, DVD or PXE.

Continue installation up to software selection

Complete the other steps up to the software selection step. The Installation Summary screen prompts the user to complete any steps still requiring attention.

⁠

Figure 2.1. The Installation Summary screen

Software Selection defaults to Minimal Install. Open the Software Selection screen to select the virtualization packages instead.

Select the server type and package groups

Red Hat Enterprise Linux 7 has two available options for installing a virtualization host: a minimal virtualization host with only the basic packages installed (Step 3.a), or a virtualization host with packages installed to allow management of guests through a graphical user interface (Step 3.b).

⁠

Selecting a minimal virtualization host

Select the Virtualization Host radio button under Base Environment, and the Virtualization Platform checkbox under Add-Ons for Selected Environment. This installs a basic virtualization environment which can be run with virsh, or remotely over the network.

⁠

Figure 2.2. Virtualization Host selected in the software selection screen

⁠

Selecting a virtualization host with a graphical user interface

Select the Server with GUI radio button under Base Environment, and the checkboxes for Virtualization Client, Virtualization Hypervisor, and Virtualization Tools under Add-Ons for Selected Environment. This installs a virtualization environment along with graphical tools for installing and managing guest virtual machines.

⁠

Figure 2.3. Server with GUI selected in the software selection screen

Finalize installation

On the Installation Summary screen, complete the steps as necessary and click Begin Installation.

When the installation is complete, reboot the system.

Important

You require a valid virtualization entitlement to receive updates for the virtualization packages.

⁠2.1.1. Installing KVM packages with Kickstart files

Kickstart files allow for large, automated installations without a user manually installing each individual host system.

To use a Kickstart file to install Red Hat Enterprise Linux with the virtualization packages, append the following package groups in the %packages section of your Kickstart file:

@virtualization-hypervisor
@virtualization-client
@virtualization-platform
@virtualization-tools

For more information about installing with Kickstart files, see the Red Hat Enterprise Linux 7 Installation Guide.

⁠2.2. Installing virtualization packages on an existing Red Hat Enterprise Linux system

This section describes the steps for installing the KVM hypervisor on an existing Red Hat Enterprise Linux 7 system.

To install the packages, your machine must be registered and subscribed to the Red Hat Customer Portal. To register via Red Hat Subscription Manager, run the subscription-manager register command and follow the prompts. Alternatively, run the Red Hat Subscription Manager application from Applications → System Tools on the desktop to register.

If you do not have a valid Red Hat subscription, visit the Red Hat online store to obtain one. For more information on registering and subscribing a system to the Red Hat Customer Portal, see https://access.redhat.com/solutions/253273.

⁠2.2.1. Installing the virtualization packages with yum

To use virtualization on Red Hat Enterprise Linux, you require at minimum the qemu-kvm, qemu-img, and libvirt packages. These packages provide the user-level KVM emulator, disk image manager, and virtualization management tools on the host Red Hat Enterprise Linux system.

The libvirt package provides the server and host-side libraries for interacting with hypervisors and host systems, and the libvirtd daemon that handles the library calls, manages virtual machines and controls the hypervisor.

To install these packages, run the following command:

# yum install qemu-kvm qemu-img libvirt

Several additional virtualization management packages are also available and are recommended when using virtualization:

⁠

Recommended virtualization packages

⁠virt-install

This package provides the virt-install command for creating virtual machines from the command line.

⁠libvirt-python

The libvirt-python package contains a module that permits applications written in the Python programming language to use the interface supplied by the libvirt API.

⁠virt-manager

virt-manager, also known as Virtual Machine Manager, provides a graphical tool for administering virtual machines. It uses the libvirt-client library as the management API.

⁠libvirt-client

The libvirt-client package provides the client-side APIs and libraries for accessing libvirt servers. The libvirt-client package includes the virsh command line tool to manage and control virtual machines and hypervisors from the command line or a special virtualization shell.

Install all of these recommended virtualization packages with the following command:

# yum install virt-install libvirt-python virt-manager python-virtinst libvirt-client

⁠2.2.2. Installing virtualization package groups

The virtualization packages can also be installed from package groups. The following table describes the virtualization package groups and what they provide.

Note

Note that the qemu-img package is installed as a dependency of the Virtualization package group if it is not already installed on the system. It can also be installed manually with the yum install qemu-img command as described previously.

⁠

Table 2.1. Virtualization Package Groups

Package Group | Description | Mandatory Packages | Optional Packages
Virtualization Hypervisor | Smallest possible virtualization host installation | libvirt, qemu-kvm | qemu-kvm-tools
Virtualization Client | Clients for installing and managing virtualization instances | gnome-boxes, virt-install, virt-manager, virt-viewer | virt-top, libguestfs-tools, libguestfs-tools-c
Virtualization Platform | Provides an interface for accessing and controlling virtual machines and containers | libvirt, libvirt-client, virt-who | fence-virtd-libvirt, fence-virtd-multicast, fence-virtd-serial, libvirt-cim, libvirt-java, libvirt-snmp, perl-Sys-Virt
Virtualization Tools | Tools for offline virtual image management | libguestfs | libguestfs-java, libguestfs-tools, libguestfs-tools-c

To install a package group, run the yum groupinstall <package group> command. For instance, to install the Virtualization Tools package group, run:

# yum groupinstall "Virtualization Tools"

⁠2.3. Registering the hypervisor and virtual machine

Red Hat Enterprise Linux 6 and 7 require that every guest virtual machine is mapped to a specific hypervisor in order to ensure that every guest is allocated the same level of subscription service. To do this you need to install a subscription agent that automatically detects all guest Virtual Machines (VMs) on each KVM hypervisor that is installed and registered, which in turn will create a mapping file that sits on the host. This mapping file ensures that all guest VMs receive the following benefits:

Subscriptions specific to virtual systems are readily available and can be applied to all of the associated guest VMs

All subscription benefits that can be inherited from the hypervisor are readily available and can be applied to all of the associated guest VMs.

Note

The information provided in this chapter is specific to Red Hat Enterprise Linux subscriptions only. If you also have a Red Hat Enterprise Virtualization subscription, or a Red Hat Satellite subscription, you should also consult the virt-who information provided with those subscriptions.

⁠2.3.1. Installing virt-who on the host physical machine

Register the KVM hypervisor

Register the KVM Hypervisor by running the subscription-manager register [options] command in a terminal as the root user on the host physical machine. More options are available using the # subscription-manager register --help menu. In cases where you are using a username and password, use the credentials that are known to the subscription manager. If this is your very first time subscribing and you do not have a user account, contact customer support. For example to register the VM as 'admin' with 'secret' as a password, you would send the following command:

[root@rhel-server ~]# subscription-manager register --username=admin --password=secret --auto-attach --type=hypervisor

Install the virt-who packages

Install the virt-who packages, by running the following command in a terminal as root on the host physical machine:

[root@rhel-server ~]# yum install virt-who

Create a virt-who configuration file

Add a configuration file in the /etc/virt-who.d/ directory. It does not matter what the name of the file is, but you should give it a name that makes sense and the file must be located in the /etc/virt-who.d/ directory. Inside that file add the following snippet and remember to save the file before closing it.

[libvirt]
type=libvirt

Start the virt-who service

Start the virt-who service by running the following command in a terminal as root on the host physical machine:

[root@virt-who ~]# systemctl start virt-who.service
[root@virt-who ~]# systemctl enable virt-who.service

Confirm virt-who service is receiving guest information

At this point, the virt-who service will start collecting a list of domains from the host. Check the /var/log/rhsm/rhsm.log file on the host physical machine to confirm that the the file contains a list of the guest VMs. For example:

2015-05-28 12:33:31,424 DEBUG: Libvirt domains found: [{'guestId': '58d59128-cfbb-4f2c-93de-230307db2ce0', 'attributes': {'active': 0, 'virtWhoType': 'libvirt', 'hypervisorType': 'QEMU'}, 'state': 5}]

⁠

Procedure 2.2. Managing the subscription on the customer portal

Subscribing the hypervisor

As the virtual machines will be receiving the same subscription benefits as the hypervisor, it is important that the hypervisor has a valid subscription and that the subscription is available for the VMs to use.

Login to the customer portal

Login to the Red Hat customer portal https://access.redhat.com/ and click the Subscriptions button at the top of the page.

Click the Systems link

In the Subscriber Inventory section (towards the bottom of the page), click Systems link.

Select the hypervisor

On the Systems page, there is a table of all subscribed systems. Click on the name of the hypervisor (localhost.localdomain for example). In the details page that opens, click Attach a subscription and select all the subscriptions listed. Click Attach Selected. This will attach the host's physical subscription to the hypervisor so that the guests can benefit from the subscription.

Subscribing the guest virtual machines - first time use

This step is for those who have a new subscription and have never subscribed a guest virtual machine before. If you are adding virtual machines, skip this step. To consume the subscription assigned to the hypervisor profile on the machine running the virt-who service, auto subscribe by running the following command in a terminal, on the guest virtual machine as root.

[root@virt-who ~]# subscription-manager attach --auto

Subscribing additional guest virtual machines

If you just subscribed a for the first time, skip this step. If you are adding additional virtual machines, it should be noted that running this command will not necessarily re-attach the same pools to the hypervisor. This is because removing all subscriptions then allowing auto attach to resolve what is necessary for a given guest virtual machine may result in different subscriptions consumed than before. This may not have any effect on your system, but it is something you should be aware about. If you used a manual attachment procedure to attach the virtual machine, which is not described below, you will need to re-attach those virtual machines manually as the auto-attach will not work. Use the following command as root in a terminal to first remove the subscriptions for the old guests and then use the auto-attach to attach subscriptions to all the guests. Run these commands on the guest virtual machine.

[root@virt-who ~]# subscription-manager remove --all
[root@virt-who ~]# subscription-manager attach --auto

Confirm subscriptions are attached

Confirm that the subscription is attached to the hypervisor by running the following command as root in a terminal on the guest virtual machine:

[root@virt-who ~]# subscription-manager list --consumed

Output similar to the following will be displayed. Pay attention to the Subscription Details. It should say 'Subscription is current'.

[root@virt-who ~]#  subscription-manager list --consumed
+-------------------------------------------+
   Consumed Subscriptions
+-------------------------------------------+
Subscription Name:	Awesome OS with unlimited virtual guests
Provides: 		Awesome OS Server Bits
SKU: 			awesomeos-virt-unlimited
Contract: 		0
Account: 		12331131231
Serial: 		7171985151317840309
Pool ID: 		2c91808451873d3501518742f556143d
Provides Management: 	No
Active: 		True
Quantity Used: 		1
Service Level:
Service Type:
Status Details:		Subscription is current
Subscription Type:
Starts: 		01/01/2015
Ends: 			12/31/2015
System Type: 		Virtual

Indicates if your subscription is current. If your subscription is not current, an error message appears. One example is Guest has not been reported on any host and is using a temporary unmapped guest subscription. In this case the guest needs to be subscribed. In other cases, use the information as indicated in Section 2.3.4.2, “I have subscription status errors, what do I do?”.

Register additional guests

When you install new guest VMs on the hypervisor, you must register the new VM and use the subscription attached to the hypervisor, by running the following commands in a terminal as root on the guest virtual machine:

[root@server1 ~]# subscription-manager register
[root@server1 ~]# subscription-manager attach --auto
[root@server1 ~]# subscription-manager list --consumed

⁠2.3.2. Registering a new guest virtual machine

In cases where a new guest virtual machine is to be created on a host that is already registered and running, the virt-who service must also be running. This ensures that the virt-who service maps the guest to a hypervisor, so the system is properly registered as a virtual system. To register the virtual machine, run the following command as root in a terminal:

[root@virt-server ~]# subscription-manager register --username=admin --password=secret --auto-attach

⁠2.3.3. Removing a guest virtual machine entry

If the guest virtual machine is running, unregister the system, by running the following command in a terminal window as root:

[root@virt-guest ~]# subscription-manager unregister

If the system has been deleted, however, the virtual service cannot tell whether the service is deleted or paused. In that case, you must manually remove the system from the server side, using the following steps:

Login to the Subscription Manager

The Subscription Manager is located on the Red Hat Customer Portal. Login to the Customer Portal using your username and password, by clicking the login icon at the top of the screen.

Click the Subscriptions tab

Click the Subscriptions tab.

Click the Systems link

Scroll down the page and click the Systems link.

Delete the system

To delete the system profile, locate the specified system's profile in the table, select the check box beside its name and click Delete.

⁠2.3.4. Troubleshooting virt-who

⁠2.3.4.1. Why is the hypervisor status red?

Scenario: On the server side, you deploy a guest on a hypervisor that does not have a subscription. 24 hours later, the hypervisor displays its status as red. To remedy this situation you must get a subscription for that hypervisor. Or, permanently migrate the guest to a hypervisor with a subscription.

⁠2.3.4.2. I have subscription status errors, what do I do?

Scenario: Any of the following error messages display:

System not properly subscribed

Status unknown

Late binding of a guest to a hypervisor through virt-who (host/guest mapping)

To find the reason for the error open the virt-who log file, named rhsm.log, located in the /var/log/rhsm/ directory.

⁠Chapter 3. Installing a virtual machine

After you have installed the virtualization packages on your Red Hat Enterprise Linux 7 host system, you can create guest operating systems. You can create guest virtual machines using the New button in virt-manager or use the virt-install command line interface to install virtual machines by a list of parameters or with a script. Both methods are covered by this chapter.

Detailed installation instructions are available in the following chapters for specific versions of Red Hat Enterprise Linux and Microsoft Windows.

⁠3.1. Guest virtual machine prerequisites and considerations

Various factors should be considered before creating any guest virtual machines. Not only should the role of a virtual machine be considered before deployment, but regular ongoing monitoring and assessment based on variable factors (load, amount of clients) should be performed. Some factors include:

⁠Performance

Guest virtual machines should be deployed and configured based on their intended tasks. Some guest systems (for instance, guests running a database server) may require special performance considerations. Guests may require more assigned CPUs or memory based on their role and projected system load.

⁠Input/Output requirements and types of Input/Output

Some guest virtual machines may have a particularly high I/O requirement or may require further considerations or projections based on the type of I/O (for instance, typical disk block size access, or the amount of clients).

⁠Storage

Some guest virtual machines may require higher priority access to storage or faster disk types, or may require exclusive access to areas of storage. The amount of storage used by guests should also be regularly monitored and taken into account when deploying and maintaining storage. Make sure to read all the considerations outlined in Chapter 13, Securing the host physical machine and improving performance. It is also important to understand that your physical storage may limit your options in your virtual storage.

⁠Networking and network infrastructure

Depending upon your environment, some guest virtual machines could require faster network links than other guests. Bandwidth or latency are often factors when deploying and maintaining guests, especially as requirements or load changes.

⁠Request requirements

SCSI requests can only be issued to guest virtual machines on virtio drives if the virtio drives are backed by whole disks, and the disk device parameter is set to lun, as shown in the following example:

<devices>
   <emulator>/usr/libexec/qemu-kvm</emulator>
   <disk type='block' device='lun'>

⁠3.2. Creating guests with virt-install

You can use the virt-install command to create virtual machines from the command line. virt-install is used either interactively (with a graphical application such as virt-viewer) or as part of a script to automate the creation of virtual machines. Using virt-install with kickstart files allows for unattended installation of virtual machines.

Note that you need root privileges in order for virt-install commands to complete successfully.

The virt-install tool provides a number of options that can be passed on the command line. Most virt-install options are not required. Minimum requirements are --name, --ram, guest storage (--disk, --filesystem or --nodisks), and an install option. To see a complete list of options, run the following command:

# virt-install --help

The virt-install man page also documents each command option, important variables, and examples.

Prior to running virt-install, qemu-img is a related command which can be used to configure storage options. See refer to Chapter 16, Using qemu-img for instructions on using qemu-img.

⁠3.2.1. Network installation with virt-install

The following examples use a network bridge (named br0 in these examples), which must be created separately, prior to running virt-install. See Section 7.4.1, “Configuring bridged networking on a Red Hat Enterprise Linux 7 host” for details on creating a network bridge.

⁠

Example 3.1. Using virt-install to install a Red Hat Enterprise Linux 6 virtual machine

This example creates a Red Hat Enterprise Linux 6 guest:

# virt-install \
   --name=guest1-rhel6-64 \
   --disk path=/var/lib/libvirt/images/guest1-rhel6-64.dsk,size=8,sparse=false,cache=none \
   --graphics spice \
   --vcpus=2 --ram=2048 \
   --location=http://example1.com/installation_tree/RHEL6.4-Server-x86_64/os \
   --network bridge=br0 \
   --os-type=linux \
   --os-variant=rhel6

The options used in this example are as follows:

--name

The name of the virtual machine.

--disk

Specifies storage configuration details for the virtual machine.

--graphics

An important option which allows graphical installation of a virtual machine. It specifies the type of graphical tool to use for interactive installation, or can be set to none for a fully automated installation by script.

--vcpus

The number of vCPUs to allocate to the guest.

--ram

The amount of memory (RAM) to allocate to the guest, in MiB.

--location

The location of the installation media. The above example uses an http:// network installation, but several other protocols can be used.

--network bridge

Specifies the network bridge to use for installation, which must be configured before running virt-install. See Section 7.4.1, “Configuring bridged networking on a Red Hat Enterprise Linux 7 host” for details on creating a network bridge.

--os-type

The guest operating system type.

--os-variant

Another important option in virt-install, used to further optimize the guest configuration. Using this option can reduce installation time and improve performance.

Running the osinfo-query os command returns a complete list of operating system variants identified by a short ID to use with the --os-variant option. For example, --os-variant=rhel7.0 configures a Red Hat Enterprise Linux 7.0 virtual machine.

In Red Hat Enterprise Linux 7, the virtio-scsi controller is available for use in guests. If both the host and guest support virtio-scsi, you can use it as follows:

⁠

Example 3.2. Using virt-install to install a guest virtual machine with the virtio-scsi controller

The items in bold are required on top of a standard installation in order to use the virtio-scsi controller.

# virt-install \
   --name=guest1-rhel7 \
   --controller type=scsi,model=virtio-scsi \
   --disk path=/var/lib/libvirt/images/guest1-rhel7.dsk,size=8,sparse=false,cache=none,bus=scsi \
   --graphics spice \
   --vcpus=2 --ram=2048 \
   --location=http://example1.com/installation_tree/RHEL7.1-Server-x86_64/os \
   --network bridge=br0 \
   --os-type=linux \
   --os-variant=rhel7

Note

Ensure that you select the correct --os-type for your operating system when running this command. This option prevents the installation disk from disconnecting when rebooting during the installation procedure. The --os-variant option further optimizes the configuration for a specific guest operating system.

⁠3.2.2. PXE installation with virt-install

virt-install PXE installations require both the --network=bridge:bridge_name parameter, where bridge_name is the name of the bridge, and the --pxe parameter.

By default, if no network is found, the guest virtual machine will attempt to boot from alternative bootable devices. If there is no other bootable device found, the guest virtual machine will pause. You can use the qemu-kvm boot parameter reboot-timeout to allow the guest to retry booting if no bootable device is found, like so:

# qemu-kvm -boot reboot-timeout=1000

⁠

Example 3.3. Fully-virtualized PXE installation with virt-install

# virt-install --hvm --connect qemu:///system \
--network=bridge:br0 --pxe --graphics spice \
--name=rhel6-machine --ram=756 --vcpus=4 \
--os-type=linux --os-variant=rhel6 \
--disk path=/var/lib/libvirt/images/rhel6-machine.img,size=10

Note that the command above cannot be executed in a text-only environment. A fully-virtualized (--hvm) guest can only be installed in a text-only environment if the --location and --extra-args "console=console_type" are provided instead of the --graphics spice parameter.

⁠3.2.3. Kickstart installation with virt-install

The following example shows using a kickstart file with virt-install:

⁠

Example 3.4. Kickstart installation with virt-install

# virt-install -n rhel7ks-guest -r 1024 --file=/var/lib/libvirt/images/rhel7ks-guest.img --file-size=10 \
 --location /var/lib/libvirt/images/rhel-server-7.1-x86_64-dvd.iso --nographics \
 --extra-args="ks=http://192.168.122.1/ks.cfg ip=dhcp \
console=tty0 console=ttyS0,115200n8” --os-variant=rhel7.0

⁠3.2.4. Guest installation with virt-install and text-based Anaconda

The following example shows using virt-install with text-based Anaconda installation:

⁠

Example 3.5. Guest installation with virt-install and text-based Anaconda

# virt-install -n rhel6anaconda-guest -r 1024 --disk=path=/path/to/rhel6anaconda-guest.img,size=10 \
--location /mnt/RHEL6DVD --nographics \
--extra-args=”console=tty0 console=ttyS0,115200n8” \
--disk=path=/path/to/rhel6-dvd.iso,device=cdrom

⁠3.3. Creating guests with virt-manager

virt-manager, also known as Virtual Machine Manager, is a graphical tool for creating and managing guest virtual machines.

This section covers how to install a Red Hat Enterprise Linux 7 guest virtual machine on a Red Hat Enterprise Linux 7 host using virt-manager.

These procedures assume that the KVM hypervisor and all other required packages are installed and the host is configured for virtualization. For more information on installing the virtualization packages, refer to Chapter 2, Installing the virtualization packages.

⁠3.3.1. virt-manager installation overview

The New VM wizard breaks down the virtual machine creation process into five steps:

Choosing the hypervisor and installation type

Locating and configuring the installation media

Configuring memory and CPU options

Configuring the virtual machine's storage

Configuring virtual machine name, networking, architecture, and other hardware settings

Ensure that virt-manager can access the installation media (whether locally or over the network) before you continue.

⁠3.3.2. Creating a Red Hat Enterprise Linux 7 guest with virt-manager

This procedure covers creating a Red Hat Enterprise Linux 7 guest virtual machine with a locally stored installation DVD or DVD image. Red Hat Enterprise Linux 7 DVD images are available from the Red Hat Customer Portal.

⁠

Procedure 3.1. Creating a Red Hat Enterprise Linux 7 guest virtual machine with virt-manager using local installation media

Optional: Preparation

Prepare the storage environment for the virtual machine. For more information on preparing storage, refer to Chapter 14, Storage pools.

Important

Various storage types may be used for storing guest virtual machines. However, for a virtual machine to be able to use migration features, the virtual machine must be created on networked storage.

Red Hat Enterprise Linux 7 requires at least 1 GB of storage space. However, Red Hat recommends at least 5 GB of storage space for a Red Hat Enterprise Linux 7 installation and for the procedures in this guide.

Open virt-manager and start the wizard

Open virt-manager by executing the virt-manager command as root or opening Applications → System Tools → Virtual Machine Manager. Alternatively, run the virt-manager command as root.

⁠

Figure 3.1. The Virtual Machine Manager window

Optionally, open a remote hypervisor by selecting the hypervisor and clicking the Connect button.

Click on the Create a new virtual machine button to start the new virtualized guest wizard.

⁠

Figure 3.2. The Create a new virtual machine button

The New VM window opens.

Specify installation type

Select an installation type:

⁠Local install media (ISO image or CDROM)

This method uses a CD-ROM, DVD, or image of an installation disk (for example, .iso).

⁠Network Install (HTTP, FTP, or NFS)

This method involves the use of a mirrored Red Hat Enterprise Linux or Fedora installation tree to install a guest. The installation tree must be accessible through either HTTP, FTP, or NFS.

If you select Network Install, provide the installation URL, and the Kickstart URL and Kernel options (if required) and continue to Step 5.

⁠Network Boot (PXE)

This method uses a Preboot eXecution Environment (PXE) server to install the guest virtual machine. Setting up a PXE server is covered in the Deployment Guide. To install via network boot, the guest must have a routable IP address or shared network device.

If you select Network Boot, continue to Step 5. After all steps are completed, a DHCP request is sent and if a valid PXE server is found the guest virtual machine's installation processes will start.

⁠Import existing disk image

This method allows you to create a new guest virtual machine and import a disk image (containing a pre-installed, bootable operating system) to it.

⁠

Figure 3.3. Virtual machine installation method

Click Forward to continue.

Select the local installation media

If you selected Local install media (ISO image or CDROM), specify your desired local installation media.

⁠

Figure 3.4. Local ISO image installation

If you wish to install from a CD-ROM or DVD, select the Use CDROM or DVD radio button, and select the appropriate disk drive from the drop-down list of drives available.

If you wish to install from an ISO image, select Use ISO image, and then click the Browse... button to open the Locate media volume window.

Select the installation image you wish to use, and click Choose Volume.

If no images are displayed in the Locate media volume window, click on the Browse Local button to browse the host machine for the installation image or DVD drive containing the installation disk. Select the installation image or DVD drive containing the installation disk and click Open; the volume is selected for use and you are returned to the Create a new virtual machine wizard.

Important

For ISO image files and guest storage images, the recommended location to use is /var/lib/libvirt/images/. Any other location may require additional configuration by SELinux. Refer to the Red Hat Enterprise Linux Virtualization Security Guide or the Red Hat Enterprise Linux SELinux User's and Administrator's Guide for more details on configuring SELinux.

Next, configure the OS type and Version of the installation. Ensure that you select the appropriate operating system type for your virtual machine. This can be specified manually or by selecting the Automatically detect operating system based on install media check box.

Click Forward to continue.

⁠

Configure memory (RAM) and virtual CPUs

Specify the number of CPUs and amount of memory (RAM) to allocate to the virtual machine. The wizard shows the number of CPUs and amount of memory you can allocate; these values affect the host's and guest's performance.

Virtual machines require sufficient physical memory (RAM) to run efficiently and effectively. Red Hat supports a minimum of 512MB of RAM for a virtual machine. Red Hat recommends at least 1024MB of RAM for each logical core.

Assign sufficient virtual CPUs for the virtual machine. If the virtual machine runs a multi-threaded application, assign the number of virtual CPUs the guest virtual machine will require to run efficiently.

You cannot assign more virtual CPUs than there are physical processors (or hyper-threads) available on the host system. The number of virtual CPUs available is noted in the Up to X available field.

⁠

Figure 3.5. Configuring Memory and CPU

After you have configured the memory and CPU settings, click Forward to continue.

Note

Memory and virtual CPUs can be overcommitted. For more information on overcommitting, refer to Chapter 8, Overcommitting with KVM.

Configure storage

Enable and assign sufficient space for your virtual machine and any applications it requires. Assign at least 5 GB for a desktop installation or at least 1 GB for a minimal installation.

⁠

Figure 3.6. Configuring virtual storage

Note

Live and offline migrations require virtual machines to be installed on shared network storage. For information on setting up shared storage for virtual machines, refer to Section 17.3, “Shared storage example: NFS for a simple migration”.

With the default local storage

Select the Create a disk image on the computer's hard drive radio button to create a file-based image in the default storage pool, the /var/lib/libvirt/images/ directory. Enter the size of the disk image to be created. If the Allocate entire disk now check box is selected, a disk image of the size specified will be created immediately. If not, the disk image will grow as it becomes filled.

Note

Although the storage pool is a virtual container it is limited by two factors: maximum size allowed to it by qemu-kvm and the size of the disk on the host physical machine. Storage pools may not exceed the size of the disk on the host physical machine. The maximum sizes are as follows:

virtio-blk = 2^63 bytes or 8 Exabytes(using raw files or disk)

Ext4 = ~ 16 TB (using 4 KB block size)

XFS = ~8 Exabytes

qcow2 and host file systems keep their own metadata and scalability should be evaluated/tuned when trying very large image sizes. Using raw disks means fewer layers that could affect scalability or max size.

Click Forward to create a disk image on the local hard drive. Alternatively, select Select managed or other existing storage, then select Browse to configure managed storage.

With a storage pool

If you select Select managed or other existing storage to use a storage pool, click Browse to open the Locate or create storage volume window.

⁠

Figure 3.7. The Locate or create storage volume window

Select a storage pool from the Storage Pools list.

Optional: Click on the New Volume button to create a new storage volume. The Add a Storage Volume screen will appear. Enter the name of the new storage volume.

Choose a format option from the Format dropdown menu. Format options include raw, qcow2, and qed. Adjust other fields as desired. Note that the qcow2 version used here is version 3. To change the qcow version refer to Section 26.20.2, “Setting target elements”

⁠

Figure 3.8. The Add a Storage Volume window

Select the new volume and click Choose volume. Next, click Finish to return to the New VM wizard. Click Forward to continue.

Name and final configuration

Name the virtual machine. Virtual machine names can contain letters, numbers and the following characters: underscores (_), periods (.), and hyphens (-). Virtual machine names must be unique for migration and cannot consist only of numbers.

Verify the settings of the virtual machine and click Finish when you are satisfied; this will create the virtual machine with default networking settings, virtualization type, and architecture.

⁠

Figure 3.9. Verifying the configuration

Or, to further configure the virtual machine's hardware, check the Customize configuration before install check box to change the guest's storage or network devices, to use the para-virtualized (virtio) drivers or to add additional devices. This opens another wizard that will allow you to add, remove, and configure the virtual machine's hardware settings.

Note

Red Hat Enterprise Linux 4 or Red Hat Enterprise Linux 5 guest virtual machines cannot be installed using graphical mode. As such, you must select "Cirrus" instead of "QXL" as a video card.

After configuring the virtual machine's hardware, click Apply. virt-manager will then create the virtual machine with your specified hardware settings.

Click on the Advanced options down arrow to inspect and modify advanced options. For a standard Red Hat Enterprise Linux 7 installation, none of these options require modification.

Click Finish to continue into the Red Hat Enterprise Linux installation sequence. For more information on installing Red Hat Enterprise Linux 7, refer to the Red Hat Enterprise Linux 7 Installation Guide.

A Red Hat Enterprise Linux 7 guest virtual machine is now created from an ISO installation disk image.

⁠3.4. Comparison of virt-install and virt-manager installation options

This table provides a quick reference to compare equivalent virt-install and virt-manager installation options for when installing a virtual machine.

Most virt-install options are not required. Minimum requirements are --name, --ram, guest storage (--disk, --filesystem or --nodisks), and an install option. These options are further specified with arguments; to see a complete list of command options and related arguments, run the following command:

# virt-install --help

In virt-manager, at minimum, a name, installation method, memory (RAM), vCPUs, storage are required.

⁠

Table 3.1. virt-install and virt-manager configuration comparison for guest installations

Configuration on virtual machine | virt-install option | virt-manager installation wizard label and step number
Virtual machine name | --name, -n | Name (step 5)
RAM to allocate (MiB) | --ram, -r | Memory (RAM) (step 3)
Storage - specify storage media | --disk | Enable storage for this virtual machine → Create a disk image on the computer's hard drive, or Select managed or other existing storage (step 4)
Storage - export a host directory to the guest | --filesystem | Enable storage for this virtual machine → Select managed or other existing storage (step 4)
Storage - configure no local disk storage on the guest | --nodisks | Deselect the Enable storage for this virtual machine checkbox (step 4)
Installation media location (local install) | --file | Local install media → Locate your install media (steps 1-2)
Installation via distribution tree (network install) | --location | Network install → URL (steps 1-2)
Install guest with PXE | --pxe | Network boot (step 1)
Number of vCPUs | --vcpus | CPUs (step 3)
Host network | --network | Advanced options dropdown menu (step 5)
Operating system | --os-type | OS type (step 2)
Operating system variant/version | --os-variant | Version (step 2)
Graphical display method | --graphics, --nographics | * virt-manager provides GUI installation only

⁠Chapter 4. Virtualizing Red Hat Enterprise Linux on Other Platforms

This chapter contains reference material for customers running Red Hat Enterprise Linux 7 as a virtualized operating system on other virtualization hosts.

⁠4.1. On VMware ESX

Red Hat Enterprise Linux 7 provides the following drivers:

vmw_balloon - a para-virtualized memory ballooning driver used when running Red Hat Enterprise Linux on VMware hosts. For further information about this driver, refer to http://kb.VMware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&externalId=1002586.

vmmouse_drv - a para-virtualized mouse driver used when running Red Hat Enterprise Linux on VMware hosts. For further information about this driver, refer to http://kb.VMware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&externalId=5739104.

vmware_drv - a para-virtualized video driver used when running Red Hat Enterprise Linux on VMware hosts. For further information about this driver, refer to http://kb.VMware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&externalId=1033557.

vmxnet3 - a para-virtualized network adapter used when running Red Hat Enterprise Linux on VMware hosts. For further information about this driver, refer to http://kb.VMware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1001805.

vmw_pvscsi - a para-virtualized SCSI adapter used when running Red Hat Enterprise Linux on VMware hosts. For further information about this driver, refer to http://kb.VMware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1010398.

⁠4.2. On Hyper-V

Red Hat Enterprise Linux 7 ships with Microsoft's Linux Integration Services, a set of drivers that enable synthetic device support in supported virtualized operating systems. Red Hat Enterprise Linux 7 provides the following drivers:

hv_vmbus - a main para-virtualized driver for communicating with the Hyper-V host

hv_netvsc - a para-virtualized network driver

hv_storvsc - a para-virtualized storage (SCSI) driver

hyperv_fb - a para-virtualized framebuffer device

hyperv_keyboard - a para-virtualized keyboard driver

hid_hyperv - a para-virtualized mouse driver

hv_balloon -a memory hotplug and ballooning driver

hv_utils - a guest integration services driver

For more information about the drivers provided refer to Microsoft's website and the Linux and FreeBSD Virtual Machines on Hyper-V article in particular. More information on the Hyper-V feature set, is located in Feature Descriptions for Linux and FreeBSD virtual machines on Hyper-V.

Another article which may be helpful is: Enabling Linux Support on Windows Server 2012 R2 Hyper-V. Access to this article may require a Microsoft account.

Note

The Hyper-V manager supports shrinking a GUID Partition Table (GPT) partitioned disk if there is free space after the last partition, by allowing the user to drop the unused last part of the disk. However, this operation will silently delete the secondary GPT header on the disk, which may trigger error messages when guest examines the partition table (for example, when printing the partition table with parted). This is a known limit of Hyper-V. As a workaround, it is possible to manually restore the secondary GPT header with the gdisk expert command "e", after shrinking the GPT disk. This also occurs when using Hyper-V's Expand option, but can also be fixed with the parted tool. Information about these commands can be viewed in the parted(8) and gdisk(8) man pages.

For more information, see the following article: Best Practices for running Linux on Hyper-V.

⁠Chapter 5. Installing a fully-virtualized Windows guest

This chapter describes how to create a fully-virtualized Windows guest using the command-line (virt-install), launch the operating system's installer inside the guest, and access the installer through virt-viewer.

Important

Red Hat Enterprise Linux 7 Windows guests are only supported under specific subscription programs such as Advanced Mission Critical (AMC). If you are unsure whether your subscription model includes support for Windows guests, please contact customer support.

To install a Windows operating system on the guest, use the virt-viewer tool. This tool allows you to display the graphical console of a virtual machine (via the VNC protocol). In doing so, virt-viewer allows you to install a fully-virtualized guest's operating system with that operating system's installer (for example, the Windows 8 installer).

Installing a Windows operating system involves two major steps:

Creating the guest virtual machine, using either virt-install or virt-manager.

Installing the Windows operating system on the guest virtual machine, using virt-viewer.

Refer to Chapter 3, Installing a virtual machine for details about creating a guest virtual machine with virt-install or virt-manager.

Note that this chapter does not describe how to install a Windows operating system on a fully-virtualized guest. Rather, it only covers how to create the guest and launch the installer within the guest. For information on how to install a Windows operating system, refer to the relevant Microsoft installation documentation.

⁠5.1. Using virt-install to create a guest

The virt-install command allows you to create a fully-virtualized guest from a terminal, for example, without a GUI.

Important

Before creating the guest, consider first if the guest needs to use KVM Windows para-virtualized (virtio) drivers. If it does, keep in mind that you can do so during or after installing the Windows operating system on the guest. For more information about virtio drivers, refer to Chapter 6, KVM Para-virtualized (virtio) Drivers.

For instructions on how to install KVM virtio drivers, refer to Section 6.1, “Installing the KVM Windows virtio drivers”.

It is possible to create a fully-virtualized guest with only a single command. To do so, run the following program (replace the values accordingly):

# virt-install \
   --name=guest-name \
   --os-type=windows \
   --network network=default \
   --disk path=path-to-disk,size=disk-size \
   --cdrom=path-to-install-disk \
   --graphics spice --ram=1024

The path-to-disk must be a device (e.g. /dev/sda3) or image file (/var/lib/libvirt/images/name.img). It must also have enough free space to support the disk-size.

The path-to-install-disk must be a path to an ISO image, or a URL from which to access a minimal boot ISO image.

Important

All image files are stored in /var/lib/libvirt/images/ by default. Other directory locations for file-based images are possible, but may require SELinux configuration. If you run SELinux in enforcing mode, refer to the Red Hat Enterprise Linux SELinux User's and Administrator's Guide for more information on SELinux.

Once the fully-virtualized guest is created, virt-viewer will launch the guest and run the operating system's installer. Refer to the relevant Microsoft installation documentation for instructions on how to install the operating system.

⁠5.2. Tips for more efficiency with Windows guest virtual machines

The following flags should be set with libvirt to make sure the Windows guest virual machine works efficiently:

hv_relaxed

hv_spinlocks=0x1fff

hv_vapic

hv_time

⁠5.2.1. Setting the Hyper-V clock flag

To set the Hyper-V clock flag, augment the Windows guest virtual machine XML to contain:

⁠

​
​
​<domain type='kvm'>
​...
​  <clock offset='utc'>
​    <timer name='hypervclock' present='yes'/>
​  </clock>
​...
​</domain>
​
​

Figure 5.1. Clock element XML

This action should not be done while the guest virtual machine is running. Shutdown the guest virtual machine, change the XML file and then re-start the guest virtual machine.

⁠Chapter 6. KVM Para-virtualized (virtio) Drivers

Para-virtualized drivers enhance the performance of guests, decreasing guest I/O latency and increasing throughput to near bare-metal levels. It is recommended to use the para-virtualized drivers for fully virtualized guests running I/O heavy tasks and applications.

Virtio drivers are KVM's para-virtualized device drivers, available for Windows guest virtual machines running on KVM hosts. These drivers are included in the virtio package. The virtio package supports block (storage) devices and network interface controllers.

Important

Red Hat Enterprise Linux 7 Windows guests are only supported under specific subscription programs such as Advanced Mission Critical (AMC). If you are unsure whether your subscription model includes support for Windows guests, please contact customer support.

The KVM virtio drivers are automatically loaded and installed on the following:

Red Hat Enterprise Linux 4.8 and newer

Red Hat Enterprise Linux 5.3 and newer

Red Hat Enterprise Linux 6 and newer

Red Hat Enterprise Linux 7 and newer

Some versions of Linux based on the 2.6.27 kernel or newer kernel versions.

Versions of Red Hat Enterprise Linux in the list above detect and install the drivers; additional installation steps are not required.

In Red Hat Enterprise Linux 3 (3.9 and above), manual installation is required.

Note

PCI devices are limited by the virtualized system architecture. Refer to Chapter 18, Guest virtual machine device configuration for additional limitations when using assigned devices.

Using KVM virtio drivers, the following Microsoft Windows versions are expected to run similarly to bare-metal-based systems.

Windows Server 2003 (32-bit and 64-bit versions)

Windows Server 2008 (32-bit and 64-bit versions)

Windows Server 2008 R2 (64-bit only)

Windows 7 (32-bit and 64-bit versions)

Windows Server 2012 (64-bit only)

Windows Server 2012 R2 (64-bit only)

Windows 8 (32-bit and 64-bit versions)

Windows 8.1 (32-bit and 64-bit versions)

Note

Network connectivity issues sometimes arise when attempting to use older virtio drivers with newer versions of QEMU. Keeping the drivers up to date is therefore recommended.

⁠6.1. Installing the KVM Windows virtio drivers

This section covers the installation process for the KVM Windows virtio drivers. The KVM virtio drivers can be loaded during the Windows installation or installed after the guest's installation.

You can install the virtio drivers on a guest virtual machine using one of the following methods:

hosting the installation files on a network accessible to the virtual machine

using a virtualized CD-ROM device of the driver installation disk .iso file

using a USB drive, by mounting the same (provided) .ISO file that you would use for the CD-ROM

using a virtualized floppy device to install the drivers during boot time (required and recommended only for Windows Server 2003)

This guide describes installation from the para-virtualized installer disk as a virtualized CD-ROM device.

⁠

Download the drivers

The virtio-win package contains the virtio block and network drivers for all supported Windows guest virtual machines.

Note

The virtio-win package can be found here: https://access.redhat.com/downloads/content/rhel---7/x86_64/2476/virtio-win/1.7.1-1.el7/noarch/fd431d51/package. It requires access to one of the following channels:

RHEL Client Supplementary (v. 7)

RHEL Server Supplementary (v. 7)

RHEL Workstation Supplementary (v. 7)

Download and install the virtio-win package on the host with the yum command.

 # yum install virtio-win

The list of virtio-win packages that are supported on Windows operating systems, and the current certified package version, can be found at the following URL: windowsservercatalog.com.

Note that the Red Hat Enterprise Virtualization Hypervisor and Red Hat Enterprise Linux are created on the same code base so the drivers for the same version (for example, Red Hat Enterprise Virtualization Hypervisor 3.3 and Red Hat Enterprise Linux 6.5) are supported for both environments.

The virtio-win package installs a CD-ROM image, virtio-win.iso, in the /usr/share/virtio-win/ directory.

⁠

Install the virtio drivers

When booting a Windows guest that uses virtio-win devices, the relevant virtio-win device drivers must already be installed on this guest. The virtio-win drivers are not provided as inbox drivers in Microsoft's Windows installation kit, so installation of a Windows guest on a virtio-win storage device (viostor/virtio-scsi) requires that you provide the appropriate driver during the installation, either directly from the virtio-win.iso or from the supplied Virtual Floppy image virtio-win<version>.vfd.

⁠6.2. Installing the drivers on an installed Windows guest virtual machine

This procedure covers installing the virtio drivers with a virtualized CD-ROM after Windows is installed.

Follow this procedure to add a CD-ROM image with virt-manager and then install the drivers.

⁠

Procedure 6.1. Installing from the driver CD-ROM image with virt-manager

Open virt-manager and the guest virtual machine

Open virt-manager, then open the guest virtual machine from the list by double-clicking the guest name.

Open the hardware window

Click the lightbulb icon on the toolbar at the top of the window to view virtual hardware details.

⁠

Figure 6.1. The virtual hardware details button

Then click the Add Hardware button at the bottom of the new view that appears.

⁠

Figure 6.2. The virtual machine hardware information window

This opens a wizard for adding the new device.

Select the ISO file

Ensure that the Select managed or other existing storage radio button is selected, and browse to the virtio driver's .iso image file. The default location for the latest version of the drivers is /usr/share/virtio-win/virtio-win.iso.

Change the Device type to IDE cdrom and click the Forward button to proceed.

⁠

Figure 6.3. The Add new virtual hardware wizard

Reboot

Reboot or start the virtual machine to begin using the driver disc. Virtualized IDE devices require a restart to for the virtual machine to recognize the new device.

Once the CD-ROM with the drivers is attached and the virtual machine has started, proceed with Procedure 6.2, “Windows installation on a Windows 7 virtual machine”.

⁠

Procedure 6.2. Windows installation on a Windows 7 virtual machine

This procedure installs the drivers on a Windows 7 virtual machine as an example. Adapt the Windows installation instructions to your guest's version of Windows.

Open the Computer Management window

On the desktop of the Windows virtual machine, click the Windows icon at the bottom corner of the screen to open the Start menu.

Right-click on Computer and select Manage from the pop-up menu.

⁠

Figure 6.4. The Computer Management window

Open the Device Manager

Select the Device Manager from the left-most pane. This can be found under Computer Management > System Tools.

⁠

Figure 6.5. The Computer Management window

Start the driver update wizard

View available system devices

Expand System devices by clicking on the arrow to its left.

⁠

Figure 6.6. Viewing available system devices in the Computer Management window

Locate the appropriate device

There are up to four drivers available: the balloon driver, the serial driver, the network driver, and the block driver.

Balloon, the balloon driver, affects the PCI standard RAM Controller in the System devices group.

vioserial, the serial driver, affects the PCI Simple Communication Controller in the System devices group.

NetKVM, the network driver, affects the Network adapters group. This driver is only available if a virtio NIC is configured. Configurable parameters for this driver are documented in Appendix E, NetKVM Driver Parameters.

viostor, the block driver, affects the Disk drives group. This driver is only available if a virtio disk is configured.

Right-click on the device whose driver you wish to update, and select Update Driver... from the pop-up menu.

This example installs the balloon driver, so right-click on PCI standard RAM Controller.

⁠

Figure 6.7. The Computer Management window

Open the driver update wizard

From the drop-down menu, select Update Driver Software... to access the driver update wizard.

⁠

Figure 6.8. Opening the driver update wizard

Specify how to find the driver

The first page of the driver update wizard asks how you want to search for driver software. Click on the second option, Browse my computer for driver software.

⁠

Figure 6.9. The driver update wizard

Select the driver to install

Open a file browser

Click on Browse...

⁠

Figure 6.10. The driver update wizard

Browse to the location of the driver

A separate driver is provided for each combination of operating systems and architectures. The drivers are arranged hierarchically according to their driver type, the operating system, and the architecture on which they will be installed: driver_type/os/arch/. For example, the Balloon driver for a Windows 7 operating system with an x86 (32-bit) architecture, resides in the Balloon/w7/x86 directory.

⁠

Figure 6.11. The Browse for driver software pop-up window

Once you have navigated to the correct location, click OK.

Click Next to continue

⁠

Figure 6.12. The Update Driver Software wizard

The following screen is displayed while the driver installs:

⁠

Figure 6.13. The Update Driver Software wizard

Close the installer

The following screen is displayed when installation is complete:

⁠

Figure 6.14. The Update Driver Software wizard

Click Close to close the installer.

Reboot

Reboot the virtual machine to complete the driver installation.

⁠6.3. Installing drivers during the Windows installation

This procedure covers installing the virtio drivers during a Windows installation.

This method allows a Windows guest virtual machine to use the virtio drivers for the default storage device.

⁠

Procedure 6.3. Installing virtio drivers during the Windows installation

Install the virtio-win package:

# yum install virtio-win

Note

The virtio-win package can be found here: https://access.redhat.com/downloads/content/rhel---7/x86_64/2476/virtio-win/1.7.1-1.el7/noarch/fd431d51/package. It requires access to one of the following channels:

RHEL Client Supplementary (v. 7)

RHEL Server Supplementary (v. 7)

RHEL Workstation Supplementary (v. 7)

Creating the guest virtual machine

Important

Create the virtual machine, as normal, without starting the virtual machine. Follow one of the procedures below.

Select one of the following guest-creation methods, and follow the instructions.

Creating the guest virtual machine with virsh

This method attaches the virtio driver floppy disk to a Windows guest before the installation.

If the virtual machine is created from an XML definition file with virsh, use the virsh define command not the virsh create command.

Create, but do not start, the virtual machine. Refer to the Red Hat Enterprise Linux Virtualization Administration Guide for details on creating virtual machines with the virsh command.

Add the driver disk as a virtualized floppy disk with the virsh command. This example can be copied and used if there are no other virtualized floppy devices attached to the guest virtual machine. Note that vm_name should be replaced with the name of the virtual machine.

# virsh attach-disk vm_name /usr/share/virtio-win/virtio-win.vfd fda --type floppy

You can now continue with Step 3.

Creating the guest virtual machine with virt-manager and changing the disk type

At the final step of the virt-manager guest creation wizard, check the Customize configuration before install checkbox.

⁠

Figure 6.15. The virt-manager guest creation wizard

Click on the Finish button to continue.

Open the Add Hardware wizard

Click the Add Hardware button in the bottom left of the new panel.

⁠

Figure 6.16. The Add Hardware button

Select storage device

Storage is the default selection in the Hardware type list.

⁠

Figure 6.17. The Add new virtual hardware wizard

Ensure the Select managed or other existing storage radio button is selected. Click Browse....

⁠

Figure 6.18. Select managed or existing storage

In the new window that opens, click Browse Local. Navigate to /usr/share/virtio-win/virtio-win.vfd, and click Select to confirm.

Change Device type to Floppy disk, and click Finish to continue.

⁠

Figure 6.19. Change the Device type

Confirm settings

Review the device settings.

⁠

Figure 6.20. The virtual machine hardware information window

You have now created a removable device accessible by your virtual machine.

Change the hard disk type

To change the hard disk type from IDE Disk to Virtio Disk, we must first remove the existing hard disk, Disk 1. Select the disk and click on the Remove button.

⁠

Figure 6.21. The virtual machine hardware information window

Add a new virtual storage device by clicking Add Hardware. Then, change the Device type from IDE disk to Virtio Disk. Click Finish to confirm the operation.

⁠

Figure 6.22. The virtual machine hardware information window

Ensure settings are correct

Review the settings for VirtIO Disk 1.

⁠

Figure 6.23. The virtual machine hardware information window

When you are satisfied with the configuration details, click the Begin Installation button.

⁠

Figure 6.24. The Begin Installation button

You can now continue with Step 3.

Creating the guest virtual machine with virt-install

Append the following parameter exactly as listed below to add the driver disk to the installation with the virt-install command:

--disk path=/usr/share/virtio-win/virtio-win.vfd,device=floppy

Important

If the device you wish to add is a disk (that is, not a floppy or a cdrom), you will also need to add the bus=virtio option to the end of the --disk parameter, like so:

--disk path=/usr/share/virtio-win/virtio-win.vfd,device=disk,bus=virtio

According to the version of Windows you are installing, append one of the following options to the virt-install command:

--os-variant win2k3

--os-variant win7

You can now continue with Step 3.

⁠

Additional steps for driver installation

During the installation, additional steps are required to install drivers, depending on the type of Windows guest.

⁠

Windows Server 2003

Before the installation blue screen repeatedly press F6 for third party drivers.

⁠

Figure 6.25. The Windows Setup screen

Press S to install additional device drivers.

⁠

Figure 6.26. The Windows Setup screen

⁠

Figure 6.27. The Windows Setup screen

Press Enter to continue the installation.

⁠

Windows Server 2008

Follow the same procedure for Windows Server 2003, but when the installer prompts you for the driver, click on Load Driver, point the installer to Drive A: and pick the driver that suits your guest operating system and architecture.

⁠6.4. Using KVM virtio drivers for existing devices

You can modify an existing hard disk device attached to the guest to use the virtio driver instead of the virtualized IDE driver. The example shown in this section edits libvirt configuration files. Note that the guest virtual machine does not need to be shut down to perform these steps, however the change will not be applied until the guest is completely shut down and rebooted.

⁠

Procedure 6.4. Using KVM virtio drivers for existing devices

Ensure that you have installed the appropriate driver (viostor), as described in Section 6.1, “Installing the KVM Windows virtio drivers”, before continuing with this procedure.

Run the virsh edit <guestname> command as root to edit the XML configuration file for your device. For example, virsh edit guest1. The configuration files are located in /etc/libvirt/qemu.

Below is a file-based block device using the virtualized IDE driver. This is a typical entry for a virtual machine not using the virtio drivers.

<disk type='file' device='disk'>
   <source file='/var/lib/libvirt/images/disk1.img'/>
   <target dev='hda' bus='ide'/>
</disk>

Change the entry to use the virtio device by modifying the bus= entry to virtio. Note that if the disk was previously IDE it will have a target similar to hda, hdb, or hdc and so on. When changing to bus=virtio the target needs to be changed to vda, vdb, or vdc accordingly.

<disk type='file' device='disk'>
   <source file='/var/lib/libvirt/images/disk1.img'/>
   <target dev='vda' bus='virtio'/>
</disk>

Remove the address tag inside the disk tags. This must be done for this procedure to work. Libvirt will regenerate the address tag appropriately the next time the virtual machine is started.

Alternatively, virt-manager, virsh attach-disk or virsh attach-interface can add a new device using the virtio drivers.

Refer to the libvirt website for more details on using Virtio: http://www.linux-kvm.org/page/Virtio

⁠6.5. Using KVM virtio drivers for new devices

This procedure covers creating new devices using the KVM virtio drivers with virt-manager.

Alternatively, the virsh attach-disk or virsh attach-interface commands can be used to attach devices using the virtio drivers.

Important

Ensure the drivers have been installed on the Windows guest before proceeding to install new devices. If the drivers are unavailable the device will not be recognized and will not work.

⁠

Procedure 6.5. Adding a storage device using the virtio storage driver

Open the guest virtual machine by double clicking on the name of the guest in virt-manager.

Open the Show virtual hardware details tab by clicking the lightbulb button.

⁠

Figure 6.28. The Show virtual hardware details tab

In the Show virtual hardware details tab, click on the Add Hardware button.

Select hardware type

Select Storage as the Hardware type.

⁠

Figure 6.29. The Add new virtual hardware wizard

Select the storage device and driver

Create a new disk image or select a storage pool volume.

Set the Device type to Virtio disk to use the virtio drivers.

⁠

Figure 6.30. The Add new virtual hardware wizard

Click Finish to complete the procedure.

⁠

Procedure 6.6. Adding a network device using the virtio network driver

Open the guest virtual machine by double clicking on the name of the guest in virt-manager.

Open the Show virtual hardware details tab by clicking the lightbulb button.

⁠

Figure 6.31. The Show virtual hardware details tab

In the Show virtual hardware details tab, click on the Add Hardware button.

Select hardware type

Select Network as the Hardware type.

⁠

Figure 6.32. The Add new virtual hardware wizard

Select the network device and driver

Set the Device model to virtio to use the virtio drivers. Choose the desired Host device.

⁠

Figure 6.33. The Add new virtual hardware wizard

Click Finish to complete the procedure.

Once all new devices are added, reboot the virtual machine. Windows virtual machines may not recognize the devices until the guest is rebooted.

⁠Chapter 7. Network configuration

This chapter provides an introduction to the common networking configurations used by libvirt based guest virtual machines. For additional information, consult the libvirt network architecture documentation: http://libvirt.org/archnetwork.html.

Red Hat Enterprise Linux 7 supports the following networking setups for virtualization:

virtual networks using Network Address Translation (NAT)

directly allocated physical devices using PCI device assignment

directly allocated virtual functions using PCIe SR-IOV

bridged networks

You must enable NAT, network bridging or directly assign a PCI device to allow external hosts access to network services on guest virtual machines.

⁠7.1. Network Address Translation (NAT) with libvirt

One of the most common methods for sharing network connections is to use Network Address Translation (NAT) forwarding (also known as virtual networks).

⁠

Host configuration

Every standard libvirt installation provides NAT-based connectivity to virtual machines as the default virtual network. Verify that it is available with the virsh net-list --all command.

# virsh net-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes

If it is missing the following could be used in the XML configuration file (such as /etc/libvirtd/qemu/myguest.xml) for the guest:

# ll /etc/libvirt/qemu/
total 12
drwx------. 3 root root 4096 Nov  7 23:02 networks
-rw-------. 1 root root 2205 Nov 20 01:20 r6.4.xml
-rw-------. 1 root root 2208 Nov  8 03:19 r6.xml

The default network is defined from /etc/libvirt/qemu/networks/default.xml

Mark the default network to automatically start:

# virsh net-autostart default
Network default marked as autostarted

Start the default network:

# virsh net-start default
Network default started

Once the libvirt default network is running, you will see an isolated bridge device. This device does not have any physical interfaces added. The new device uses NAT and IP forwarding to connect to the physical network. Do not add new interfaces.

# brctl show
bridge name     bridge id               STP enabled     interfaces
virbr0          8000.000000000000       yes

libvirt adds iptables rules which allow traffic to and from guest virtual machines attached to the virbr0 device in the INPUT, FORWARD, OUTPUT and POSTROUTING chains. libvirt then attempts to enable the ip_forward parameter. Some other applications may disable ip_forward, so the best option is to add the following to /etc/sysctl.conf.

 net.ipv4.ip_forward = 1

⁠

Guest virtual machine configuration

Once the host configuration is complete, a guest virtual machine can be connected to the virtual network based on its name. To connect a guest to the 'default' virtual network, the following could be used in the XML configuration file (such as /etc/libvirtd/qemu/myguest.xml) for the guest:

<interface type='network'>
   <source network='default'/>
</interface>

Note

Defining a MAC address is optional. If you do not define one, a MAC address is automatically generated and used as the MAC address of the bridge device used by the network. Manually setting the MAC address may be useful to maintain consistency or easy reference throughout your environment, or to avoid the very small chance of a conflict.

<interface type='network'>
  <source network='default'/>
  <mac address='00:16:3e:1a:b3:4a'/>
</interface>

⁠7.2. Disabling vhost-net

The vhost-net module is a kernel-level back end for virtio networking that reduces virtualization overhead by moving virtio packet processing tasks out of user space (the QEMU process) and into the kernel (the vhost-net driver). vhost-net is only available for virtio network interfaces. If the vhost-net kernel module is loaded, it is enabled by default for all virtio interfaces, but can be disabled in the interface configuration in the case that a particular workload experiences a degradation in performance when vhost-net is in use.

Specifically, when UDP traffic is sent from a host machine to a guest virtual machine on that host, performance degradation can occur if the guest virtual machine processes incoming data at a rate slower than the host machine sends it. In this situation, enabling vhost-net causes the UDP socket's receive buffer to overflow more quickly, which results in greater packet loss. It is therefore better to disable vhost-net in this situation to slow the traffic, and improve overall performance.

To disable vhost-net, edit the <interface> sub-element in the guest virtual machine's XML configuration file and define the network as follows:

<interface type="network">
   ...
   <model type="virtio"/>
   <driver name="qemu"/>
   ...
</interface>

Setting the driver name to qemu forces packet processing into QEMU user space, effectively disabling vhost-net for that interface.

⁠7.3. Enabling vhost-net zero-copy

In Red Hat Enterprise Linux 7, vhost-net zero-copy is disabled by default. To enable this action on a permanent basis, add a new file vhost-net.conf to /etc/modprobe.d with the following content:

options vhost_net  experimental_zcopytx=1

If you want to disable this again, you can run the following:

modprobe -r vhost_net

modprobe vhost_net experimental_zcopytx=0

The first command removes the old file, the second one makes a new file (like above) and disables zero-copy. You can use this to enable as well but the change will not be permanent.

To confirm that this has taken effect, check the output of cat /sys/module/vhost_net/parameters/experimental_zcopytx. It should show:

$ cat /sys/module/vhost_net/parameters/experimental_zcopytx
0

⁠7.4. Bridged networking

Bridged networking (also known as virtual network switching) is used to place virtual machine network interfaces on the same network as the physical interface. Bridges require minimal configuration and make a virtual machine appear on an existing network, which reduces management overhead and network complexity. As bridges contain few components and configuration variables, they provide a transparent set-up which is straightforward to understand and troubleshoot, if required.

Bridging can be configured in a virtualized environment using standard Red Hat Enterprise Linux tools, virt-manager, or libvirt, and is described in the following sections.

However, even in a virtualized environment, bridges may be more easily created using the host operating system's networking tools. More information about this bridge creation method can be found in the Red Hat Enterprise Linux 7 Networking Guide.

⁠7.4.1. Configuring bridged networking on a Red Hat Enterprise Linux 7 host

Bridged networking can be configured for virtual machines on a Red Hat Enterprise Linux host, independent to the virtualization management tools. This configuration may be more appropriate when the virtualization bridge is the host's only network interface, or is the host's management network interface.

The Red Hat Enterprise Linux 7 Networking Guide contains detailed instructions on configuring bridged networking. See the Red Hat Enterprise Linux 7 Networking Guide - Configure Network Bridging for instructions on configuring network bridging outside of the virtualization tools.

⁠7.4.2. Bridged networking with Virtual Machine Manager

This section provides instructions on creating a bridge from a host machine's interface to a guest virtual machine using virt-manager.

Note

Depending on your environment, setting up a bridge with libvirt tools in Red Hat Enterprise Linux 7 may require disabling Network Manager, which is not recommended by Red Hat. A bridge created with libvirt also requires libvirtd to be running for the bridge to maintain network connectivity.

It is recommended to configure bridged networking on the physical Red Hat Enterprise Linux host as described in the Red Hat Enterprise Linux 7 Networking Guide, while using libvirt after bridge creation to add virtual machine interfaces to the bridges.

⁠

Procedure 7.1. Creating a bridge with virt-manager

From the virt-manager main menu, click Edit > Connection Details to open the Connection Details window.

Click the Network Interfaces tab.

Click the + at the bottom of the window to configure a new network interface.

In the Interface type drop-down menu, select Bridge, and then click Forward to continue.

⁠

Figure 7.1. Adding a bridge

In the Name field, enter a name for the bridge, such as br0.

Select a Start mode from the drop-down menu. Choose from one of the following:

none - deactivates the bridge

onboot - activates the bridge on the next guest virtual machine reboot

hotplug - activates the bridge even if the guest virtual machine is running

Check the Activate now check box to activate the bridge immediately.

To configure either the IP settings or Bridge settings, click the appropriate Configure button. A separate window will open to specify the desired settings. Make any necessary changes and click OK when done.

Select the physical interface to connect to your virtual machines. If the interface is currently in use by another guest virtual machine, you will receive a warning message.

Click Finish and the wizard closes, taking you back to the Connections menu.

⁠

Figure 7.2. Adding a bridge

Select the bridge to use, and click Apply to exit the wizard.

To stop the interface, click the Stop Interface key. Once the bridge is stopped, to delete the interface, click the Delete Interface key.

⁠7.4.3. Bridged networking with libvirt

Depending on your environment, setting up a bridge with libvirt in Red Hat Enterprise Linux 7 may require disabling Network Manager, which is not recommended by Red Hat. This also requires libvirtd to be running for the bridge to operate.

It is recommended to configure bridged networking on the physical Red Hat Enterprise Linux host as described in the Red Hat Enterprise Linux 7 Networking Guide.

Important

libvirt is now able to take advantage of new kernel tunable parameters to manage host bridge forwarding database (FDB) entries, thus potentially improving system network performance when bridging multiple virtual machines. Set the macTableManager attribute of a network's <bridge> element to 'libvirt' in the host's XML configuration file:

<bridge name='br0' macTableManager='libvirt'/>

This will turn off learning (flood) mode on all bridge ports, and libvirt will add or remove entries to the FDB as necessary. Along with removing the overhead of learning the proper forwarding ports for MAC addresses, this also allows the kernel to disable promiscuous mode on the physical device that connects the bridge to the network, which further reduces overhead.

⁠Chapter 8. Overcommitting with KVM

⁠8.1. Introduction

The KVM hypervisor supports overcommitting CPUs and overcommitting memory. Overcommitting is allocating more virtualized CPUs (vCPUs) or memory than there are physical resources on the system. With CPU overcommit, under-utilized virtualized servers or desktops can run on fewer servers which saves a number of system resources, with the net effect of less power, cooling, and investment in server hardware.

As most processes do not access 100% of their allocated memory all the time, KVM can use this behavior to its advantage and allocate more memory for guest virtual machines than the host machine actually has available, in a process called overcommiting of resources.

⁠8.2. Overcommitting Memory

Important

Overcommitting is not an ideal solution for all memory issues, as the recommended methods to deal with memory shortage are to allocate less memory per guest, add more physical memory to the host, or utilize swap space. If you decide to overcommit memory, ensure sufficient testing is performed. Contact Red Hat support for assistance with overcommitting.

Virtual machines running on a KVM hypervisor do not have dedicated blocks of physical RAM assigned to them. Instead, each guest functions as a Linux process where the host machine's Linux kernel allocates memory only when requested. In addition, the host's memory manager can move the guest's memory between its own physical memory and swap space. This is why overcommitting requires allotting sufficient swap space on the host physical machine to accommodate all guests, as well as enough memory for the host's processes. As a basic rule, the host's operating system requires a maximum of 4GB of memory along with a minimum of 4GB of swap space. Refer to Example 8.1, “Memory overcommit example” for more information.

Red Hat Knowledgebase has an article on safely and efficiently determining the size of the swap partition.

Note

The example below is provided as a guide for configuring swap only. The settings listed may not be appropriate for your environment.

⁠

Example 8.1. Memory overcommit example

ExampleServer1 has 32GB of physical RAM. The system is being configured to run 50 guest virtual machines, each requiring 1GB of virtualized memory. As mentioned above, the host machine's system itself needs a maximum of 4GB (apart from the guests) as well as an additional 4GB as a swap space minimum.

The swap space is calculated as follows:

Calculate the amount of memory needed for the sum of all the virtual machines - In this example: (50 virtual machines * 1GB of memory per virtual machine) = 50GB

Add the virtual machine's memory amount to the amount needed for the host's operating system and for the host machine's minimum swap space - In this example: 50GB guest memory + 4GB host's OS + 4GB minimal swap = 58GB

Subtract this amount from the amount of physical RAM there is on the system - In this example 58GB - 32GB = 26GB

The answer is the amount of swap space that needs to be allocated - In this example 26GB.

Note

Overcommitting does not work with all virtual machines, but has been found to work in a desktop virtualization setup with minimal intensive usage or running several identical guests with KSM. Proceed with caution before changing these settings.

For more information on KSM and overcommitting, see the Red Hat Enterprise Linux 7 Virtualization Tuning and Optimization Guide.

⁠8.3. Overcommitting virtualized CPUs (vCPUs)

The KVM hypervisor supports overcommitting virtualized CPUs (vCPUs). Virtualized CPUs can be overcommitted as far as load limits of guest virtual machines allow. Use caution when overcommitting vCPUs, as loads near 100% may cause dropped requests or unusable response times.

In Red Hat Enterprise Linux 7, it is possible to overcommit guests with more than one vCPU (known as symmetric multiprocessing virtual machines), however, you may experience performance deterioration when running more cores on the virtual machine than are present on your physical CPU. For example, a virtual machine with four vCPUs should not be run on a host machine with a dual core processor. Overcommitting symmetric multiprocessing (SMP) virtual machines beyond the physical number of processing cores will cause significant performance degradation, due to programs getting less CPU time than required.

With SMP guests, some processing overhead is inherent. CPU overcommitting can increase the SMP overhead, because time-slicing guests can make inter-CPU communication inside a guest slower. This overhead increases with guests that have a larger number of vCPUs, or a larger overcommit ratio.

Virtualized CPUs are overcommitted best when when a single host has multiple guests, and each guest has a small number of vCPUs, compared to the number of host CPUs. The Linux scheduler is very efficient with this type of load. KVM should safely support guests with loads under 100% at a ratio of five vCPUs (on 5 virtual machines) to one physical CPU on one single host. The KVM hypervisor will switch between all of the virtual machines, making sure that the load is balanced.

For best performance, Red Hat recommends assigning guests only as many vCPUs as are required to run the programs that are inside each guest.

Important

Applications which use 100% of memory or processing resources may become unstable in overcommitted environments. Do not overcommit memory or CPUs in a production environment without extensive testing, as the amount of SMP and the CPU overcommit ratio is workload dependent.

⁠Chapter 9. KVM guest timing management

Virtualization involves several intrinsic challenges for time keeping in guest virtual machines. Interrupts cannot always be delivered simultaneously and instantaneously to all guest virtual machines, because interrupts in virtual machines are not true interrupts; they are injected into the guest virtual machine by the host machine. The host may be running another guest virtual machine, or a different process, meaning that the precise timing typically required by interrupts may not always be possible.

Guest virtual machines without accurate time keeping may experience issues with network applications and processes, as session validity, migration, and other network activities rely on timestamps to remain correct.

KVM avoids these issues by providing guest virtual machines with a para-virtualized clock (kvm-clock). However, it is still vital to test timing before attempting activities that may be affected by time keeping inaccuracies.

Note

Red Hat Enterprise Linux 5.5 and newer, Red Hat Enterprise Linux 6.0 and newer, and Red Hat Enterprise Linux 7 use kvm-clock as their default clock source. Running without kvm-clock requires special configuration, and is not recommended.

Important

The Network Time Protocol (NTP) daemon should be running on the host and the guest virtual machines. Make sure to install ntp and enable the ntpd service:

Enable the ntpd service and add it to the default startup sequence:

# systemctl enable ntpd

Start the service:

# systemctl start ntpd

The ntpd service will correct the effects of clock skew as long as the clock runs no more than 0.05% faster or slower than the reference time source. The ntp startup script adjusts the clock offset from the reference time by adjusting the system clock at startup time, if required.

⁠

Constant Time Stamp Counter (TSC)

Modern Intel and AMD CPUs provide a constant Time Stamp Counter (TSC). The count frequency of the constant TSC does not vary when the CPU core itself changes frequency, for example, to comply with a power saving policy. A CPU with a constant TSC frequency is necessary in order to use the TSC as a clock source for KVM guests.

Your CPU has a constant Time Stamp Counter if the constant_tsc flag is present. To determine if your CPU has the constant_tsc flag run the following command:

$ cat /proc/cpuinfo | grep constant_tsc

If any output is given your CPU has the constant_tsc bit. If no output is given follow the instructions below.

⁠

Configuring hosts without a constant Time Stamp Counter

Systems without a constant TSC frequency cannot use the TSC as a clock source for virtual machines, and require additional configuration. Power management features interfere with accurate time keeping and must be disabled for guest virtual machines to accurately keep time with KVM.

Important

These instructions are for AMD revision F CPUs only.

If the CPU lacks the constant_tsc bit, disable all power management features (BZ#513138). Each system has several timers it uses to keep time. The TSC is not stable on the host, which is sometimes caused by cpufreq changes, deep C state, or migration to a host with a faster TSC. Deep C sleep states can stop the TSC. To prevent the kernel using deep C states append processor.max_cstate=1 to the kernel boot. To make this change persistent, edit values of the GRUB_CMDLINE_LINUX key in the /etc/default/grubfile. For example. if you want to enable emergency mode for each boot, edit the entry as follows:

GRUB_CMDLINE_LINUX="emergency"

Note that you can specify multiple parameters for the GRUB_CMDLINE_LINUX key, similarly to adding the parameters in the GRUB 2 boot menu.

To disnable cpufreq (only necessary on hosts without the constant_tsc), install kernel-tools and enable the cpupower.service (systemctl disable cpupower.service). If you want to disable this service every time the guest virtual machine boots, change the configuration file in /etc/sysconfig/cpupower and change the CPUPOWER_START_OPTS and CPUPOWER_STOP_OPTS. Valid limits can be found in the /sys/devices/system/cpu/[cpuid]/cpufreq/scaling_available_governors files. For more information on this package or on power management and governors, refer to the Red Hat Enterprise Linux 7 Power Management Guide.

⁠9.1. Required parameters for Red Hat Enterprise Linux guests

For certain Red Hat Enterprise Linux guest virtual machines, additional kernel parameters are required. These parameters can be set by appending them to the end of the /kernel line in the /boot/grub2/grub.cfg file of the guest virtual machine.

The table below lists versions of Red Hat Enterprise Linux and the parameters required on the specified systems.

⁠

Table 9.1. Kernel parameter requirements

Red Hat Enterprise Linux version | Additional guest kernel parameters
7.0 AMD64/Intel 64 with the para-virtualized clock | Additional parameters are not required
6.1 and higher AMD64/Intel 64 with the para-virtualized clock | Additional parameters are not required
6.0 AMD64/Intel 64 with the para-virtualized clock | Additional parameters are not required
6.0 AMD64/Intel 64 without the para-virtualized clock | notsc lpj=n
5.5 AMD64/Intel 64 with the para-virtualized clock | Additional parameters are not required
5.5 AMD64/Intel 64 without the para-virtualized clock | notsc lpj=n
5.5 x86 with the para-virtualized clock | Additional parameters are not required
5.5 x86 without the para-virtualized clock | clocksource=acpi_pm lpj=n
5.4 AMD64/Intel 64 | notsc
5.4 x86 | clocksource=acpi_pm
5.3 AMD64/Intel 64 | notsc
5.3 x86 | clocksource=acpi_pm
4.8 AMD64/Intel 64 | notsc
4.8 x86 | clock=pmtmr
3.9 AMD64/Intel 64 | Additional parameters are not required

Note

The lpj parameter requires a numeric value equal to the loops per jiffy value of the specific CPU on which the guest virtual machine runs. If you do not know this value, do not set the lpj parameter.

Warning

The divider kernel parameter was previously recommended for Red Hat Enterprise Linux 4 and 5 guest virtual machines that did not have high responsiveness requirements, or exist on systems with high guest density. It is no longer recommended for use with guests running Red Hat Enterprise Linux 4, or Red Hat Enterprise Linux 5 versions prior to version 5.8.

divider can improve throughput on Red Hat Enterprise Linux 5 versions equal to or later than 5.8 by lowering the frequency of timer interrupts. For example, if HZ=1000, and divider is set to 10 (that is, divider=10), the number of timer interrupts per period changes from the default value (1000) to 100 (the default value, 1000, divided by the divider value, 10).

BZ#698842 details a bug in the way that the divider parameter interacts with interrupt and tick recording. This bug is fixed as of Red Hat Enterprise Linux 5.8. However, the divider parameter can still cause kernel panic in guests using Red Hat Enterprise Linux 4, or Red Hat Enterprise Linux 5 versions prior to version 5.8.

Red Hat Enterprise Linux 6 and newer does not have a fixed-frequency clock interrupt; it operates in tickless mode and uses the timer dynamically as required. The divider parameter is therefore not useful for Red Hat Enterprise Linux 6 and Red Hat Enterprise Linux 7, and guests on these systems are not affected by this bug.

⁠9.2. Steal time accounting

Steal time is the amount of CPU time desired by a guest virtual machine that is not provided by the host. Steal time occurs when the host allocates these resources elsewhere: for example, to another guest.

Steal time is reported in the CPU time fields in /proc/stat. It is automatically reported by utilities such as top and vmstat. It is displayed as "%st", or in the "st" column. Note that it cannot be switched off.

Large amounts of steal time indicate CPU contention, which can reduce guest performance. To relieve CPU contention, increase the guest's CPU priority or CPU quota, or run fewer guests on the host.

⁠Chapter 10. Network booting with libvirt

Guest virtual machines can be booted with PXE enabled. PXE allows guest virtual machines to boot and load their configuration off the network itself. This section demonstrates some basic configuration steps to configure PXE guests with libvirt.

This section does not cover the creation of boot images or PXE servers. It is used to explain how to configure libvirt, in a private or bridged network, to boot a guest virtual machine with PXE booting enabled.

Warning

These procedures are provided only as an example. Ensure that you have sufficient backups before proceeding.

⁠10.1. Preparing the boot server

To perform the steps in this chapter you will need:

A PXE Server (DHCP and TFTP) - This can be a libvirt internal server, manually-configured dhcpd and tftpd, dnsmasq, a server configured by Cobbler, or some other server.

Boot images - for example, PXELINUX configured manually or by Cobbler.

⁠10.1.1. Setting up a PXE boot server on a private libvirt network

This example uses the default network. Perform the following steps:

⁠

Procedure 10.1. Configuring the PXE boot server

Place the PXE boot images and configuration in /var/lib/tftp.

Run the following commands:

# virsh net-destroy default
# virsh net-edit default

Edit the <ip> element in the configuration file for the default network to include the appropriate address, network mask, DHCP address range, and boot file, where BOOT_FILENAME represents the file name you are using to boot the guest virtual machine.

<ip address='192.168.122.1' netmask='255.255.255.0'>
   <tftp root='/var/lib/tftp' />
   <dhcp>
      <range start='192.168.122.2' end='192.168.122.254' />
      <bootp file='BOOT_FILENAME' />
   </dhcp>
</ip>

Run:

# virsh net-start default

Boot the guest using PXE (refer to Section 10.2, “Booting a guest using PXE”).

⁠10.2. Booting a guest using PXE

This section demonstrates how to boot a guest virtual machine with PXE.

⁠10.2.1. Using bridged networking

⁠

Procedure 10.2. Booting a guest using PXE and bridged networking

Ensure bridging is enabled such that the PXE boot server is available on the network.

Boot a guest virtual machine with PXE booting enabled. You can use the virt-install command to create a new virtual machine with PXE booting enabled, as shown in the following example command:

virt-install --pxe --network bridge=breth0 --prompt

Alternatively, ensure that the guest network is configured to use your bridged network, and that the XML guest configuration file has a <boot dev='network'/> element inside the <os> element, as shown in the following example:

<os>
   <type arch='x86_64' machine='rhel6.2.0'>hvm</type>
   <boot dev='network'/>
   <boot dev='hd'/>
</os>
<interface type='bridge'>
   <mac address='52:54:00:5a:ad:cb'/>
   <source bridge='breth0'/>
   <target dev='vnet0'/>
   <alias name='net0'/>
   <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>

⁠10.2.2. Using a private libvirt network

⁠

Procedure 10.3. Using a private libvirt network

Configure PXE booting on libvirt as shown in Section 10.1.1, “Setting up a PXE boot server on a private libvirt network”.

Boot a guest virtual machine using libvirt with PXE booting enabled. You can use the virt-install command to create/install a new virtual machine using PXE:

virt-install --pxe --network network=default --prompt

Alternatively, ensure that the guest network is configured to use your bridged network, and that the XML guest configuration file has a <boot dev='network'/> element inside the <os> element, as shown in the following example:

<os>
   <type arch='x86_64' machine='rhel6.2.0'>hvm</type>
   <boot dev='network'/>
   <boot dev='hd'/>
</os>

Also ensure that the guest virtual machine is connected to the private network:

<interface type='network'>
   <mac address='52:54:00:66:79:14'/>
   <source network='default'/>
   <target dev='vnet0'/>
   <alias name='net0'/>
   <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>

⁠Chapter 11. Enhancing virtualization with the QEMU guest agent and SPICE agent

Agents in Red Hat Enterprise Linux such as the QEMU guest agent and the SPICE agent can be deployed to help the virtualization tools run more optimally on your system. These agents are described in this chapter.

Note

To further optimize and tune host and guest performance, see the Red Hat Enterprise Linux 7 Virtualization Tuning and Optimization Guide.

⁠11.1. QEMU guest agent

The QEMU guest agent runs inside the guest and allows the host machine to issue commands to the guest operating system using libvirt, helping with functions such as freezing and thawing filesystems. The guest operating system then responds to those commands asynchronously. The QEMU guest agent package, qemu-guest-agent, is installed by default in Red Hat Enterprise Linux 7.

CPU hotplugging and hot-unplugging are supported with the help of the QEMU guest agent on Linux guests; CPUs can be enabled or disabled while the guest is running, thus implementing the hotplug feature and mimicking the unplug feature. Refer to Section 23.16.6, “Configuring virtual CPU count” for more information.

This section covers the libvirt commands and options available to the guest agent.

Important

Note that it is only safe to rely on the QEMU guest agent when run by trusted guests. An untrusted guest may maliciously ignore or abuse the guest agent protocol, and although built-in safeguards exist to prevent a denial of service attack on the host, the host requires guest co-operation for operations to run as expected.

⁠11.1.1. Setting up communication between the QEMU guest agent and host

The host machine communicates with the QEMU guest agent through a VirtIO serial connection between the host and guest machines. A VirtIO serial channel is connected to the host via a character device driver (typically a Unix socket), and the guest listens on this serial channel.

Note

The qemu-guest-agent does not detect if the host is listening to the VirtIO serial channel. However, as the current use for this channel is to listen for host-to-guest events, the probability of a guest virtual machine running into problems by writing to the channel with no listener is very low. Additionally, the qemu-guest-agent protocol includes synchronization markers which allow the host physical machine to force a guest virtual machine back into sync when issuing a command, and libvirt already uses these markers, so that guest virtual machines are able to safely discard any earlier pending undelivered responses.

⁠11.1.1.1. Configuring the QEMU guest agent on a Linux guest

The QEMU guest agent can be configured on a running or shut down virtual machine. If configured on a running guest, the guest will start using the guest agent immediately. If the guest is shut down, the QEMU guest agent will be enabled at next boot.

Either virsh or virt-manager can be used to configure communication between the guest and the QEMU guest agent. The following instructions describe how to configure the QEMU guest agent on a Linux guest.

⁠

Procedure 11.1. Setting up communication between guest agent and host with virsh on a shut down Linux guest

Shut down the virtual machine

Ensure the virtual machine (named rhel7 in this example) is shut down before configuring the QEMU guest agent:

# virsh shutdown rhel7

Add the QEMU guest agent channel to the guest XML configuration

Edit the guest's XML file to add the QEMU guest agent details:

# virsh edit rhel7

Add the following to the guest's XML file and save the changes:

​<channel type='unix'>
​   <target type='virtio' name='org.qemu.guest_agent.0'/>
​</channel>

Start the virtual machine

# virsh start rhel7

Install the QEMU guest agent on the guest

Install the QEMU guest agent if not yet installed in the guest virtual machine:

# yum install qemu-guest-agent

Start the QEMU guest agent in the guest

Start the QEMU guest agent service in the guest:

# systemctl start qemu-guest-agent

Alternatively, the QEMU guest agent can be configured on a running guest with the following steps:

⁠

Procedure 11.2. Setting up communication between guest agent and host on a running Linux guest

Create an XML file for the QEMU guest agent

​# cat agent.xml
​<channel type='unix'>
​   <target type='virtio' name='org.qemu.guest_agent.0'/>
​</channel>

Attach the QEMU guest agent to the virtual machine

Attach the QEMU guest agent to the running virtual machine (named rhel7 in this example) with this command:

# virsh attach-device rhel7 agent.xml

Install the QEMU guest agent on the guest

Install the QEMU guest agent if not yet installed in the guest virtual machine:

# yum install qemu-guest-agent

Start the QEMU guest agent in the guest

Start the QEMU guest agent service in the guest:

# systemctl start qemu-guest-agent

⁠

Procedure 11.3. Setting up communication between the QEMU guest agent and host with virt-manager

Shut down the virtual machine

Ensure the virtual machine is shut down before configuring the QEMU guest agent.

To shut down the virtual machine, select it from the list of virtual machines in Virtual Machine Manager, then click the light switch icon from the menu bar.

Add the QEMU guest agent channel to the guest

Open the virtual machine's hardware details by clicking the lightbulb icon at the top of the guest window.

Click the Add Hardware button to open the Add New Virtual Hardware window, and select Channel.

Select the QEMU guest agent from the Name drop-down list and click Finish:

⁠

Figure 11.1. Selecting the QEMU guest agent channel device

Start the virtual machine

To start the virtual machine, select it from the list of virtual machines in Virtual Machine Manager, then click the triangle (play) icon from the menu bar.

Install the QEMU guest agent on the guest

Open the guest with virt-manager and install the QEMU guest agent if not yet installed in the guest virtual machine:

# yum install qemu-guest-agent

Start the QEMU guest agent in the guest

Start the QEMU guest agent service in the guest:

# systemctl start qemu-guest-agent

The QEMU guest agent is now configured on the rhel7 virtual machine.

⁠11.1.2. Configuring the QEMU guest agent on a Windows guest

The following instructions describe how to configure the QEMU guest agent on a Windows guest running on a Red Hat Enterprise Linux host.

Note

Windows guest virtual machines require the QEMU guest agent package for Windows, qemu-guest-agent-win. This agent is required for VSS (Volume Shadow Copy Service) support for Windows guest virtual machines running on Red Hat Enterprise Linux. More information can be found here.

⁠

Procedure 11.4. Configuring the QEMU guest agent on a Windows guest

(Optional) Prepare the Red Hat Enterprise Linux host machine

While the virtio-win package is not required to run the QEMU guest agent, these drivers improve performance.

Check for the virtio-win package on the host

Verify the following packages are installed on the Red Hat Enterprise Linux host machine:

virtio-win, located in /usr/share/virtio-win/

Create an *.iso file to copy the drivers to the Windows guest

To copy the drivers to the Windows guest, make an *.iso file for the virtio-win drivers using the following command:

# mkisofs -o /var/lib/libvirt/images/virtiowin.iso /usr/share/virtio-win/drivers

Install the drivers on the Windows guest

Install the virtio-serial drivers in the guest by mounting the *.iso to the Windows guest to update the driver. Start the guest (in this example, the guest's name is win7x86), then attach the driver .iso file to the guest as shown:

# virsh attach-disk win7x86 /var/lib/libvirt/images/virtiowin.iso hdb

To install the drivers using the Windows Control Panel, navigate to the following menus:

To install the virtio-win driver - Select Hardware and Sound > Device manager > virtio-serial driver.

Shutdown the guest virtual machine

Gracefully shutdown the guest virtual machine by running the

# virsh shutdown win7x86

command.

Add the QEMU guest agent channel to the guest from the host

Add the following elements to the guest's XML file using the # virsh edit win7x86 command and save the changes. The source socket name must be unique in the host, named win7x86.agent in this example:

​<channel type='unix'>
​      <source mode='bind' path='/var/lib/libvirt/qemu/win7x86.agent'/>
​      <target type='virtio' name='org.qemu.guest_agent.0'/>
​   </channel>

Alternatively, this step can be completed with virt-manager:

To add the QEMU guest agent channel to the guest with virt-manager, click the lightbulb icon at the top of the guest window to show the virtual machine hardware details.

Click the Add Hardware button to open the Add New Virtual Hardware window and select Channel.

Select the QEMU guest agent from the Name drop-down list and click Finish:

⁠

Figure 11.2. Selecting the QEMU guest agent channel device

Boot the Windows guest

Boot the Windows guest from the host machine to apply the changes:

# virsh start win7x86

Preparing the QEMU guest agent in the Windows guest

To prepare the guest agent in the Windows guest:

Install the latest virtio-win package

Run the following command on the host to install the virtio-win package:

# yum install virtio-win

Confirm the installation completed

After the virtio-win package finishes installing, check the /usr/share/virtio-win/guest-agent/ folder and you will find an file named qemu-ga-x64.msi or qemu-ga-x86.msi as shown:

# ls -l /usr/share/virtio-win/guest-agent/

total 1544

-rw-r--r--. 1 root root 856064 Oct 23 04:58 qemu-ga-x64.msi

-rw-r--r--. 1 root root 724992 Oct 23 04:58 qemu-ga-x86.msi

Install the .msi file

From the Windows guest, install the qemu-ga-x64.msi or qemu-ga-x86.msi file by double clicking on the file. Once installed, it will be shown as a qemu-guest-agent service in the Windows guest within the System Manager. This same manager can be used to monitor the status of the service.

⁠11.2. Using the QEMU guest agent with libvirt

Installing the QEMU guest agent allows various libvirt commands to become more powerful. The guest agent enhances the following virsh commands:

virsh shutdown --mode=agent - This shutdown method is more reliable than virsh shutdown --mode=acpi, as virsh shutdown used with the QEMU guest agent is guaranteed to shut down a cooperative guest in a clean state. If the agent is not present, libvirt must instead rely on injecting an ACPI shutdown event, but some guests ignore that event and thus will not shut down.

Can be used with the same syntax for virsh reboot.

virsh snapshot-create --quiesce - Allows the guest to flush its I/O into a stable state before the snapshot is created, which allows use of the snapshot without having to perform a fsck or losing partial database transactions. The guest agent allows a high level of disk contents stability by providing guest co-operation.

virsh domfsfreeze and virsh domfsthaw - Quiesces the guest filesystem in isolation.

virsh domfstrim - Instructs the guest to trim its filesystem.

virsh domtime - Queries or sets the guest's clock.

virsh setvcpus --guest - Instructs the guest to take CPUs offline.

virsh domifaddr --source agent - Queries the guest operating system's IP address via the guest agent.

virsh domfsinfo - Shows a list of mounted filesystems within the running guest.

virsh set-user-password - Sets the password for a user account in the guest.

⁠11.2.1. Creating a guest disk backup

libvirt can communicate with qemu-guest-agent to ensure that snapshots of guest virtual machine file systems are consistent internally and ready to use as needed. Guest system administrators can write and install application-specific freeze/thaw hook scripts. Before freezing the filesystems, the qemu-guest-agent invokes the main hook script (included in the qemu-guest-agent package). The freezing process temporarily deactivates all guest virtual machine applications.

The snapshot process is comprised of the following steps:

File system applications / databases flush working buffers to the virtual disk and stop accepting client connections

Applications bring their data files into a consistent state

Main hook script returns

qemu-guest-agent freezes the filesystems and the management stack takes a snapshot

Snapshot is confirmed

Filesystem function resumes

Thawing happens in reverse order.

To create a snapshot of the guest's file system, run the virsh snapshot-create --quiesce --disk-only command (alternatively, run virsh snapshot-create-as guest_name --quiesce --disk-only, explained in further detail in Section 23.19.2, “Creating a snapshot for the current guest virtual machine”).

Note

An application-specific hook script might need various SELinux permissions in order to run correctly, as is done when the script needs to connect to a socket in order to talk to a database. In general, local SELinux policies should be developed and installed for such purposes. Accessing file system nodes should work out of the box, after issuing the restorecon -FvvR command listed in Table 11.1, “QEMU guest agent package contents” in the table row labeled /etc/qemu-ga/fsfreeze-hook.d/.

The qemu-guest-agent binary RPM includes the following files:

⁠

Table 11.1. QEMU guest agent package contents

File name | Description
/usr/lib/systemd/system/qemu-guest-agent.service | Service control script (start/stop) for the QEMU guest agent.
/etc/sysconfig/qemu-ga | Configuration file for the QEMU guest agent, as it is read by the /usr/lib/systemd/system/qemu-guest-agent.service control script. The settings are documented in the file with shell script comments.
/usr/bin/qemu-ga | QEMU guest agent binary file.
/etc/qemu-ga | Root directory for hook scripts.
/etc/qemu-ga/fsfreeze-hook | Main hook script. No modifications are needed here.
/etc/qemu-ga/fsfreeze-hook.d | Directory for individual, application-specific hook scripts. The guest system administrator should copy hook scripts manually into this directory, ensure proper file mode bits for them, and then run restorecon -FvvR on this directory.
/usr/share/qemu-kvm/qemu-ga/ | Directory with sample scripts (for example purposes only). The scripts contained here are not executed.

The main hook script, /etc/qemu-ga/fsfreeze-hook logs its own messages, as well as the application-specific script's standard output and error messages, in the following log file: /var/log/qemu-ga/fsfreeze-hook.log. For more information, refer to the qemu-guest-agent wiki page at wiki.qemu.org or libvirt.org.

⁠11.3. SPICE agent

The SPICE agent helps run graphical applications such as virt-manager more smoothly, by helping integrate the guest operating system with the SPICE client.

For example, when resizing a window in virt-manager, the SPICE agent allows for automatic X session resolution adjustment to the client resolution. The SPICE agent also provides support for copy and paste between the host and guest, and prevents mouse cursor lag.

For system-specific information on the SPICE agent's capabilities, see the spice-vdagent package's README file.

⁠11.3.1. Setting up communication between the SPICE agent and host

The SPICE agent can be configured on a running or shut down virtual machine. If configured on a running guest, the guest will start using the guest agent immediately. If the guest is shut down, the SPICE agent will be enabled at next boot.

Either virsh or virt-manager can be used to configure communication between the guest and the SPICE agent. The following instructions describe how to configure the SPICE agent on a Linux guest.

⁠

Procedure 11.5. Setting up communication between guest agent and host with virsh on a Linux guest

Shut down the virtual machine

Ensure the virtual machine (named rhel7 in this example) is shut down before configuring the SPICE agent:

# virsh shutdown rhel7

Add the SPICE agent channel to the guest XML configuration

Edit the guest's XML file to add the SPICE agent details:

# virsh edit rhel7

Add the following to the guest's XML file and save the changes:

​<channel type='spicevmc'>
​   <target type='virtio' name='com.redhat.spice.0'/>
​</channel>

Start the virtual machine

# virsh start rhel7

Install the SPICE agent on the guest

Install the SPICE agent if not yet installed in the guest virtual machine:

# yum install spice-vdagent

Start the SPICE agent in the guest

Start the SPICE agent service in the guest:

# systemctl start spice-vdagent

Alternatively, the SPICE agent can be configured on a running guest with the following steps:

⁠

Procedure 11.6. Setting up communication between SPICE agent and host on a running Linux guest

Create an XML file for the SPICE agent

​# cat agent.xml
​<channel type='spicevmc'>
​   <target type='virtio' name='com.redhat.spice.0'/>
​</channel>

Attach the SPICE agent to the virtual machine

Attach the SPICE agent to the running virtual machine (named rhel7 in this example) with this command:

# virsh attach-device rhel7 agent.xml

Install the SPICE agent on the guest

Install the SPICE agent if not yet installed in the guest virtual machine:

# yum install spice-vdagent

Start the SPICE agent in the guest

Start the SPICE agent service in the guest:

# systemctl start spice-vdagent

⁠

Procedure 11.7. Setting up communication between the SPICE agent and host with virt-manager

Shut down the virtual machine

Ensure the virtual machine is shut down before configuring the SPICE agent.

To shut down the virtual machine, select it from the list of virtual machines in Virtual Machine Manager, then click the light switch icon from the menu bar.

Add the SPICE agent channel to the guest

Open the virtual machine's hardware details by clicking the lightbulb icon at the top of the guest window.

Click the Add Hardware button to open the Add New Virtual Hardware window, and select Channel.

Select the SPICE agent from the Name drop-down list and click Finish:

⁠

Figure 11.3. Selecting the SPICE agent channel device

Start the virtual machine

To start the virtual machine, select it from the list of virtual machines in Virtual Machine Manager, then click the triangle (play) icon from the menu bar.

Install the SPICE agent on the guest

Open the guest with virt-manager and install the SPICE agent if not yet installed in the guest virtual machine:

# yum install spice-vdagent

Start the SPICE agent in the guest

Start the SPICE agent service in the guest:

# systemctl start spice-vdagent

The SPICE agent is now configured on the rhel7 virtual machine.

⁠Chapter 12. Nested Virtualization

⁠12.1. Overview

As of Red Hat Enterprise Linux 7.2, nested virtualization is available as a Technology Preview for KVM guest virtual machines. With this feature, a guest virtual machine (also referred to as level 1 or L1) running on a physical host (level 0 or L0) can act as a hypervisor, and create its own (L2) guest virtual machines.

Nested virtualization is useful in a variety of scenarios, such as debugging hypervisors in a constrained environment and testing larger virtual deployments on a limited amount of physical resources.

Nested virtualization is supported on Intel and AMD processors.

⁠12.2. Setup

Follow these steps to enable, configure, and start using nested virtualization:

The feature is disabled by default. To enable it, use the following procedure on the L0 host physical machine.

For Intel:

Check whether nested virtualization is available on your system.

$ cat /sys/module/kvm_intel/parameters/nested

If this command returns Y or 1, the feature is enabled.

If the command returns 0 or N, use steps 2 and 3.

Unload the kvm_intel module:

# modprobe -r kvm_intel

Activate the nesting feature:

# modprobe kvm_intel nested=1

The nesting feature is now enabled only until the next reboot of the L0 host. To enable it permanently, add the following line to the /etc/modprobe.d/kvm.conf file:

options kvm_intel nested=1

For AMD:

Check whether nested virtualization is available on your system:

$catq /sys/module/kvm_amd/parameters/nested

If this command returns "Y" or "1", the feature is enabled.

If the command returns "0" or "N", use steps 2 and 3.

Unload the kvm_amd module

# modprobe -r kvm_amd

Activate the nesting feature

# modprobe kvm_amd nested=1

The nesting feature is now enabled only until the next reboot of the L0 host. To enable it permanently, add the following line to the /etc/modprobe.d/kvm.conf file:

options kvm_amd nested=1

Afterwards, configure your L1 virtual machine for nested virtualization using one of the following methods:

virt-manager

Open the GUI of the desired guest and click the Show Virtual Hardware Details icon.

Select the Processor menu, and in the Configuration section, type host-passthrough in the Model field (do not use the drop-down selection), and click Apply.

Domain XML

Add the following line to the domain XML file of the guest:

<cpu mode='host-passthrough'/>

If the XML file already contains a <cpu> element, rewrite it.

To start using nested virtualization, install an L2 guest within the L1 guest. To do this, follow the same procedure as when installing the L1 guest - see Chapter 3, Installing a virtual machine for more information.

⁠12.3. Restrictions and Limitations

As of Red Hat Enterprise Linux 7.2, it is strongly recommended to run Red Hat Enterprise Linux 7.2 or later in the L0 host and the L1 guests. L2 guests can contain any of the supported systems.

It is not supported to migrate L2 guests.

Use of L2 guests as hypervisors and creating L3 guests is not supported.

Not all features available on the host are available to be utilized by the L1 hypervisor. For instance, IOMMU/VT-d or APICv cannot be used by the L1 hypervisor.

It is not possible to use nested virtualization or a subset of it if the host CPU is missing the necessary feature flags. For example, if the host CPU does not support Extended Page Tables (EPT), then the L1 hypervisor will not be able to use it.

To determine if the L0 and L1 hypervisors are set up correctly, use the $ cat /proc/cpuinfo command on both L0 and L1, and make sure that the following flags are listed for the respective CPUs on both hypervisors:

For Intel - vmx (Hardware Virtualization)

For AMD - svm (equivalent to vmx)

⁠Part II. Administration

⁠Chapter 13. Securing the host physical machine and improving performance

The following tasks and tips can assist you with increasing the performance of your Red Hat Enterprise Linux host.

Run SELinux in enforcing mode. Set SELinux to run in enforcing mode with the setenforce command.

# setenforce 1

Remove or disable any unnecessary services such as AutoFS, NFS, FTP, HTTP, NIS, telnetd, sendmail and so on.

Only add the minimum number of user accounts needed for platform management on the server and remove unnecessary user accounts.

Avoid running any unessential applications on your host. Running applications on the host may impact virtual machine performance and can affect server stability. Any application which may crash the server will also cause all virtual machines on the server to go down.

Use a central location for virtual machine installations and images. Virtual machine images should be stored under /var/lib/libvirt/images/. If you are using a different directory for your virtual machine images make sure you add the directory to your SELinux policy and relabel it before starting the installation. Use of shareable, network storage in a central location is highly recommended.

Note

Additional performance tips can be found in the Red Hat Enterprise Linux Virtualization Tuning and Optimization Guide.

Additional security tips can be found in the Red Hat Enterprise Linux Virtualization Security Guide.

Both of these guides can be found at https://access.redhat.com/site/documentation/.

⁠13.1. Security Deployment Plan

When deploying virtualization technologies, you must ensure that the host physical machine and its operating system cannot be compromised. In this case the host physical machine is a Red Hat Enterprise Linux system that manages the system, devices, memory and networks as well as all guest virtual machines. If the host physical machine is insecure, all guest virtual machines in the system are vulnerable. There are several ways to enhance security on systems using virtualization. You or your organization should create a Deployment Plan. This plan needs to contain the following:

Operating specifications

Specifies which services are needed on your guest virtual machines

Specifies the host physical servers as well as what support is required for these services

Here are a few security issues to consider while developing a deployment plan:

Run only necessary services on host physical machines. The fewer processes and services running on the host physical machine, the higher the level of security and performance.

Enable SELinux on the hypervisor. Refer to the Red Hat Enterprise Linux Virtualization Security Guide for more information on using SELinux and virtualization.

Use a firewall to restrict traffic to the host physical machine. You can setup a firewall with default-reject rules that will help secure the host physical machine from attacks. It is also important to limit network-facing services.

Do not allow normal users to access the host operating system. If the host operating system is privileged, granting access to unprivileged accounts may compromise the level of security.

⁠13.2. Client access control

libvirt's client access control framework allows system administrators to setup fine grained permission rules across client users, managed objects, and API operations. This allows client connections to be locked down to a minimal set of privileges.

In a default configuration, the libvirtd daemon has three levels of access control. All connections start off in an unauthenticated state, where the only API operations allowed are those required to complete authentication. After successful authentication, a connection either has full, unrestricted access to all libvirt API calls, or is locked down to only "read only" operations, according to what socket the client connection originated on. The access control framework allows authenticated connections to have fine grained permission rules to be defined by the administrator. Every API call in libvirt has a set of permissions that will be validated against the object being used. Further permissions will also be checked if certain flags are set in the API call. In addition to checks on the object passed in to an API call, some methods will filter their results.

⁠13.2.1. Access control drivers

The access control framework is designed as a pluggable system to enable future integration with arbitrary access control technologies. By default, the none driver is used, which does no access control checks at all. At this time, libvirt ships with support for using polkit as a real access control driver. To learn how to use the polkit access driver refer to the configuration documentation.

The access driver is configured in the libvirtd.conf configuration file, using the access_drivers parameter. This parameter accepts an array of access control driver names. If more than one access driver is requested, then all must succeed in order for access to be granted. To enable 'polkit' as the driver run the command:

# augtool -s set '/files/etc/libvirt/libvirtd.conf/access_drivers[1]' polkit

To set the driver back to the default (no access control), run the following command:

# augtool -s rm /files/etc/libvirt/libvirtd.conf/access_drivers

It should be noted that changes made to libvirtd.conf require that the libvirtd daemon be restarted.

⁠13.2.2. Objects and permissions

libvirt applies access control to all the main object types in its API. Each object type, in turn, has a set of permissions defined. To determine what permissions are checked for specific API call, consult the API reference manual documentation for the API in question. For the complete list of objects and permissions, refer to libvirt.org.

⁠Chapter 14. Storage pools

This chapter includes instructions on creating storage pools of assorted types. A storage pool is a quantity of storage set aside by an administrator, often a dedicated storage administrator, for use by guest virtual machines. Storage pools are divided into storage volumes either by the storage administrator or the system administrator, and the volumes are then assigned to guest virtual machines as block devices.

For example, the storage administrator responsible for an NFS server creates a shared disk to store all of the guest virtual machines' data. The system administrator would define a storage pool on the virtualization host using the details of the shared disk. In this example, the administrator may want nfs.example.com:/path/to/share to be mounted on /vm_data). When the storage pool is started, libvirt mounts the share on the specified directory, just as if the system administrator logged in and executed mount nfs.example.com:/path/to/share /vmdata. If the storage pool is configured to autostart, libvirt ensures that the NFS shared disk is mounted on the directory specified when libvirt is started.

Once the storage pool is started, the files in the NFS shared disk are reported as storage volumes, and the storage volumes' paths may be queried using the libvirt APIs. The storage volumes' paths can then be copied into the section of a guest virtual machine's XML definition describing the source storage for the guest virtual machine's block devices.In the case of NFS, an application using the libvirt APIs can create and delete storage volumes in the storage pool (files in the NFS share) up to the limit of the size of the pool (the storage capacity of the share). Not all storage pool types support creating and deleting volumes. Stopping the storage pool (pool-destroy) undoes the start operation, in this case, unmounting the NFS share. The data on the share is not modified by the destroy operation, despite what the name of the command suggests. See man virsh for more details.

A second example is an iSCSI storage pool. A storage administrator provisions an iSCSI target to present a set of LUNs to the host running the virtual machines. When libvirt is configured to manage that iSCSI target as a storage pool, libvirt will ensure that the host logs into the iSCSI target and libvirt can then report the available LUNs as storage volumes. The storage volumes' paths can be queried and used in virtual machines' XML definitions as in the NFS example. In this case, the LUNs are defined on the iSCSI server, and libvirt cannot create and delete volumes.

Storage pools and volumes are not required for the proper operation of guest virtual machines. Storage pools and volumes provide a way for libvirt to ensure that a particular piece of storage will be available for a guest virtual machine. On systems that do not use storage pools, system administrators must ensure the availability of the guest virtual machine's storage, for example, adding the NFS share to the host physical machine's fstab so that the share is mounted at boot time.

One of the advantages of using libvirt to manage storage pools and volumes is libvirt's remote protocol, so it is possible to manage all aspects of a guest virtual machine's life cycle, as well as the configuration of the resources required by the guest virtual machine. These operations can be performed on a remote host entirely within the libvirt API. As a result, a management application using libvirt can enable a user to perform all the required tasks for configuring the host physical machine for a guest virtual machine such as: allocating resources, running the guest virtual machine, shutting it down and de-allocating the resources, without requiring shell access or any other control channel.

Although the storage pool is a virtual container it is limited by two factors: maximum size allowed to it by qemu-kvm and the size of the disk on the host machine. Storage pools may not exceed the size of the disk on the host machine. The maximum sizes are as follows:

virtio-blk = 2^63 bytes or 8 Exabytes(using raw files or disk)

Ext4 = ~ 16 TB (using 4 KB block size)

XFS = ~8 Exabytes

qcow2 and host file systems keep their own metadata and scalability should be evaluated/tuned when trying very large image sizes. Using raw disks means fewer layers that could affect scalability or max size.

libvirt uses a directory-based storage pool, the /var/lib/libvirt/images/ directory, as the default storage pool. The default storage pool can be changed to another storage pool.

Local storage pools - Local storage pools are directly attached to the host physical machine server. Local storage pools include: local directories, directly attached disks, physical partitions, and LVM volume groups. These storage volumes store guest virtual machine images or are attached to guest virtual machines as additional storage. As local storage pools are directly attached to the host physical machine server, they are useful for development, testing and small deployments that do not require migration or large numbers of guest virtual machines. Local storage pools are not suitable for many production environments as local storage pools do not support live migration.

Networked (shared) storage pools - Networked storage pools include storage devices shared over a network using standard protocols. Networked storage is required when migrating virtual machines between host physical machines with virt-manager, but is optional when migrating with virsh. Networked storage pools are managed by libvirt. Supported protocols for networked storage pools include:

Fibre Channel-based LUNs

iSCSI

NFS

GFS2

SCSI RDMA protocols (SCSI RCP), the block export protocol used in InfiniBand and 10GbE iWARP adapters.

Note

Multi-path storage pools should not be created or used as they are not fully supported.

⁠

Example 14.1. NFS storage pool

Suppose a storage administrator responsible for an NFS server creates a share to store guest virtual machines' data. The system administrator defines a pool on the host physical machine with the details of the share (nfs.example.com:/path/to/share should be mounted on /vm_data). When the pool is started, libvirt mounts the share on the specified directory, just as if the system administrator logged in and executed mount nfs.example.com:/path/to/share /vmdata. If the pool is configured to autostart, libvirt ensures that the NFS share is mounted on the directory specified when libvirt is started.

Once the pool starts, the files that the NFS share, are reported as volumes, and the storage volumes' paths are then queried using the libvirt APIs. The volumes' paths can then be copied into the section of a guest virtual machine's XML definition file describing the source storage for the guest virtual machine's block devices. With NFS, applications using the libvirt APIs can create and delete volumes in the pool (files within the NFS share) up to the limit of the size of the pool (the maximum storage capacity of the share). Not all pool types support creating and deleting volumes. Stopping the pool negates the start operation, in this case, unmounts the NFS share. The data on the share is not modified by the destroy operation, despite the name. See man virsh for more details.

Note

Storage pools and volumes are not required for the proper operation of guest virtual machines. Pools and volumes provide a way for libvirt to ensure that a particular piece of storage will be available for a guest virtual machine, but some administrators will prefer to manage their own storage and guest virtual machines will operate properly without any pools or volumes defined. On systems that do not use pools, system administrators must ensure the availability of the guest virtual machines' storage using whatever tools they prefer, for example, adding the NFS share to the host physical machine's fstab so that the share is mounted at boot time.

⁠14.1. Disk-based storage pools

This section covers creating disk based storage devices for guest virtual machines.

Warning

Guests should not be given write access to whole disks or block devices (for example, /dev/sdb). Use partitions (for example, /dev/sdb1) or LVM volumes.

If you pass an entire block device to the guest, the guest will likely partition it or create its own LVM groups on it. This can cause the host physical machine to detect these partitions or LVM groups and cause errors.

⁠14.1.1. Creating a disk based storage pool using virsh

This procedure creates a new storage pool using a disk device with the virsh command.

Warning

Dedicating a disk to a storage pool will reformat and erase all data presently stored on the disk device. It is strongly recommended to back up the data on the storage device before commencing with the following procedure:

Create a GPT disk label on the disk

The disk must be relabeled with a GUID Partition Table (GPT) disk label. GPT disk labels allow for creating a large numbers of partitions, up to 128 partitions, on each device. GPT partition tables can store partition data for far more partitions than the MS-DOS partition table.

# parted /dev/sdb
GNU Parted 2.1
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) mklabel
New disk label type? gpt
(parted) quit
Information: You may need to update /etc/fstab.
#

Create the storage pool configuration file

Create a temporary XML text file containing the storage pool information required for the new device.

The file must be in the format shown below, and contain the following fields:

⁠<name>guest_images_disk</name>

The name parameter determines the name of the storage pool. This example uses the name guest_images_disk in the example below.

⁠ <device path='/dev/sdb'/>

The device parameter with the path attribute specifies the device path of the storage device. This example uses the device /dev/sdb.

⁠<target> <path>/dev</path></target>

The file system target parameter with the path sub-parameter determines the location on the host physical machine file system to attach volumes created with this storage pool.

For example, sdb1, sdb2, sdb3. Using /dev/, as in the example below, means volumes created from this storage pool can be accessed as /dev/sdb1, /dev/sdb2, /dev/sdb3.

⁠<format type='gpt'/>

The format parameter specifies the partition table type. This example uses the gpt in the example below, to match the GPT disk label type created in the previous step.

Create the XML file for the storage pool device with a text editor.

⁠

Example 14.2. Disk based storage device storage pool

<pool type='disk'>
  <name>guest_images_disk</name>
  <source>
    <device path='/dev/sdb'/>
    <format type='gpt'/>
  </source>
  <target>
    <path>/dev</path>
  </target>
</pool>

Attach the device

Add the storage pool definition using the virsh pool-define command with the XML configuration file created in the previous step.

# virsh pool-define ~/guest_images_disk.xml
Pool guest_images_disk defined from /root/guest_images_disk.xml
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images_disk    inactive   no

Start the storage pool

Start the storage pool with the virsh pool-start command. Verify the pool is started with the virsh pool-list --all command.

# virsh pool-start guest_images_disk
Pool guest_images_disk started
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images_disk    active     no

Turn on autostart

Turn on autostart for the storage pool. Autostart configures the libvirtd service to start the storage pool when the service starts.

# virsh pool-autostart guest_images_disk
Pool guest_images_disk marked as autostarted
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images_disk    active     yes

Verify the storage pool configuration

Verify the storage pool was created correctly, the sizes reported correctly, and the state reports as running.

# virsh pool-info guest_images_disk
Name:           guest_images_disk
UUID:           551a67c8-5f2a-012c-3844-df29b167431c
State:          running
Capacity:       465.76 GB
Allocation:     0.00
Available:      465.76 GB
# ls -la /dev/sdb
brw-rw----. 1 root disk 8, 16 May 30 14:08 /dev/sdb
# virsh vol-list guest_images_disk
Name                 Path
-----------------------------------------

Optional: Remove the temporary configuration file

Remove the temporary storage pool XML configuration file if it is not needed anymore.

# rm ~/guest_images_disk.xml

A disk based storage pool is now available.

⁠14.1.2. Deleting a storage pool using virsh

The following demonstrates how to delete a storage pool using virsh:

To avoid any issues with other guest virtual machines using the same pool, it is best to stop the storage pool and release any resources in use by it.

# virsh pool-destroy guest_images_disk

Remove the storage pool's definition

# virsh pool-undefine guest_images_disk

⁠14.2. Partition-based storage pools

This section covers using a pre-formatted block device, a partition, as a storage pool.

For the following examples, a host physical machine has a 500GB hard drive (/dev/sdc) partitioned into one 500GB, ext4 formatted partition (/dev/sdc1). We set up a storage pool for it using the procedure below.

⁠14.2.1. Creating a partition-based storage pool using virt-manager

This procedure creates a new storage pool using a partition of a storage device.

⁠

Procedure 14.1. Creating a partition-based storage pool with virt-manager

Open the storage pool settings

In the virt-manager graphical interface, select the host physical machine from the main window.

Open the Edit menu and select Connection Details

⁠

Figure 14.1. Connection Details

Click on the Storage tab of the Connection Details window.

⁠

Figure 14.2. Storage tab

Create the new storage pool

Add a new pool (part 1)

Press the + button (the add pool button). The Add a New Storage Pool wizard appears.

Choose a Name for the storage pool. This example uses the name guest_images_fs. Change the Type to fs: Pre-Formatted Block Device.

⁠

Figure 14.3. Storage pool name and type

Press the Forward button to continue.

Add a new pool (part 2)

Change the Target Path, Format, and Source Path fields.

⁠

Figure 14.4. Storage pool path and format

⁠Target Path

Enter the location to mount the source device for the storage pool in the Target Path field. If the location does not already exist, virt-manager will create the directory.

⁠Format

Select a format from the Format list. The device is formatted with the selected format.

This example uses the ext4 file system, the default Red Hat Enterprise Linux file system.

⁠Source Path

Enter the device in the Source Path field.

This example uses the /dev/sdc1 device.

Verify the details and press the Finish button to create the storage pool.

Verify the new storage pool

The new storage pool appears in the storage list on the left after a few seconds. Verify the size is reported as expected, 458.20 GB Free in this example. Verify the State field reports the new storage pool as Active.

Select the storage pool. In the Autostart field, click the On Boot checkbox. This will make sure the storage device starts whenever the libvirtd service starts.

⁠

Figure 14.5. Storage list confirmation

The storage pool is now created, close the Connection Details window.

⁠14.2.2. Deleting a storage pool using virt-manager

This procedure demonstrates how to delete a storage pool.

To avoid any issues with other guest virtual machines using the same pool, it is best to stop the storage pool and release any resources in use by it. To do this, select the storage pool you want to stop and click the red X icon at the bottom of the Storage window.

⁠

Figure 14.6. Stop Icon

Delete the storage pool by clicking the Trash can icon. This icon is only enabled if you stop the storage pool first.

⁠14.2.3. Creating a partition-based storage pool using virsh

This section covers creating a partition-based storage pool with the virsh command.

Warning

Do not use this procedure to assign an entire disk as a storage pool (for example, /dev/sdb). Guests should not be given write access to whole disks or block devices. Only use this method to assign partitions (for example, /dev/sdb1) to storage pools.

⁠

Procedure 14.2. Creating pre-formatted block device storage pools using virsh

Create the storage pool definition

Use the virsh pool-define-as command to create a new storage pool definition. There are three options that must be provided to define a pre-formatted disk as a storage pool:

⁠Partition name

The name parameter determines the name of the storage pool. This example uses the name guest_images_fs in the example below.

⁠device

The device parameter with the path attribute specifies the device path of the storage device. This example uses the partition /dev/sdc1.

⁠mountpoint

The mountpoint on the local file system where the formatted device will be mounted. If the mount point directory does not exist, the virsh command can create the directory.

The directory /guest_images is used in this example.

# virsh pool-define-as guest_images_fs fs - - /dev/sdc1 - "/guest_images"
Pool guest_images_fs defined

The new pool is now created.

Verify the new pool

List the present storage pools.

# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images_fs      inactive   no

Create the mount point

Use the virsh pool-build command to create a mount point for a pre-formatted file system storage pool.

# virsh pool-build guest_images_fs
Pool guest_images_fs built
# ls -la /guest_images
total 8
drwx------.  2 root root 4096 May 31 19:38 .
dr-xr-xr-x. 25 root root 4096 May 31 19:38 ..
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images_fs      inactive   no

Start the storage pool

Use the virsh pool-start command to mount the file system onto the mount point and make the pool available for use.

# virsh pool-start guest_images_fs
Pool guest_images_fs started
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images_fs      active     no

Turn on autostart

By default, a storage pool is defined with virsh is not set to automatically start each time libvirtd starts. Turn on automatic start with the virsh pool-autostart command. The storage pool is now automatically started each time libvirtd starts.

# virsh pool-autostart guest_images_fs
Pool guest_images_fs marked as autostarted

# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images_fs      active     yes

Verify the storage pool

Verify the storage pool was created correctly, the sizes reported are as expected, and the state is reported as running. Verify there is a "lost+found" directory in the mount point on the file system, indicating the device is mounted.

# virsh pool-info guest_images_fs
Name:           guest_images_fs
UUID:           c7466869-e82a-a66c-2187-dc9d6f0877d0
State:          running
Persistent:     yes
Autostart:      yes
Capacity:       458.39 GB
Allocation:     197.91 MB
Available:      458.20 GB
# mount | grep /guest_images
/dev/sdc1 on /guest_images type ext4 (rw)
# ls -la /guest_images
total 24
drwxr-xr-x.  3 root root  4096 May 31 19:47 .
dr-xr-xr-x. 25 root root  4096 May 31 19:38 ..
drwx------.  2 root root 16384 May 31 14:18 lost+found

⁠14.2.4. Deleting a storage pool using virsh

To avoid any issues with other guest virtual machines using the same pool, it is best to stop the storage pool and release any resources in use by it.

# virsh pool-destroy guest_images_disk

Optionally, if you want to remove the directory where the storage pool resides use the following command:

# virsh pool-delete guest_images_disk

Remove the storage pool's definition

# virsh pool-undefine guest_images_disk

⁠14.3. Directory-based storage pools

This section covers storing guest virtual machines in a directory on the host physical machine.

Directory-based storage pools can be created with virt-manager or the virsh command line tools.

⁠14.3.1. Creating a directory-based storage pool with virt-manager

Create the local directory

Optional: Create a new directory for the storage pool

Create the directory on the host physical machine for the storage pool. This example uses a directory named /guest virtual machine_images.

# mkdir /guest_images

Set directory ownership

Change the user and group ownership of the directory. The directory must be owned by the root user.

# chown root:root /guest_images

Set directory permissions

Change the file permissions of the directory.

# chmod 700 /guest_images

Verify the changes

Verify the permissions were modified. The output shows a correctly configured empty directory.

# ls -la /guest_images
total 8
drwx------.  2 root root 4096 May 28 13:57 .
dr-xr-xr-x. 26 root root 4096 May 28 13:57 ..

Configure SELinux file contexts

Configure the correct SELinux context for the new directory. Note that the name of the pool and the directory do not have to match. However, when you shutdown the guest virtual machine, libvirt has to set the context back to a default value. The context of the directory determines what this default value is. It is worth explicitly labeling the directory virt_image_t, so that when the guest virtual machine is shutdown, the images get labeled 'virt_image_t' and are thus isolated from other processes running on the host physical machine.

# semanage fcontext -a -t virt_image_t '/guest_images(/.*)?'
# restorecon -R /guest_images

Open the storage pool settings

In the virt-manager graphical interface, select the host physical machine from the main window.

Open the Edit menu and select Connection Details

⁠

Figure 14.7. Connection details window

Click on the Storage tab of the Connection Details window.

⁠

Figure 14.8. Storage tab

Create the new storage pool

Add a new pool (part 1)

Press the + button (the add pool button). The Add a New Storage Pool wizard appears.

Choose a Name for the storage pool. This example uses the name guest_images. Change the Type to dir: Filesystem Directory.

⁠

Figure 14.9. Name the storage pool

Press the Forward button to continue.

Add a new pool (part 2)

Change the Target Path field. For example, /guest_images.

Verify the details and press the Finish button to create the storage pool.

Verify the new storage pool

The new storage pool appears in the storage list on the left after a few seconds. Verify the size is reported as expected, 36.41 GB Free in this example. Verify the State field reports the new storage pool as Active.

Select the storage pool. In the Autostart field, confirm that the On Boot checkbox is checked. This will make sure the storage pool starts whenever the libvirtd service starts.

⁠

Figure 14.10. Verify the storage pool information

The storage pool is now created, close the Connection Details window.

⁠14.3.2. Deleting a storage pool using virt-manager

This procedure demonstrates how to delete a storage pool.

To avoid any issues with other guest virtual machines using the same pool, it is best to stop the storage pool and release any resources in use by it. To do this, select the storage pool you want to stop and click the red X icon at the bottom of the Storage window.

⁠

Figure 14.11. Stop Icon

Delete the storage pool by clicking the Trash can icon. This icon is only enabled if you stop the storage pool first.

⁠14.3.3. Creating a directory-based storage pool with virsh

Create the storage pool definition

Use the virsh pool-define-as command to define a new storage pool. There are two options required for creating directory-based storage pools:

The name of the storage pool.

This example uses the name guest_images. All further virsh commands used in this example use this name.

The path to a file system directory for storing guest image files. If this directory does not exist, virsh will create it.

This example uses the /guest_images directory.

 # virsh pool-define-as guest_images dir - - - - "/guest_images"
Pool guest_images defined

Verify the storage pool is listed

Verify the storage pool object is created correctly and the state reports it as inactive.

# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images     inactive   no

Create the local directory

Use the virsh pool-build command to build the directory-based storage pool for the directory guest_images (for example), as shown:

# virsh pool-build guest_images
Pool guest_images built
# ls -la /guest_images
total 8
drwx------.  2 root root 4096 May 30 02:44 .
dr-xr-xr-x. 26 root root 4096 May 30 02:44 ..
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images     inactive   no

Start the storage pool

Use the virsh command pool-start to enable a directory storage pool, thereby allowing allowing volumes of the pool to be used as guest disk images.

# virsh pool-start guest_images
Pool guest_images started
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default             active     yes
guest_images    active     no

Turn on autostart

Turn on autostart for the storage pool. Autostart configures the libvirtd service to start the storage pool when the service starts.

# virsh pool-autostart guest_images
Pool guest_images marked as autostarted
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images         active     yes

Verify the storage pool configuration

Verify the storage pool was created correctly, the size is reported correctly, and the state is reported as running. If you want the pool to be accessible even if the guest virtual machine is not running, make sure that Persistent is reported as yes. If you want the pool to start automatically when the service starts, make sure that Autostart is reported as yes.

# virsh pool-info guest_images
Name:           guest_images
UUID:           779081bf-7a82-107b-2874-a19a9c51d24c
State:          running
Persistent:     yes
Autostart:      yes
Capacity:       49.22 GB
Allocation:     12.80 GB
Available:      36.41 GB

# ls -la /guest_images
total 8
drwx------.  2 root root 4096 May 30 02:44 .
dr-xr-xr-x. 26 root root 4096 May 30 02:44 ..
#

A directory-based storage pool is now available.

⁠14.3.4. Deleting a storage pool using virsh

The following demonstrates how to delete a storage pool using virsh:

To avoid any issues with other guest virtual machines using the same pool, it is best to stop the storage pool and release any resources in use by it.

# virsh pool-destroy guest_images_disk

Optionally, if you want to remove the directory where the storage pool resides use the following command:

# virsh pool-delete guest_images_disk

Remove the storage pool's definition

# virsh pool-undefine guest_images_disk

⁠14.4. LVM-based storage pools

This chapter covers using LVM volume groups as storage pools.

LVM-based storage groups provide the full flexibility of LVM.

Note

Thin provisioning is currently not possible with LVM based storage pools.

Note

Please refer to the Red Hat Enterprise Linux Storage Administration Guide for more details on LVM.

Warning

LVM-based storage pools require a full disk partition. If activating a new partition/device with these procedures, the partition will be formatted and all data will be erased. If using the host's existing Volume Group (VG) nothing will be erased. It is recommended to back up the storage device before commencing the following procedure.

⁠14.4.1. Creating an LVM-based storage pool with virt-manager

LVM-based storage pools can use existing LVM volume groups or create new LVM volume groups on a blank partition.

Optional: Create new partition for LVM volumes

These steps describe how to create a new partition and LVM volume group on a new hard disk drive.

Warning

This procedure will remove all data from the selected storage device.

Create a new partition

Use the fdisk command to create a new disk partition from the command line. The following example creates a new partition that uses the entire disk on the storage device /dev/sdb.

# fdisk /dev/sdb
Command (m for help):

Press n for a new partition.

Press p for a primary partition.

Command action
   e   extended
   p   primary partition (1-4)

Choose an available partition number. In this example the first partition is chosen by entering 1.

Partition number (1-4): 1

Enter the default first cylinder by pressing Enter.

First cylinder (1-400, default 1):

Select the size of the partition. In this example the entire disk is allocated by pressing Enter.

Last cylinder or +size or +sizeM or +sizeK (2-400, default 400):

Set the type of partition by pressing t.

Command (m for help): t

Choose the partition you created in the previous steps. In this example, the partition number is 1.

Partition number (1-4): 1

Enter 8e for a Linux LVM partition.

Hex code (type L to list codes): 8e

write changes to disk and quit.

Command (m for help): w
Command (m for help): q

Create a new LVM volume group

Create a new LVM volume group with the vgcreate command. This example creates a volume group named guest_images_lvm.

# vgcreate guest_images_lvm /dev/sdb1
  Physical volume "/dev/vdb1" successfully created
  Volume group "guest_images_lvm" successfully created

The new LVM volume group, guest_images_lvm, can now be used for an LVM-based storage pool.

Open the storage pool settings

In the virt-manager graphical interface, select the host from the main window.

Open the Edit menu and select Connection Details

⁠

Figure 14.12. Connection details

Click on the Storage tab.

⁠

Figure 14.13. Storage tab

Create the new storage pool

Start the Wizard

Press the + button (the add pool button). The Add a New Storage Pool wizard appears.

Choose a Name for the storage pool. We use guest_images_lvm for this example. Then change the Type to logical: LVM Volume Group, and

⁠

Figure 14.14. Add LVM storage pool

Press the Forward button to continue.

Add a new pool (part 2)

Change the Target Path field. This example uses /guest_images.

Now fill in the Target Path and Source Path fields, then tick the Build Pool check box.

Use the Target Path field to either select an existing LVM volume group or as the name for a new volume group. The default format is /dev/storage_pool_name.

This example uses a new volume group named /dev/guest_images_lvm.

The Source Path field is optional if an existing LVM volume group is used in the Target Path.

For new LVM volume groups, input the location of a storage device in the Source Path field. This example uses a blank partition /dev/sdc.

The Build Pool checkbox instructs virt-manager to create a new LVM volume group. If you are using an existing volume group you should not select the Build Pool checkbox.

This example is using a blank partition to create a new volume group so the Build Pool checkbox must be selected.

⁠

Figure 14.15. Add target and source

Verify the details and press the Finish button format the LVM volume group and create the storage pool.

Confirm the device to be formatted

A warning message appears.

⁠

Figure 14.16. Warning message

Press the Yes button to proceed to erase all data on the storage device and create the storage pool.

Verify the new storage pool

The new storage pool will appear in the list on the left after a few seconds. Verify the details are what you expect, 465.76 GB Free in our example. Also verify the State field reports the new storage pool as Active.

It is generally a good idea to have the Autostart check box enabled, to ensure the storage pool starts automatically with libvirtd.

⁠

Figure 14.17. Confirm LVM storage pool details

Close the Host Details dialog, as the task is now complete.

⁠14.4.2. Deleting a storage pool using virt-manager

This procedure demonstrates how to delete a storage pool.

To avoid any issues with other guest virtual machines using the same pool, it is best to stop the storage pool and release any resources in use by it. To do this, select the storage pool you want to stop and click the red X icon at the bottom of the Storage window.

⁠

Figure 14.18. Stop Icon

Delete the storage pool by clicking the Trash can icon. This icon is only enabled if you stop the storage pool first.

⁠14.4.3. Creating an LVM-based storage pool with virsh

This section outlines the steps required to create an LVM-based storage pool with the virsh command. It uses the example of a pool named guest_images_lvm from a single drive (/dev/sdc). This is only an example and your settings should be substituted as appropriate.

⁠

Procedure 14.3. Creating an LVM-based storage pool with virsh

Define the pool name guest_images_lvm.

# virsh pool-define-as guest_images_lvm logical - - /dev/sdc libvirt_lvm \ /dev/libvirt_lvm
Pool guest_images_lvm defined

Build the pool according to the specified name. If you are using an already existing volume group, skip this step.

# virsh pool-build guest_images_lvm

Pool guest_images_lvm built

Initialize the new pool.

# virsh pool-start guest_images_lvm

Pool guest_images_lvm started

Show the volume group information with the vgs command.

# vgs
VG          #PV #LV #SN Attr   VSize   VFree
libvirt_lvm   1   0   0 wz--n- 465.76g 465.76g

Set the pool to start automatically.

# virsh pool-autostart guest_images_lvm
Pool guest_images_lvm marked as autostarted

List the available pools with the virsh command.

# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
guest_images_lvm     active     yes

The following commands demonstrate the creation of three volumes (volume1, volume2 and volume3) within this pool.

# virsh vol-create-as guest_images_lvm volume1 8G
Vol volume1 created

# virsh vol-create-as guest_images_lvm volume2 8G
Vol volume2 created

# virsh vol-create-as guest_images_lvm volume3 8G
Vol volume3 created

List the available volumes in this pool with the virsh command.

# virsh vol-list guest_images_lvm
Name                 Path
-----------------------------------------
volume1              /dev/libvirt_lvm/volume1
volume2              /dev/libvirt_lvm/volume2
volume3              /dev/libvirt_lvm/volume3

The following two commands (lvscan and lvs) display further information about the newly created volumes.

# lvscan
ACTIVE            '/dev/libvirt_lvm/volume1' [8.00 GiB] inherit
ACTIVE            '/dev/libvirt_lvm/volume2' [8.00 GiB] inherit
ACTIVE            '/dev/libvirt_lvm/volume3' [8.00 GiB] inherit

# lvs
LV       VG            Attr     LSize   Pool Origin Data%  Move Log Copy%  Convert
volume1  libvirt_lvm   -wi-a-   8.00g
volume2  libvirt_lvm   -wi-a-   8.00g
volume3  libvirt_lvm   -wi-a-   8.00g

⁠14.4.4. Deleting a storage pool using virsh

The following demonstrates how to delete a storage pool using virsh:

To avoid any issues with other guests using the same pool, it is best to stop the storage pool and release any resources in use by it.

# virsh pool-destroy guest_images_disk

Optionally, if you want to remove the directory where the storage pool resides use the following command:

# virsh pool-delete guest_images_disk

Remove the storage pool's definition

# virsh pool-undefine guest_images_disk

⁠14.5. iSCSI-based storage pools

This section covers using iSCSI-based devices to store guest virtual machines. This allows for more flexible storage options such as using iSCSI as a block storage device. The iSCSI devices use an LIO target, which is a multi-protocol SCSI target for Linux. In addition to iSCSI, LIO also supports Fibre Channel and Fibre Channel over Ethernet (FCoE).

iSCSI (Internet Small Computer System Interface) is a network protocol for sharing storage devices. iSCSI connects initiators (storage clients) to targets (storage servers) using SCSI instructions over the IP layer.

⁠14.5.1. Configuring a software iSCSI target

Introduced in Red Hat Enterprise Linux 7, iSCSI targets are created with the targetcli package, which provides a command set for creating software-backed iSCSI targets.

⁠

Procedure 14.4. Creating an iSCSI target

Install the required package

Install the targetcli package and all dependencies:

# yum install targetcli

Launch targetcli

Launch the targetcli command set:

# targetcli

⁠

Create storage objects

Create three storage objects as follows, using the device created in Section 14.4, “LVM-based storage pools”:

Create a block storage object, by changing into the /backstores/block directory and running the following command:

# create [block-name][filepath]

For example:

 # create block1 dev=/dev/vdb1

Create a fileio object, by changing into the fileio directory and running the following command:

# create [fileioname] [imagename] [image-size]

For example:

# create fileio1 /foo.img 50M

Create a ramdisk object by changing into the ramdisk directory, and running the following command:

# create [ramdiskname] [size]

For example:

# create ramdisk1 1M

Remember the names of the disks you created in this step, as you will need them later.

Navigate to the /iscsi directory

Change into the iscsi directory:

#cd /iscsi

Create iSCSI target

Create an iSCSI target in one of two ways:

create with no additional parameters, automatically generates the IQN.

create iqn.2010-05.com.example.server1:iscsirhel7guest creates a specific IQN on a specific server.

⁠

Define the target portal group (TPG)

Each iSCSI target needs to have a target portal group (TPG) defined. In this example, the default tpg1 will be used, but you can add additional tpgs as well. As this is the most common configuration, the example will configure tpg1. To do this, make sure you are still in the /iscsi directory and change to the /tpg1 directory.

# /iscsi>iqn.iqn.2010-05.com.example.server1:iscsirhel7guest/tpg1

Define the portal IP address

In order to export the block storage over iSCSI, the portals, LUNs, and ACLs must all be configured first.

The portal includes the IP address and TCP port that the target will listen on, and the initiators will connect to. iSCSI uses port 3260, which is the port that will be configured by default. To connect to this port, run the following command from the /tpg directory:

# portals/ create

This command will have all available IP addresses listening to this port. To specify that only one specific IP address will listen on the port, run portals/ create [ipaddress], and the specified IP address will be configured to listen to port 3260.

Configure the LUNs and assign the storage objects to the fabric

This step uses the storage devices created in Step 3. Make sure you change into the luns directory for the TPG you created in Step 6, or iscsi>iqn.iqn.2010-05.com.example.server1:iscsirhel7guest, for example.

Assign the first LUN to the ramdisk as follows:

# create /backstores/ramdisk/ramdisk1

Assign the second LUN to the block disk as follows:

# create /backstores/block/block1

Assign the third LUN to the fileio disk as follows:

# create /backstores/fileio/file1

Listing the resulting LUNs should resemble this screen output:

/iscsi/iqn.20...csirhel7guest/tpg1 ls

o- tgp1 ............................................................................[enabled, auth]
   o- acls..................................................................................[0 ACL]
   o- luns.................................................................................[3 LUNs]
   | o- lun0.....................................................................[ramdisk/ramdisk1]
   | o- lun1..............................................................[block/block1 (dev/vdb1)]
   | o- lun2...............................................................[fileio/file1 (foo.img)]
   o- portals............................................................................[1 Portal]
     o- IP-ADDRESS:3260........................................................................[OK]

⁠

Creating ACLs for each initiator

This step allows for the creation of authentication when the initiator connects, and it also allows for restriction of specified LUNs to specified initiators. Both targets and initiators have unique names. iSCSI initiators use an IQN.

To find the IQN of the iSCSI initiator, run the following command, replacing the name of the initiator:

# cat /etc/iscsi/initiatorname.iscsi

Use this IQN to create the ACLs.

Change to the acls directory.

Run the command create [iqn], or to create specific ACLs, refer to the following example:

# create iqn.2010-05.com.example.foo:888

Alternatively, to configure the kernel target to use a single user ID and password for all initiators, and allow all initiators to log in with that user ID and password, use the following commands (replacing userid and password):

# set auth userid=redhat
# set auth password=password123
# set attribute authentication=1
# set attribute generate_node_acls=1

Make the configuration persistent with the saveconfig command. This will overwrite the previous boot settings. Alternatively, running exit from the targetcli saves the target configuration by default.

Enable the service with systemctl enable target.service to apply the saved settings on next boot.

⁠

Procedure 14.5. Optional steps

Create LVM volumes

LVM volumes are useful for iSCSI backing images. LVM snapshots and re-sizing can be beneficial for guest virtual machines. This example creates an LVM image named virtimage1 on a new volume group named virtstore on a RAID5 array for hosting guest virtual machines with iSCSI.

Create the RAID array

Creating software RAID5 arrays is covered by the Red Hat Enterprise Linux Deployment Guide.

Create the LVM volume group

Create a logical volume group named virtstore with the vgcreate command.

# vgcreate virtstore /dev/md1

Create a LVM logical volume

Create a logical volume group named virtimage1 on the virtstore volume group with a size of 20GB using the lvcreate command.

# lvcreate **size 20G -n virtimage1 virtstore

The new logical volume, virtimage1, is ready to use for iSCSI.

Important

Using LVM volumes for kernel target backstores can cause issues if the initiator also partitions the exported volume with LVM. This can be solved by adding global_filter = ["r|^/dev/vg0|"] to /etc/lvm/lvm.conf

Optional: Test discovery

Test whether the new iSCSI device is discoverable.

# iscsiadm --mode discovery --type sendtargets --portal server1.example.com
127.0.0.1:3260,1 iqn.2010-05.com.example.server1:iscsirhel7guest

Optional: Test attaching the device

Attach the new device (iqn.2010-05.com.example.server1:iscsirhel7guest) to determine whether the device can be attached.

# iscsiadm -d2 -m node --login
scsiadm: Max file limits 1024 1024

Logging in to [iface: default, target: iqn.2010-05.com.example.server1:iscsirhel7guest, portal: 10.0.0.1,3260]
Login to [iface: default, target: iqn.2010-05.com.example.server1:iscsirhel7guest, portal: 10.0.0.1,3260] successful.

Detach the device.

# iscsiadm -d2 -m node --logout
scsiadm: Max file limits 1024 1024

Logging out of session [sid: 2, target: iqn.2010-05.com.example.server1:iscsirhel7guest, portal: 10.0.0.1,3260
Logout of [sid: 2, target: iqn.2010-05.com.example.server1:iscsirhel7guest, portal: 10.0.0.1,3260] successful.

An iSCSI device is now ready to use for virtualization.

⁠14.5.2. Creating an iSCSI storage pool in virt-manager

This procedure covers creating a storage pool with an iSCSI target in virt-manager.

⁠

Procedure 14.6. Adding an iSCSI device to virt-manager

Open the host machine's storage details

In virt-manager, click the Edit and select Connection Details from the dropdown menu.

⁠

Figure 14.19. Connection details

Click on the Storage tab.

⁠

Figure 14.20. Storage menu

Add a new pool (Step 1 of 2)

Press the + button (the add pool button). The Add a New Storage Pool wizard appears.

⁠

Figure 14.21. Add an iSCSI storage pool name and type

Choose a name for the storage pool, change the Type to iSCSI, and press Forward to continue.

Add a new pool (Step 2 of 2)

You will need the information you used in Section 14.5, “iSCSI-based storage pools” and Step 6 to complete the fields in this menu.

Enter the iSCSI source and target. The Format option is not available as formatting is handled by the guest virtual machines. It is not advised to edit the Target Path. The default target path value, /dev/disk/by-path/, adds the drive path to that directory. The target path should be the same on all host physical machines for migration.

Enter the hostname or IP address of the iSCSI target. This example uses host1.example.com.

In the Source Path field, enter the iSCSI target IQN. If you look at Step 6 in Section 14.5, “iSCSI-based storage pools”, this is the information you added in the /etc/target/targets.conf file. This example uses iqn.2010-05.com.example.server1:iscsirhel7guest.

Check the IQN checkbox to enter the IQN for the initiator. This example uses iqn.2010-05.com.example.host1:iscsirhel7.

Click Finish to create the new storage pool.

⁠

Figure 14.22. Create an iSCSI storage pool

⁠14.5.3. Deleting a storage pool using virt-manager

This procedure demonstrates how to delete a storage pool.

To avoid any issues with other guest virtual machines using the same pool, it is best to stop the storage pool and release any resources in use by it. To do this, select the storage pool you want to stop and click the red X icon at the bottom of the Storage window.

⁠

Figure 14.23. Stop Icon

Delete the storage pool by clicking the Trash can icon. This icon is only enabled if you stop the storage pool first.

⁠14.5.4. Creating an iSCSI-based storage pool with virsh

Optional: Secure the storage pool

If desired, set up authentication with the steps in Section 14.5.5, “Securing an iSCSI storage pool”.

Define the storage pool

Storage pool definitions can be created with the virsh command line tool. Creating storage pools with virsh is useful for system administrators using scripts to create multiple storage pools.

The virsh pool-define-as command has several parameters which are accepted in the following format:

virsh pool-define-as name type source-host source-path source-dev source-name target

The parameters are explained as follows:

⁠type

defines this pool as a particular type, iSCSI for example

⁠name

sets the name for the storage pool; must be unique

⁠source-host and source-path

the hostname and iSCSI IQN respectively

⁠source-dev and source-name

these parameters are not required for iSCSI-based pools; use a - character to leave the field blank.

⁠target

defines the location for mounting the iSCSI device on the host machine

The example below creates the same iSCSI-based storage pool as the virsh pool-define-as example above:

# virsh pool-define-as --name iscsirhel7pool --type iscsi \
     --source-host server1.example.com \
     --source-dev iqn.2010-05.com.example.server1:iscsirhel7guest \
     --target /dev/disk/by-path
Pool iscsirhel7pool defined

Verify the storage pool is listed

Verify the storage pool object is created correctly and the state is inactive.

# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
iscsirhel7pool       inactive   no

Optional: Establish a direct connection to the iSCSI storage pool

This step is optional, but it allows you to establish a direct connection to the iSCSI storage pool. By default this is enabled, but if the connection is to the host machine (and not direct to the network) you can change it back by editing the domain XML for the virtual machine to reflect this example:

⁠

​
​   ...
​   <disk type='volume' device='disk'>
​      <driver name='qemu'/>
​      <source pool='iscsi' volume='unit:0:0:1' mode='direct'/>
​      <target dev='vda' bus='virtio'/>
​      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
​   </disk>
​      ...
​

Figure 14.24. Disk type element XML example

Note

The same iSCSI storage pool can be used for a LUN or a disk, by specifying the disk device as either a disk or lun. See Section 15.5.3, “Adding SCSI LUN-based storage to a guest” for XML configuration examples for adding SCSI LUN-based storage to a guest.

Additionally, the source mode can be specified as mode='host' for a connection to the host machine.

If you have configured authentication on the iSCSI server as detailed in Step 9, then the following XML used as a <disk> sub-element will provide the authentication credentials for the disk. Section 14.5.5, “Securing an iSCSI storage pool” describes how to configure the libvirt secret.

         <auth type='chap' username='redhat'>
            <secret usage='iscsirhel7secret'/>
         </auth>

Start the storage pool

Use the virsh pool-start to enable a directory storage pool. This allows the storage pool to be used for volumes and guest virtual machines.

# virsh pool-start iscsirhel7pool
Pool iscsirhel7pool started
# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
iscsirhel7pool       active     no

Turn on autostart

Turn on autostart for the storage pool. Autostart configures the libvirtd service to start the storage pool when the service starts.

# virsh pool-autostart iscsirhel7pool
Pool iscsirhel7pool marked as autostarted

Verify that the iscsirhel7pool pool has autostart enabled:

# virsh pool-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes
iscsirhel7pool       active     yes

Verify the storage pool configuration

Verify the storage pool was created correctly, the sizes report correctly, and the state reports as running.

# virsh pool-info iscsirhel7pool
Name:           iscsirhel7pool
UUID:           afcc5367-6770-e151-bcb3-847bc36c5e28
State:          running
Persistent:     unknown
Autostart:      yes
Capacity:       100.31 GB
Allocation:     0.00
Available:      100.31 GB

An iSCSI-based storage pool called iscsirhel7pool is now available.

⁠14.5.5. Securing an iSCSI storage pool

Username and password parameters can be configured with virsh to secure an iSCSI storage pool. This can be configured before or after the pool is defined, but the pool must be started for the authentication settings to take effect.

⁠

Procedure 14.7. Configuring authentication for a storage pool with virsh

Create a libvirt secret file

Create a libvirt secret XML file called secret.xml, using the following example:

# cat secret.xml
<secret ephemeral='no' private='yes'>
    <description>Passphrase for the iSCSI example.com server</description>
    <auth type='chap' username='redhat'/>
    <usage type='iscsi'>
        <target>iscsirhel7secret</target>
    </usage>
</secret>

Define the secret file

Define the secret.xml file with virsh:

# virsh secret-define secret.xml

Verify the secret file's UUID

Verify the UUID in secret.xml:

# virsh secret-list

 UUID                                  Usage
--------------------------------------------------------------------------------
 2d7891af-20be-4e5e-af83-190e8a922360  iscsi iscsirhel7secret

Assign a secret to the UUID

Assign a secret to that UUID, using the following command syntax as an example:

# MYSECRET=`printf %s "password123" | base64`
# virsh secret-set-value 2d7891af-20be-4e5e-af83-190e8a922360 $MYSECRET

This ensures the CHAP username and password are set in a libvirt-controlled secret list.

Add an authentication entry to the storage pool

Modify the <source> entry in the storage pool's XML file using virsh edit and add an <auth> element, specifying authentication type, username, and secret usage.

The following shows an example of a storage pool XML definition with authentication configured:

# cat iscsirhel7pool.xml
  <pool type='iscsi'>
    <name>iscsirhel7pool</name>
      <source>
         <host name='192.168.122.1'/>
         <device path='iqn.2010-05.com.example.server1:iscsirhel7guest'/>
         <auth type='chap' username='redhat'>
            <secret usage='iscsirhel7secret'/>
         </auth>
      </source>
    <target>
      <path>/dev/disk/by-path</path>
    </target>
  </pool>

Note

The <auth> sub-element exists in different locations within the guest XML's <pool> and <disk> elements. For a <pool>, <auth> is specified within the <source> element, as this describes where to find the pool sources, since authentication is a property of some pool sources (iSCSI and RBD). For a <disk>, which is a sub-element of a domain, the authentication to the iSCSI or RBD disk is a property of the disk. See Section 14.5.4, “Creating an iSCSI-based storage pool with virsh”Creating an iSCSI-based storage pool with virsh for an example of <disk> configured in the guest XML.

Activate the changes in the storage pool

The storage pool must be started to activate these changes.

If the storage pool has not yet been started, follow the steps in Section 14.5.4, “Creating an iSCSI-based storage pool with virsh” to define and start the storage pool.

If the pool has already been started, run the following commands to stop and restart the storage pool:

# virsh pool-destroy iscsirhel7pool
# virsh pool-start iscsirhel7pool

⁠14.5.6. Deleting a storage pool using virsh

The following demonstrates how to delete a storage pool using virsh:

To avoid any issues with other guest virtual machines using the same pool, it is best to stop the storage pool and release any resources in use by it.

# virsh pool-destroy iscsirhel7pool

Remove the storage pool's definition

# virsh pool-undefine iscsirhel7pool

⁠14.6. NFS-based storage pools

This procedure covers creating a storage pool with a NFS mount point in virt-manager.

⁠14.6.1. Creating a NFS-based storage pool with virt-manager

Open the host physical machine's storage tab

Open the Storage tab in the Host Details window.

Open virt-manager.

Select a host physical machine from the main virt-manager window. Click Edit menu and select Connection Details.

⁠

Figure 14.25. Connection details

Click on the Storage tab.

⁠

Figure 14.26. Storage tab

Create a new pool (part 1)

Press the + button (the add pool button). The Add a New Storage Pool wizard appears.

⁠

Figure 14.27. Add an NFS name and type

Choose a name for the storage pool and press Forward to continue.

Create a new pool (part 2)

Enter the target path for the device, the hostname and the NFS share path. Set the Format option to NFS or auto (to detect the type). The target path must be identical on all host physical machines for migration.

Enter the hostname or IP address of the NFS server. This example uses server1.example.com.

Enter the NFS path. This example uses /nfstrial.

⁠

Figure 14.28. Create an NFS storage pool

Press Finish to create the new storage pool.

⁠14.6.2. Deleting a storage pool using virt-manager

This procedure demonstrates how to delete a storage pool.

To avoid any issues with other guests using the same pool, it is best to stop the storage pool and release any resources in use by it. To do this, select the storage pool you want to stop and click the red X icon at the bottom of the Storage window.

⁠

Figure 14.29. Stop Icon

Delete the storage pool by clicking the Trash can icon. This icon is only enabled if you stop the storage pool first.

⁠14.7. Using a NPIV virtual adapter (vHBA) with SCSI devices

NPIV (N_Port ID Virtualization) is a software technology that allows sharing of a single physical Fibre Channel host bus adapter (HBA).

This allows multiple guests to see the same storage from multiple physical hosts, and thus allows for easier migration paths for the storage. As a result, there is no need for the migration to create or copy storage, as long as the correct storage path is specified.

In virtualization, the virtual host bus adapter, or vHBA, controls the LUNs for virtual machines. For a host to share one Fibre Channel device path between multiple KVM guests, a vHBA must be created for each virtual machine. A single vHBA must not be used by multiple KVM guests.

Each vHBA is identified by its own WWNN (World Wide Node Name) and WWPN (World Wide Port Name). The path to the storage is determined by the WWNN and WWPN values.

This section provides instructions for configuring a vHBA persistently on a virtual machine.

Note

Before creating a vHBA, it is recommended to configure storage array (SAN)-side zoning in the host LUN to provide isolation between guests and prevent the possibility of data corruption.

⁠14.7.1. Creating a vHBA

⁠

Procedure 14.8. Creating a vHBA

Locate HBAs on the host system

To locate the HBAs on your host system, use the virsh nodedev-list --cap vports command.

For example, the following output shows a host that has two HBAs that support vHBA:

# virsh nodedev-list --cap vports
scsi_host3
scsi_host4

Check the HBA's details

Use the virsh nodedev-dumpxml HBA_device command to see the HBA's details.

The XML output from the virsh nodedev-dumpxml command will list the fields <name>, <wwnn>, and <wwpn>, which are used to create a vHBA. The <max_vports> value shows the maximum number of supported vHBAs.

​
​ # virsh nodedev-dumpxml scsi_host3
​<device>
​  <name>scsi_host3</name>
​  <path>/sys/devices/pci0000:00/0000:00:04.0/0000:10:00.0/host3</path>
​  <parent>pci_0000_10_00_0</parent>
​  <capability type='scsi_host'>
​    <host>3</host>
​    <unique_id>0</unique_id>
​    <capability type='fc_host'>
​      <wwnn>20000000c9848140</wwnn>
​      <wwpn>10000000c9848140</wwpn>
​      <fabric_wwn>2002000573de9a81</fabric_wwn>
​    </capability>
​    <capability type='vport_ops'>
​      <max_vports>127</max_vports>
​      <vports>0</vports>
​    </capability>
​  </capability>
​</device>

In this example, the <max_vports> value shows there are a total 127 virtual ports available for use in the HBA configuration. The <vports> value shows the number of virtual ports currently being used. These values update after creating a vHBA.

Create a vHBA host device

Create an XML file similar to the following (in this example, named vhba_host3.xml) for the vHBA host.

​# cat vhba_host3.xml
​   <device>
​     <parent>scsi_host3</parent>
​     <capability type='scsi_host'>
​       <capability type='fc_host'>
​       </capability>
​     </capability>
​   </device>

The <parent> field specifies the HBA device to associate with this vHBA device. The details in the <device> tag are used in the next step to create a new vHBA device for the host. See http://libvirt.org/formatnode.html for more information on the nodedev XML format.

Create a new vHBA on the vHBA host device

To create a vHBA on vhba_host3, use the virsh nodedev-create command:

# virsh nodedev-create vhba_host3.xml
Node device scsi_host5 created from vhba_host3.xml

Verify the vHBA

Verify the new vHBA's details (scsi_host5) with the virsh nodedev-dumpxml command:

​# virsh nodedev-dumpxml scsi_host5
​<device>
​  <name>scsi_host5</name>
​  <path>/sys/devices/pci0000:00/0000:00:04.0/0000:10:00.0/host3/vport-3:0-0/host5</path>
​  <parent>scsi_host3</parent>
​  <capability type='scsi_host'>
​    <host>5</host>
​    <unique_id>2</unique_id>
​    <capability type='fc_host'>
​      <wwnn>5001a4a93526d0a1</wwnn>
​      <wwpn>5001a4ace3ee047d</wwpn>
​      <fabric_wwn>2002000573de9a81</fabric_wwn>
​    </capability>
​  </capability>
​</device>

⁠14.7.2. Creating a storage pool using the vHBA

It is recommended to define a libvirt storage pool based on the vHBA in order to preserve the vHBA configuration.

Using a storage pool has two primary advantages:

the libvirt code can easily find the LUN's path via virsh command output, and

virtual machine migration requires only defining and starting a storage pool with the same vHBA name on the target machine. To do this, the vHBA LUN, libvirt storage pool and volume name must be specified in the virtual machine's XML configuration. Refer to Section 14.7.3, “Configuring the virtual machine to use a vHBA LUN” for an example.

Create a SCSI storage pool

To create a persistent vHBA configuration, first create a libvirt 'scsi' storage pool XML file using the format below. It is recommended to use a stable location for the <path> value, such as one of the /dev/disk/by-{path|id|uuid|label} locations on your system. More information on <path> and the elements within <target> can be found at http://libvirt.org/formatstorage.html.

In this example, the 'scsi' storage pool is named vhbapool_host3.xml:

​  <pool type='scsi'>
​      <name>vhbapool_host3</name>
​      <source>
​        <adapter type='fc_host' wwnn='5001a4a93526d0a1' wwpn='5001a4ace3ee047d'/>
​      </source>
​      <target>
​        <path>/dev/disk/by-path</path>
​        <permissions>
​          <mode>0700</mode>
​          <owner>0</owner>
​          <group>0</group>
​        </permissions>
​      </target>
​    </pool>

Important

The pool must be type='scsi' and the source adapter type must be 'fc_host'. For a persistent configuration across host reboots, the wwnn and wwpn attributes must be the values assigned to the vHBA (scsi_host5 in this example) by libvirt.

Optionally, the 'parent' attribute can be used in the <adapter> field to identify the parent scsi_host device as the vHBA. Note, the value is not the scsi_host of the vHBA created by virsh nodedev-create, but it is the parent of that vHBA.

Providing the 'parent' attribute is also useful for duplicate pool definition checks. This is more important in environments where both the 'fc_host' and 'scsi_host' source adapter pools are being used, to ensure a new definition does not duplicate using the same scsi_host of another existing storage pool.

The following example shows the optional 'parent' attribute used in the <adapter> field in a storage pool configuration:

<adapter type='fc_host' parent='scsi_host3' wwnn='5001a4a93526d0a1' wwpn='5001a4ace3ee047d'/>

Define the pool

To define the storage pool (named vhbapool_host3 in this example) persistently, use the virsh pool-define command:

# virsh pool-define vhbapool_host3.xml
Pool vhbapool_host3 defined from vhbapool_host3.xml

Start the pool

Start the storage pool with the following command:

# virsh pool-start vhbapool_host3
Pool vhbapool_host3 started

Note

When starting the pool, libvirt will check if the vHBA with same wwpn:wwnn already exists. If it does not yet exist, a new vHBA with the provided wwpn:wwnn will be created and the configuration will not be persistent. Correspondingly, when destroying the pool, libvirt will destroy the vHBA using the same wwpn:wwnn values as well.

Enable autostart

Finally, to ensure that subsequent host reboots will automatically define vHBAs for use in virtual machines, set the storage pool autostart feature (in this example, for a pool named vhbapool_host3):

# virsh pool-autostart vhbapool_host3

⁠14.7.3. Configuring the virtual machine to use a vHBA LUN

After a storage pool is created for a vHBA, add the vHBA LUN to the virtual machine configuration.

Find available LUNs

First, use the virsh vol-list command in order to generate a list of available LUNs on the vHBA. For example:

# virsh vol-list vhbapool_host3
 Name                 Path
------------------------------------------------------------------------------
 unit:0:4:0           /dev/disk/by-path/pci-0000:10:00.0-fc-0x5006016844602198-lun-0
 unit:0:5:0           /dev/disk/by-path/pci-0000:10:00.0-fc-0x5006016044602198-lun-0

The list of LUN names displayed will be available for use as disk volumes in virtual machine configurations.

Add the vHBA LUN to the virtual machine

Add the vHBA LUN to the virtual machine by creating a disk volume on the virtual machine in the virtual machine's XML. Specify the storage pool and the volume in the <source> parameter, using the following as an example:

​   <disk type='volume' device='disk'>
​     <driver name='qemu' type='raw'/>
​     <source pool='vhbapool_host3' volume='unit:0:4:0'/>
​     <target dev='hda' bus='ide'/>
​   </disk>

To specify a lun device instead of a disk, refer to the following example:

​    <disk type='volume' device='lun' sgio='unfiltered'>
​      <driver name='qemu' type='raw'/>
​      <source pool='vhbapool_host3' volume='unit:0:4:0' mode='host'/>
​      <target dev='sda' bus='scsi'/>
​      <shareable />
​    </disk>

See Section 15.5.3, “Adding SCSI LUN-based storage to a guest” for XML configuration examples for adding SCSI LUN-based storage to a guest.

⁠14.7.4. Destroying the vHBA storage pool

A vHBA created by the storage pool can be destroyed by the virsh pool-destroy command:

# virsh pool-destroy vhbapool_host3

Note that executing the virsh pool-destroy command will also remove the vHBA that was created in Section 14.7.1, “Creating a vHBA”.

To verify the pool and vHBA have been destroyed, run:

# virsh nodedev-list --cap scsi_host

scsi_host5 will no longer appear in the list of results.

⁠Chapter 15.  Storage Volumes

⁠15.1. Introduction

Storage pools are divided into storage volumes. Storage volumes are an abstraction of physical partitions, LVM logical volumes, file-based disk images and other storage types handled by libvirt. Storage volumes are presented to guest virtual machines as local storage devices regardless of the underlying hardware. Note the sections below do not contain all of the possible commands and arguments that virsh allows, for more information refer to Section 23.15, “Storage Volume Commands”.

⁠15.1.1. Referencing volumes

For more additional parameters and arguments, refer to Section 23.15.4, “Listing volume information”.

To reference a specific volume, three approaches are possible:

⁠The name of the volume and the storage pool

A volume may be referred to by name, along with an identifier for the storage pool it belongs in. On the virsh command line, this takes the form --pool storage_pool volume_name.

For example, a volume named firstimage in the guest_images pool.

# virsh vol-info --pool guest_images firstimage
Name:           firstimage
Type:           block
Capacity:       20.00 GB
Allocation:     20.00 GB

virsh #

⁠The full path to the storage on the host physical machine system

A volume may also be referred to by its full path on the file system. When using this approach, a pool identifier does not need to be included.

For example, a volume named secondimage.img, visible to the host physical machine system as /images/secondimage.img. The image can be referred to as /images/secondimage.img.

# virsh vol-info /images/secondimage.img
Name:           secondimage.img
Type:           file
Capacity:       20.00 GB
Allocation:     136.00 kB

⁠The unique volume key

When a volume is first created in the virtualization system, a unique identifier is generated and assigned to it. The unique identifier is termed the volume key. The format of this volume key varies upon the storage used.

When used with block based storage such as LVM, the volume key may follow this format:

c3pKz4-qPVc-Xf7M-7WNM-WJc8-qSiz-mtvpGn

When used with file based storage, the volume key may instead be a copy of the full path to the volume storage.

/images/secondimage.img

For example, a volume with the volume key of Wlvnf7-a4a3-Tlje-lJDa-9eak-PZBv-LoZuUr:

# virsh vol-info Wlvnf7-a4a3-Tlje-lJDa-9eak-PZBv-LoZuUr
Name:           firstimage
Type:           block
Capacity:       20.00 GB
Allocation:     20.00 GB

virsh provides commands for converting between a volume name, volume path, or volume key:

⁠ vol-name

Returns the volume name when provided with a volume path or volume key.

# virsh vol-name /dev/guest_images/firstimage
firstimage
# virsh vol-name Wlvnf7-a4a3-Tlje-lJDa-9eak-PZBv-LoZuUr

⁠vol-path

Returns the volume path when provided with a volume key, or a storage pool identifier and volume name.

# virsh vol-path Wlvnf7-a4a3-Tlje-lJDa-9eak-PZBv-LoZuUr
/dev/guest_images/firstimage
# virsh vol-path --pool guest_images firstimage
/dev/guest_images/firstimage

⁠The vol-key command

Returns the volume key when provided with a volume path, or a storage pool identifier and volume name.

# virsh vol-key /dev/guest_images/firstimage
Wlvnf7-a4a3-Tlje-lJDa-9eak-PZBv-LoZuUr
# virsh vol-key --pool guest_images firstimage
Wlvnf7-a4a3-Tlje-lJDa-9eak-PZBv-LoZuUr

For more information refer to Section 23.15.4, “Listing volume information”.

⁠15.2. Creating volumes

This section shows how to create disk volumes inside a block based storage pool. In the example below, the virsh vol-create-as command will create a storage volume with a specific size in GB within the guest_images_disk storage pool. As this command is repeated per volume needed, three volumes are created as shown in the example. For additional parameters and arguments refer to Section 23.15.1, “Creating storage volumes”

# virsh vol-create-as guest_images_disk volume1 8G
Vol volume1 created

# virsh vol-create-as guest_images_disk volume2 8G
Vol volume2 created

# virsh vol-create-as guest_images_disk volume3 8G
Vol volume3 created

# virsh vol-list guest_images_disk
Name                 Path
-----------------------------------------
volume1              /dev/sdb1
volume2              /dev/sdb2
volume3              /dev/sdb3

# parted -s /dev/sdb print
Model: ATA ST3500418AS (scsi)
Disk /dev/sdb: 500GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system  Name     Flags
2      17.4kB  8590MB  8590MB               primary
3      8590MB  17.2GB  8590MB               primary
1      21.5GB  30.1GB  8590MB               primary

⁠15.3. Cloning volumes

The new volume will be allocated from storage in the same storage pool as the volume being cloned. The virsh vol-clone must have the --pool argument which dictates the name of the storage pool that contains the volume to be cloned. The rest of the command names the volume to be cloned (volume3) and the name of the new volume that was cloned (clone1). The virsh vol-list command lists the volumes that are present in the storage pool (guest_images_disk). For additional commands and arguments refer to Section 23.15.1.2, “Cloning a storage volume”

# virsh vol-clone --pool guest_images_disk volume3 clone1
Vol clone1 cloned from volume3

# virsh vol-list guest_images_disk
Name                 Path
-----------------------------------------
volume1              /dev/sdb1
volume2              /dev/sdb2
volume3              /dev/sdb3
clone1               /dev/sdb4

# parted -s /dev/sdb print
Model: ATA ST3500418AS (scsi)
Disk /dev/sdb: 500GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    File system  Name     Flags
1      4211MB  12.8GB  8595MB  primary
2      12.8GB  21.4GB  8595MB  primary
3      21.4GB  30.0GB  8595MB  primary
4      30.0GB  38.6GB  8595MB  primary

⁠15.4. Deleting and removing volumes

For the virsh commands you need to delete and remove a volume, refer to Section 23.15.2, “Deleting storage volumes”.

⁠15.5. Adding storage devices to guests

This section covers adding storage devices to a guest. Additional storage can only be added as needed. The following types of storage is discussed in this section:

File based storage. Refer to Section 15.5.1, “Adding file based storage to a guest”.

Block devices - including CD-ROM, DVD and floppy devices. Refer to Section 15.5.2, “Adding hard drives and other block devices to a guest”.

SCSI controllers and devices. If your host physical machine can accommodate it, up to 100 SCSI controllers can be added to any guest virtual machine. Refer to Section 15.5.4, “Managing storage controllers in a guest virtual machine”.

⁠15.5.1. Adding file based storage to a guest

File-based storage is a collection of files that are stored on the host physical machines file system that act as virtualized hard drives for guests. To add file-based storage, perform the following steps:

⁠

Procedure 15.1. Adding file-based storage

Create a storage file or use an existing file (such as an IMG file). Note that both of the following commands create a 4GB file which can be used as additional storage for a guest:

Pre-allocated files are recommended for file-based storage images. Create a pre-allocated file using the following dd command as shown:

# dd if=/dev/zero of=/var/lib/libvirt/images/FileName.img bs=1G count=4

Alternatively, create a sparse file instead of a pre-allocated file. Sparse files are created much faster and can be used for testing, but are not recommended for production environments due to data integrity and performance issues.

# dd if=/dev/zero of=/var/lib/libvirt/images/FileName.img bs=1G seek=4096 count=4

Create the additional storage by writing a <disk> element in a new file. In this example, this file will be known as NewStorage.xml.

A <disk> element describes the source of the disk, and a device name for the virtual block device. The device name should be unique across all devices in the guest, and identifies the bus on which the guest will find the virtual block device. The following example defines a virtio block device whose source is a file-based storage container named FileName.img:

<disk type='file' device='disk'>
   <driver name='qemu' type='raw' cache='none'/>
   <source file='/var/lib/libvirt/images/FileName.img'/>
   <target dev='vdb'/>
</disk>

Device names can also start with "hd" or "sd", identifying respectively an IDE and a SCSI disk. The configuration file can also contain an <address> sub-element that specifies the position on the bus for the new device. In the case of virtio block devices, this should be a PCI address. Omitting the <address> sub-element lets libvirt locate and assign the next available PCI slot.

Attach the CD-ROM as follows:

<disk type='file' device='cdrom'>
   <driver name='qemu' type='raw' cache='none'/>
   <source file='/var/lib/libvirt/images/FileName.img'/>
   <readonly/>
   <target dev='hdc'/>
</disk >

Add the device defined in NewStorage.xml with your guest (Guest1):

# virsh attach-device --config Guest1 ~/NewStorage.xml

Note

This change will only apply after the guest has been destroyed and restarted. In addition, persistent devices can only be added to a persistent domain, that is a domain whose configuration has been saved with virsh define command.

If the guest is running, and you want the new device to be added temporarily until the guest is destroyed, omit the --config option:

# virsh attach-device Guest1 ~/NewStorage.xml

Note

The virsh command allows for an attach-disk command that can set a limited number of parameters with a simpler syntax and without the need to create an XML file. The attach-disk command is used in a similar manner to the attach-device command mentioned previously, as shown:

# virsh attach-disk Guest1 /var/lib/libvirt/images/FileName.img vdb --cache none

Note that the virsh attach-disk command also accepts the --config option.

Start the guest machine (if it is currently not running):

# virsh start Guest1

Note

The following steps are Linux guest specific. Other operating systems handle new storage devices in different ways. For other systems, refer to that operating system's documentation.

⁠

Partitioning the disk drive

The guest now has a hard disk device called /dev/vdb. If required, partition this disk drive and format the partitions. If you do not see the device that you added, then it indicates that there is an issue with the disk hotplug in your guest's operating system.

Start fdisk for the new device:

# fdisk /dev/vdb
Command (m for help):

Type n for a new partition.

The following appears:

Command action
e   extended
p   primary partition (1-4)

Type p for a primary partition.

Choose an available partition number. In this example, the first partition is chosen by entering 1.

Partition number (1-4): 1

Enter the default first cylinder by pressing Enter.

First cylinder (1-400, default 1):

Select the size of the partition. In this example the entire disk is allocated by pressing Enter.

Last cylinder or +size or +sizeM or +sizeK (2-400, default 400):

Enter t to configure the partition type.

Command (m for help): t

Select the partition you created in the previous steps. In this example, the partition number is 1 as there was only one partition created and fdisk automatically selected partition 1.

Partition number (1-4): 1

Enter 83 for a Linux partition.

Hex code (type L to list codes): 83

Enter w to write changes and quit.

Command (m for help): w

Format the new partition with the ext3 file system.

# mke2fs -j /dev/vdb1

Create a mount directory, and mount the disk on the guest. In this example, the directory is located in myfiles.

# mkdir /myfiles
# mount /dev/vdb1 /myfiles

The guest now has an additional virtualized file-based storage device. Note however, that this storage will not mount persistently across reboot unless defined in the guest's /etc/fstab file:

/dev/vdb1    /myfiles    ext3     defaults    0 0

⁠15.5.2. Adding hard drives and other block devices to a guest

System administrators have the option to use additional hard drives to provide increased storage space for a guest, or to separate system data from user data.

⁠

Procedure 15.2. Adding physical block devices to guests

This procedure describes how to add a hard drive on the host physical machine to a guest. It applies to all physical block devices, including CD-ROM, DVD and floppy devices.

Physically attach the hard disk device to the host physical machine. Configure the host physical machine if the drive is not accessible by default.

Do one of the following:

Create the additional storage by writing a disk element in a new file. In this example, this file will be known as NewStorage.xml. The following example is a configuration file section which contains an additional device-based storage container for the host physical machine partition /dev/sr0:

<disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/dev/sr0'/>
      <target dev='vdc' bus='virtio'/>
</disk>

Follow the instruction in the previous section to attach the device to the guest virtual machine. Alternatively, you can use the virsh attach-disk command, as shown:

# virsh attach-disk Guest1 /dev/sr0 vdc

Note that the following options are available:

The virsh attach-disk command also accepts the --config, --type, and --mode options, as shown:

# virsh attach-disk Guest1 /dev/sr0 vdc --config --type cdrom --mode readonly

Additionally, --type also accepts --type disk in cases where the device is a hard drive.

The guest virtual machine now has a new hard disk device called /dev/vdc on Linux (or something similar, depending on what the guest virtual machine OS chooses) . You can now initialize the disk from the guest virtual machine, following the standard procedures for the guest virtual machine's operating system. Refer to Procedure 15.1, “Adding file-based storage” and Step 6 for an example.

Warning

The host physical machine should not use filesystem labels to identify file systems in the fstab file, the initrd file or on the kernel command line. Doing so presents a security risk if less guest virtual machines, have write access to whole partitions or LVM volumes, because a guest virtual machine could potentially write a filesystem label belonging to the host physical machine, to its own block device storage. Upon reboot of the host physical machine, the host physical machine could then mistakenly use the guest virtual machine's disk as a system disk, which would compromise the host physical machine system.

It is preferable to use the UUID of a device to identify it in the fstab file, the initrd file or on the kernel command line. While using UUIDs is still not completely secure on certain file systems, a similar compromise with UUID is significantly less feasible.

Warning

Guest virtual machines should not be given write access to whole disks or block devices (for example, /dev/sdb). Guest virtual machines with access to whole block devices may be able to modify volume labels, which can be used to compromise the host physical machine system. Use partitions (for example, /dev/sdb1) or LVM volumes to prevent this issue. Refer to https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Logical_Volume_Manager_Administration/LVM_CLI.html, or https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Logical_Volume_Manager_Administration/LVM_examples.html for information on LVM administration and configuration examples. If you are using raw access to partitions, for example, /dev/sdb1 or raw disks such as /dev/sdb, you should configure LVM to only scan disks that are safe, using the global_filter setting. Refer to https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Logical_Volume_Manager_Administration/lvmconf_file.html for an example of an LVM configuration script using the global_filter command.

⁠15.5.3. Adding SCSI LUN-based storage to a guest

A host SCSI LUN device can be exposed entirely to the guest using three mechanisms, depending on your host configuration. Exposing the SCSI LUN device in this way allows for SCSI commands to be executed directly to the LUN on the guest. This is useful as a means to share a LUN between guests, as well as to share Fibre Channel storage between hosts.

Important

The optional sgio attribute controls whether unprivileged SCSI Generical I/O (SG_IO) commands are filtered for a device='lun' disk. The sgio attribute can be specified as 'filtered' or 'unfiltered', but must be set to 'unfiltered' to allow SG_IO ioctl commands to be passed through on the guest in a persistent reservation.

In addition to setting sgio='unfiltered', the <shareable> element must be set to share a LUN between guests. The sgio attribute defaults to 'filtered' if not specified.

The <disk> XML attribute device='lun' is valid for the following guest disk configurations:

type='block' for <source dev='/dev/sdX'... />

​   <disk type='block' device='lun' sgio='unfiltered'>
​      ​ <driver name='qemu' type='raw'/>
​      ​ <source dev='/dev/disk/by-path/pci-0000\:04\:00.1-fc-0x203400a0b85ad1d7-lun-0'/>
​      ​ <target dev='sda' bus='scsi'/>
​      <shareable />
​   ​</disk>

Note

The backslashes prior to the colons in the <source> device name are required.

type='network' for <source protocol='iscsi'... />

​    <disk type='network' device='disk' sgio='unfiltered'>
​      <driver name='qemu' type='raw'/>
​      <source protocol='iscsi' name='iqn.2013-07.com.example:iscsi-net-pool/1'>
​        <host name='example.com' port='3260'/>
​      </source>
​      <auth username='myuser'>
​        <secret type='iscsi' usage='libvirtiscsi'/>
​      </auth>
​      <target dev='sda' bus='scsi'/>
​      <shareable />
​    </disk>

type='volume' when using an iSCSI or a NPIV/vHBA source pool as the SCSI source pool.

The following example XML shows a guest using an iSCSI source pool (named iscsi-net-pool) as the SCSI source pool:

​    <disk type='volume' device='lun' sgio='unfiltered'>
​      <driver name='qemu' type='raw'/>
​      <source pool='iscsi-net-pool' volume='unit:0:0:1' mode='host'/>
​      <target dev='sda' bus='scsi'/>
​      <shareable />
​    </disk>

Note

The mode= option within the <source> tag is optional, but if used, it must be set to 'host' and not 'direct'. When set to 'host', libvirt will find the path to the device on the local host. When set to 'direct', libvirt will generate the path to the device using the source pool's source host data.

The iSCSI pool (iscsi-net-pool) in the example above will have a similar configuration to the following:

​# virsh pool-dumpxml iscsi-net-pool
​<pool type='iscsi'>
​  <name>iscsi-net-pool</name>
​  <capacity unit='bytes'>11274289152</capacity>
​  <allocation unit='bytes'>11274289152</allocation>
​  <available unit='bytes'>0</available>
​  <source>
​    <host name='192.168.122.1' port='3260'/>
​    <device path='iqn.2013-12.com.example:iscsi-chap-netpool'/>
​    <auth type='chap' username='redhat'>
​      <secret usage='libvirtiscsi'/>
​    </auth>
​  </source>
​  <target>
​    <path>/dev/disk/by-path</path>
​    <permissions>
​      <mode>0755</mode>
​    </permissions>
​  </target>
​</pool>

To verify the details of the available LUNs in the iSCSI source pool, run the following command:

​# virsh vol-list iscsi-net-pool
​ Name                 Path
​------------------------------------------------------------------------------
​ unit:0:0:1           /dev/disk/by-path/ip-192.168.122.1:3260-iscsi-iqn.2013-12.com.example:iscsi-chap-netpool-lun-1
​ unit:0:0:2           /dev/disk/by-path/ip-192.168.122.1:3260-iscsi-iqn.2013-12.com.example:iscsi-chap-netpool-lun-2

type='volume' when using a NPIV/vHBA source pool as the SCSI source pool.

The following example XML shows a guest using a NPIV/vHBA source pool (named vhbapool_host3) as the SCSI source pool:

​<disk type='volume' device='lun' sgio='unfiltered'>
​  <driver name='qemu' type='raw'/>
​  <source pool='vhbapool_host3' volume='unit:0:1:0'/>
​  <target dev='sda' bus='scsi'/>
​  <shareable />
​</disk>

The NPIV/vHBA pool (vhbapool_host3) in the example above will have a similar configuration to:

​# virsh pool-dumpxml vhbapool_host3
​<pool type='scsi'>
​  <name>vhbapool_host3</name>
​  <capacity unit='bytes'>0</capacity>
​  <allocation unit='bytes'>0</allocation>
​  <available unit='bytes'>0</available>
​  <source>
​    <adapter type='fc_host' parent='scsi_host3' managed='yes' wwnn='5001a4a93526d0a1' wwpn='5001a4ace3ee045d'/>
​  </source>
​  <target>
​    <path>/dev/disk/by-path</path>
​    <permissions>
​      <mode>0700</mode>
​      <owner>0</owner>
​      <group>0</group>
​    </permissions>
​  </target>
​</pool>

To verify the details of the available LUNs on the vHBA, run the following command:

​# virsh vol-list vhbapool_host3
​ Name                 Path
​------------------------------------------------------------------------------
​ unit:0:0:0           /dev/disk/by-path/pci-0000:10:00.0-fc-0x5006016044602198-lun-0
​ unit:0:1:0           /dev/disk/by-path/pci-0000:10:00.0-fc-0x5006016844602198-lun-0

For more information on using a NPIV vHBA with SCSI devices, see Section 14.7.3, “Configuring the virtual machine to use a vHBA LUN”.

The following procedure shows an example of adding a SCSI LUN-based storage device to a guest. Any of the above <disk device='lun'> guest disk configurations can be attached with this method. Substitute configurations according to your environment.

⁠

Procedure 15.3. Attaching SCSI LUN-based storage to a guest

Create the device file by writing a <disk> element in a new file, and save this file with an XML extension (in this example, sda.xml):

​# cat sda.xml
​<disk type='volume' device='lun' sgio='unfiltered'>
​  <driver name='qemu' type='raw'/>
​  <source pool='vhbapool_host3' volume='unit:0:1:0'/>
​  <target dev='sda' bus='scsi'/>
​  <shareable />
​</disk>

Associate the device created in sda.xml with your guest virtual machine (Guest1, for example):

# virsh attach-device --config Guest1 ~/sda.xml

Note

Running the virsh attach-device command with the --config option requires a guest reboot to add the device permanently to the guest. Alternatively, the --persistent option can be used instead of --config, which can also be used to hotplug the device to a guest.

Alternatively, the SCSI LUN-based storage can be attached or configured on the guest using virt-manager. To configure this using virt-manager, click the Add Hardware button and add a virtual disk with the desired parameters, or change the settings of an existing SCSI LUN device from this window. In Red Hat Enterprise Linux 7.2, the SGIO value can also be configured in virt-manager:

⁠

Figure 15.1. Configuring SCSI LUN storage with virt-manager

⁠15.5.4. Managing storage controllers in a guest virtual machine

Unlike virtio disks, SCSI devices require the presence of a controller in the guest virtual machine. This section details the necessary steps to create a virtual SCSI controller (also known as "Host Bus Adapter", or HBA), and to add SCSI storage to the guest virtual machine.

⁠

Procedure 15.4. Creating a virtual SCSI controller

Display the configuration of the guest virtual machine (Guest1) and look for a pre-existing SCSI controller:

# virsh dumpxml Guest1 | grep controller.*scsi

If a device controller is present, the command will output one or more lines similar to the following:

<controller type='scsi' model='virtio-scsi' index='0'/>

If the previous step did not show a device controller, create the description for one in a new file and add it to the virtual machine, using the following steps:

Create the device controller by writing a <controller> element in a new file and save this file with an XML extension. virtio-scsi-controller.xml, for example.

<controller type='scsi' model='virtio-scsi'/>

Associate the device controller you just created in virtio-scsi-controller.xml with your guest virtual machine (Guest1, for example):

# virsh attach-device --config Guest1 ~/virtio-scsi-controller.xml

In this example the --config option behaves the same as it does for disks. Refer to Procedure 15.2, “Adding physical block devices to guests” for more information.

Add a new SCSI disk or CD-ROM. The new disk can be added using the methods in sections Section 15.5.1, “Adding file based storage to a guest” and Section 15.5.2, “Adding hard drives and other block devices to a guest”. In order to create a SCSI disk, specify a target device name that starts with sd. The supported limit for each controller is 1024 virtio-scsi disks, but it is possible that other available resources in the host (such as file descriptors) are exhausted with fewer disks.

For more information refer to the following Red Hat Enterprise Linux 6 whitepaper: The next-generation storage interface for the Red Hat Enterprise Linux Kernel Virtual Machine: virtio-scsi.

# virsh attach-disk Guest1 /var/lib/libvirt/images/FileName.img sdb --cache none

Depending on the version of the driver in the guest virtual machine, the new disk may not be detected immediately by a running guest virtual machine. Follow the steps in the Red Hat Enterprise Linux Storage Administration Guide.

⁠Chapter 16. Using qemu-img

The qemu-img command line tool is used for formatting, modifying, and verifying various file systems used by KVM. qemu-img options and usages are highlighted in the sections that follow.

⁠16.1. Checking the disk image

To perform a consistency check on a disk image with the file name imgname.

# qemu-img check [-f format] imgname

Note

Only the qcow2, qcow2 version3, and vdi formats support consistency checks.

⁠16.2. Committing changes to an image

Commit any changes recorded in the specified image file (imgname) to the file's base image with the qemu-img commit command. Optionally, specify the file's format type (fmt).

 # qemu-img commit [-f qcow2] [-t cache] imgname

⁠16.3. Converting an existing image to another format

The convert option is used to convert one recognized image format to another image format. Refer to Section 16.9, “Supported qemu-img formats” for a list of accepted formats.

# qemu-img convert [-c] [-p] [-f fmt] [-t cache] [-O output_fmt] [-o options] [-S sparse_size] filename output_filename

The -p parameter shows the progress of the command (optional and not for every command) and -S flag allows for the creation of a sparse file, which is included within the disk image. Sparse files in all purposes function like a standard file, except that the physical blocks that only contain zeros (i.e., nothing). When the Operating System sees this file, it treats it as it exists and takes up actual disk space, even though in reality it doesn't take any. This is particularly helpful when creating a disk for a guest virtual machine as this gives the appearance that the disk has taken much more disk space than it has. For example, if you set -S to 50Gb on a disk image that is 10Gb, then your 10Gb of disk space will appear to be 60Gb in size even though only 10Gb is actually being used.

Convert the disk image filename to disk image output_filename using format output_format. The disk image can be optionally compressed with the -c option, or encrypted with the -o option by setting -o encryption. Note that the options available with the -o parameter differ with the selected format.

Only the qcow2 and qcow2 format supports encryption or compression. qcow2 encryption uses the AES format with secure 128-bit keys. qcow2 compression is read-only, so if a compressed sector is converted from qcow2 format, it is written to the new format as uncompressed data.

Image conversion is also useful to get a smaller image when using a format which can grow, such as qcow or cow. The empty sectors are detected and suppressed from the destination image.

⁠16.4. Creating and formatting new images or devices

Create the new disk image filename of size size and format format.

# qemu-img create [-f format] [-o options] filename [size]

If a base image is specified with -o backing_file=filename, the image will only record differences between itself and the base image. The backing file will not be modified unless you use the commit command. No size needs to be specified in this case.

⁠16.5. Displaying image information

The info parameter displays information about a disk image filename. The format for the info option is as follows:

# qemu-img info [-f format] filename

This command is often used to discover the size reserved on disk which can be different from the displayed size. If snapshots are stored in the disk image, they are displayed also. This command will show for example, how much space is being taken by a qcow2 image on a block device. This is done by running the qemu-img. You can check that the image in use is the one that matches the output of the qemu-img info command with the qemu-img check command.

# qemu-img info /dev/vg-90.100-sluo/lv-90-100-sluo
image: /dev/vg-90.100-sluo/lv-90-100-sluo
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 0
cluster_size: 65536

⁠16.6. Re-basing a backing file of an image

The qemu-img rebase changes the backing file of an image.

# qemu-img rebase [-f fmt] [-t cache] [-p] [-u] -b backing_file [-F backing_fmt] filename

The backing file is changed to backing_file and (if the format of filename supports the feature), the backing file format is changed to backing_format.

Note

Only the qcow2 format supports changing the backing file (rebase).

There are two different modes in which rebase can operate: safe and unsafe.

safe mode is used by default and performs a real rebase operation. The new backing file may differ from the old one and the qemu-img rebase command will take care of keeping the guest virtual machine-visible content of filename unchanged. In order to achieve this, any clusters that differ between backing_file and old backing file of filename are merged into filename before making any changes to the backing file.

Note that safe mode is an expensive operation, comparable to converting an image. The old backing file is required for it to complete successfully.

unsafe mode is used if the -u option is passed to qemu-img rebase. In this mode, only the backing file name and format of filename is changed, without any checks taking place on the file contents. Make sure the new backing file is specified correctly or the guest-visible content of the image will be corrupted.

This mode is useful for renaming or moving the backing file. It can be used without an accessible old backing file. For instance, it can be used to fix an image whose backing file has already been moved or renamed.

⁠16.7. Re-sizing the disk image

Change the disk image filename as if it had been created with size size. Only images in raw format can be re-sized in both directions, whereas qcow2 version 2 or qcow2 version 3 images can be grown but cannot be shrunk.

Use the following to set the size of the disk image filename to size bytes:

# qemu-img resize filename size

You can also re-size relative to the current size of the disk image. To give a size relative to the current size, prefix the number of bytes with + to grow, or - to reduce the size of the disk image by that number of bytes. Adding a unit suffix allows you to set the image size in kilobytes (K), megabytes (M), gigabytes (G) or terabytes (T).

# qemu-img resize filename [+|-]size[K|M|G|T]

Warning

Before using this command to shrink a disk image, you must use file system and partitioning tools inside the VM itself to reduce allocated file systems and partition sizes accordingly. Failure to do so will result in data loss.

After using this command to grow a disk image, you must use file system and partitioning tools inside the VM to actually begin using the new space on the device.

⁠16.8. Listing, creating, applying, and deleting a snapshot

Using different parameters from the qemu-img snapshot command you can list, apply, create, or delete an existing snapshot (snapshot) of specified image (filename).

# qemu-img snapshot [ -l | -a snapshot | -c snapshot | -d snapshot ] filename

The accepted arguments are as follows:

-l lists all snapshots associated with the specified disk image.

The apply option, -a, reverts the disk image (filename) to the state of a previously saved snapshot.

-c creates a snapshot (snapshot) of an image (filename).

-d deletes the specified snapshot.

⁠16.9. Supported qemu-img formats

When a format is specified in any of the qemu-img commands, the following format types may be used:

raw - Raw disk image format (default). This can be the fastest file-based format. If your file system supports holes (for example in ext2 or ext3 ), then only the written sectors will reserve space. Use qemu-img info to obtain the real size used by the image or ls -ls on Unix/Linux. Although Raw images give optimal performance, only very basic features are available with a Raw image (no snapshots etc.).

qcow2 - QEMU image format, the most versatile format with the best feature set. Use it to have optional AES encryption, zlib-based compression, support of multiple VM snapshots, and smaller images, which are useful on file systems that do not support holes . Note that this expansive feature set comes at the cost of performance.

Although only the formats above can be used to run on a guest virtual machine or host physical machine machine, qemu-img also recognizes and supports the following formats in order to convert from them into either raw , or qcow2 format. The format of an image is usually detected automatically. In addition to converting these formats into raw or qcow2 , they can be converted back from raw or qcow2 to the original format. Note that the qcow2 version supplied with Red Hat Enterprise Linux 7 is 1.1. The format that is supplied with previous versions of Red Hat Enterprise Linux will be 0.10. You can revert image files to previous versions of qcow2. To know which version you are using, run qemu-img info qcow2 [imagefilename.img] command. To change the qcow version refer to Section 26.20.2, “Setting target elements”.

bochs - Bochs disk image format.

cloop - Linux Compressed Loop image, useful only to reuse directly compressed CD-ROM images present for example in the Knoppix CD-ROMs.

cow - User Mode Linux Copy On Write image format. The cow format is included only for compatibility with previous versions.

dmg - Mac disk image format.

nbd - Network block device.

parallels - Parallels virtualization disk image format.

qcow - Old QEMU image format. Only included for compatibility with older versions.

vdi - Oracle VM VirtualBox hard disk image format.

vmdk - VMware 3 and 4 compatible image format.

vvfat - Virtual VFAT disk image format.

⁠Chapter 17. KVM live migration

This chapter covers migrating guest virtual machines running on one host physical machine to another. In both instances, the host physical machines are running the KVM hypervisor.

Migration describes the process of moving a guest virtual machine from one host physical machine to another. This is possible because guest virtual machines are running in a virtualized environment instead of directly on the hardware. Migration is useful for:

Load balancing - guest virtual machines can be moved to host physical machines with lower usage when their host physical machine becomes overloaded, or another host physical machine is under-utilized.

Hardware independence - when we need to upgrade, add, or remove hardware devices on the host physical machine, we can safely relocate guest virtual machines to other host physical machines. This means that guest virtual machines do not experience any downtime for hardware improvements.

Energy saving - guest virtual machines can be redistributed to other host physical machines and can thus be powered off to save energy and cut costs in low usage periods.

Geographic migration - guest virtual machines can be moved to another location for lower latency or in serious circumstances.

Migration works by sending the state of the guest virtual machine's memory and any virtualized devices to a destination host physical machine. It is recommended to use shared, networked storage to store the guest virtual machine's images to be migrated. It is also recommended to use libvirt-managed storage pools for shared storage when migrating virtual machines.

Migrations can be performed live or not.

In a live migration, the guest virtual machine continues to run on the source host physical machine, while its memory pages are transferred to the destination host physical machine. During migration, KVM monitors the source for any changes in pages it has already transferred, and begins to transfer these changes when all of the initial pages have been transferred. KVM also estimates transfer speed during migration, so when the remaining amount of data to transfer will take a certain configurable period of time (10ms by default), KVM suspends the original guest virtual machine, transfers the remaining data, and resumes the same guest virtual machine on the destination host physical machine.

In contrast, a non-live migration (offline migration) suspends the guest virtual machine and then copies the guest virtual machine's memory to the destination host physical machine. The guest virtual machine is then resumed on the destination host physical machine and the memory the guest virtual machine used on the source host physical machine is freed. The time it takes to complete such a migration only depends on network bandwidth and latency. If the network is experiencing heavy use or low bandwidth, the migration will take much longer. It should be noted that if the original guest virtual machine modifies pages faster than KVM can transfer them to the destination host physical machine, offline migration must be used, as live migration would never complete.

Note

If you are migrating a guest virtual machine that has virtio devices on it please adhere to the warning explained in Important

⁠17.1. Live migration requirements

Migrating guest virtual machines requires the following:

⁠

Migration requirements

A guest virtual machine installed on shared storage using one of the following protocols:

Fibre Channel-based LUNs

iSCSI

FCoE

NFS

GFS2

SCSI RDMA protocols (SCSI RCP): the block export protocol used in Infiniband and 10GbE iWARP adapters

Make sure that the libvirtd service is enabled.

# systemctl enable libvirtd

Make sure that the libvirtd service is running.

# systemctl restart libvirtd

. It is also important to note that the ability to migrate effectively is dependent on the parameter settings in the /etc/libvirt/libvirtd.conf configuration file.

The migration platforms and versions should be checked against table Table 17.1, “Live Migration Compatibility”

Both systems must have the appropriate TCP/IP ports open. In cases where a firewall is used refer to the Red Hat Enterprise Linux Virtualization Security Guide for detailed port information.

A separate system exporting the shared storage medium. Storage should not reside on either of the two host physical machines being used for migration.

Shared storage must mount at the same location on source and destination systems. The mounted directory names must be identical. Although it is possible to keep the images using different paths, it is not recommended. Note that, if you are intending to use virt-manager to perform the migration, the path names must be identical. If however you intend to use virsh to perform the migration, different network configurations and mount directories can be used with the help of --xml option or pre-hooks when doing migrations (refer to Live Migration Limitations). For more information on prehooks, refer to libvirt.org, and for more information on the XML option, refer to Chapter 26, Manipulating the domain XML.

When migration is attempted on an existing guest virtual machine in a public bridge+tap network, the source and destination host physical machines must be located in the same network. Otherwise, the guest virtual machine network will not operate after migration.

Live Migration Limitations

Guest virtual machine migration has the following limitations when used on Red Hat Enterprise Linux with virtualization technology based on KVM:

Point to point migration – must be done manually to designate destination hypervisor from originating hypervisor

No validation or roll-back is available

Determination of target may only be done manually

Storage migration cannot be performed live on Red Hat Enterprise Linux 7, but you can migrate storage while the guest virtual machine is powered down. Live storage migration is available on Red Hat Enterprise Virtualization . Call your service representative for details.

⁠

Procedure 17.1. Configuring libvirtd.conf

Opening the libvirtd.conf requires running the command as root:

# vim /etc/libvirt/libvirtd.conf

Change the parameters as needed and save the file.

Restart the libvirtd service:

# systemctl restart libvirtd

⁠17.2. Live migration and Red Hat Enterprise Linux version compatibility

Live Migration is supported as shown in table Table 17.1, “Live Migration Compatibility”:

⁠

Table 17.1. Live Migration Compatibility

Migration Method | Release Type | Example | Live Migration Support | Notes
Forward | Major release | 6.5+ → 7.x | Fully supported | Any issues should be reported
Backward | Major release | 7.x → 6.y | Not supported |
Forward | Minor release | 7.x → 7.y (7.0 → 7.1) | Fully supported | Any issues should be reported
Backward | Minor release | 7.y → 7.x (7.1 → 7.0) | Fully supported | Any issues should be reported

⁠

Troubleshooting problems with migration

Issues with the migration protocol — If backward migration ends with "unknown section error", repeating the migration process can repair the issue as it may be a transient error. If not, please report the problem.

⁠

Configuring network storage

Configure shared storage and install a guest virtual machine on the shared storage.

Alternatively, use the NFS example in Section 17.3, “Shared storage example: NFS for a simple migration”

⁠17.3. Shared storage example: NFS for a simple migration

Important

This example uses NFS to share guest virtual machine images with other KVM host physical machines. Although not practical for large installations, it is presented to demonstrate migration techniques only. Do not use this example for migrating or running more than a few guest virtual machines. In addition, it is required that the synch parameter is enabled. This is required for proper export of the NFS storage.

iSCSI storage is a better choice for large deployments. Refer to Section 14.5, “iSCSI-based storage pools” for configuration details.

Also note, that the instructions provided herin are not meant to replace the detailed instructions found in Red Hat Linux Storage Administration Guide. Refer to this guide for information on configuring NFS, opening IP tables, and configuring the firewall.

Make sure that NFS file locking is not used as it is not supported in KVM.

Export your libvirt image directory

Migration requires storage to reside on a system that is separate to the migration target systems. On this separate system, export the storage by adding the default image directory to the /etc/exports file:

/var/lib/libvirt/images *.example.com(rw,no_root_squash,sync)

Change the hostname parameter as required for your environment.

Start NFS

Install the NFS packages if they are not yet installed:

# yum install nfs-utils

Make sure that the ports for NFS in iptables (2049, for example) are opened and add NFS to the /etc/hosts.allow file.

Start the NFS service:

# systemctl restart nfs-server

Mount the shared storage on the destination

On the migration destination system, mount the /var/lib/libvirt/images directory:

# mount storage_host:/var/lib/libvirt/images /var/lib/libvirt/images

Warning

Whichever directory is chosen for the source host physical machine must be exactly the same as that on the destination host physical machine. This applies to all types of shared storage. The directory must be the same or the migration with virt-manager will fail.

⁠17.4. Live KVM migration with virsh

A guest virtual machine can be migrated to another host physical machine with the virsh command. The migrate command accepts parameters in the following format:

# virsh migrate --live GuestName DestinationURL

Note that the --live option may be eliminated when live migration is not desired. Additional options are listed in Section 17.4.2, “Additional options for the virsh migrate command”.

The GuestName parameter represents the name of the guest virtual machine which you want to migrate.

The DestinationURL parameter is the connection URL of the destination host physical machine. The destination system must run the same version of Red Hat Enterprise Linux, be using the same hypervisor and have libvirt running.

Note

The DestinationURL parameter for normal migration and peer2peer migration has different semantics:

normal migration: the DestinationURL is the URL of the target host physical machine as seen from the source guest virtual machine.

peer2peer migration: DestinationURL is the URL of the target host physical machine as seen from the source host physical machine.

Once the command is entered, you will be prompted for the root password of the destination system.

Important

Name resolution must be working on both sides (source and destination) in order for migration to succeed. Each side must be able to find the other. Make sure that you can ping one side to the other to check that the name resolution is working.

⁠

Example: live migration with virsh

This example migrates from host1.example.com to host2.example.com. Change the host physical machine names for your environment. This example migrates a virtual machine named guest1-rhel6-64.

This example assumes you have fully configured shared storage and meet all the prerequisites (listed here: Migration requirements).

⁠

Verify the guest virtual machine is running

From the source system, host1.example.com, verify guest1-rhel6-64 is running:

[root@host1 ~]# virsh list
Id Name                 State
----------------------------------
 10 guest1-rhel6-64     running

⁠

Migrate the guest virtual machine

Execute the following command to live migrate the guest virtual machine to the destination, host2.example.com. Append /system to the end of the destination URL to tell libvirt that you need full access.

# virsh migrate --live guest1-rhel7-64 qemu+ssh://host2.example.com/system

Once the command is entered you will be prompted for the root password of the destination system.

⁠

Wait

The migration may take some time depending on load and the size of the guest virtual machine. virsh only reports errors. The guest virtual machine continues to run on the source host physical machine until fully migrated.

⁠

Verify the guest virtual machine has arrived at the destination host

From the destination system, host2.example.com, verify guest1-rhel7-64 is running:

[root@host2 ~]# virsh list
Id Name                 State
----------------------------------
 10 guest1-rhel7-64     running

The live migration is now complete.

Note

libvirt supports a variety of networking methods including TLS/SSL, UNIX sockets, SSH, and unencrypted TCP. Refer to Chapter 21, Remote management of guests for more information on using other methods.

Note

Non-running guest virtual machines cannot be migrated with the virsh migrate command. To migrate a non-running guest virtual machine, the following script should be used:

 virsh -c qemu+ssh://<target-system-FQDN> migrate --offline --persistent

⁠17.4.1. Additional tips for migration with virsh

It is possible to perform multiple, concurrent live migrations where each migration runs in a separate command shell. However, this should be done with caution and should involve careful calculations as each migration instance uses one MAX_CLIENT from each side (source and target). As the default setting is 20, there is enough to run 10 instances without changing the settings. Should you need to change the settings, refer to the procedure Procedure 17.1, “Configuring libvirtd.conf”.

Open the libvirtd.conf file as described in Procedure 17.1, “Configuring libvirtd.conf”.

Look for the Processing controls section.

#################################################################
#
# Processing controls
#

# The maximum number of concurrent client connections to allow
# over all sockets combined.
#max_clients = 20

# The minimum limit sets the number of workers to start up
# initially. If the number of active clients exceeds this,
# then more threads are spawned, upto max_workers limit.
# Typically you'd want max_workers to equal maximum number
# of clients allowed
#min_workers = 5
#max_workers = 20

# The number of priority workers. If all workers from above
# pool will stuck, some calls marked as high priority
# (notably domainDestroy) can be executed in this pool.
#prio_workers = 5

# Total global limit on concurrent RPC calls. Should be
# at least as large as max_workers. Beyond this, RPC requests
# will be read into memory and queued. This directly impact
# memory usage, currently each request requires 256 KB of
# memory. So by default upto 5 MB of memory is used
#
# XXX this isn't actually enforced yet, only the per-client
# limit is used so far
#max_requests = 20

# Limit on concurrent requests from a single client
# connection. To avoid one client monopolizing the server
# this should be a small fraction of the global max_requests
# and max_workers parameter
#max_client_requests = 5

#################################################################

Change the max_clients and max_workers parameters settings. It is recommended that the number be the same in both parameters. The max_clients will use 2 clients per migration (one per side) and max_workers will use 1 worker on the source and 0 workers on the destination during the perform phase and 1 worker on the destination during the finish phase.

Important

The max_clients and max_workers parameters settings are effected by all guest virtual machine connections to the libvirtd service. This means that any user that is using the same guest virtual machine and is performing a migration at the same time will also beholden to the limits set in the the max_clients and max_workers parameters settings. This is why the maximum value needs to be considered carefully before performing a concurrent live migration.

Important

The max_clients parameter controls how many clients are allowed to connect to libvirt. When a large number of containers are started at once, this limit can be easily reached and exceeded. The value of the max_clients parameter could be increased to avoid this, but doing so can leave the system more vulnerable to denial of service (DoS) attacks against instances. To alleviate this problem, a new max_anonymous_clients setting has been introduced in Red Hat Enterprise Linux 7.0 that specifies a limit of connections which are accepted but not yet authenticated. You can implement a combination of max_clients and max_anonymous_clients to suit your workload.

Save the file and restart the service.

Note

There may be cases where a migration connection drops because there are too many ssh sessions that have been started, but not yet authenticated. By default, sshd allows only 10 sessions to be in a "pre-authenticated state" at any time. This setting is controlled by the MaxStartups parameter in the sshd configuration file (located here: /etc/ssh/sshd_config), which may require some adjustment. Adjusting this parameter should be done with caution as the limitation is put in place to prevent DoS attacks (and over-use of resources in general). Setting this value too high will negate its purpose. To change this parameter, edit the file /etc/ssh/sshd_config, remove the # from the beginning of the MaxStartups line, and change the 10 (default value) to a higher number. Remember to save the file and restart the sshd service. For more information, refer to the sshd_config man page.

⁠17.4.2. Additional options for the virsh migrate command

In addition to --live, virsh migrate accepts the following options:

--direct - used for direct migration

--p2p - used for peer-2-peer migration

--tunneled - used for tunneled migration

--offline - migrates domain definition without starting the domain on destination and without stopping it on source host. Offline migration may be used with inactive domains and it must be used with the --persistent option.

--persistent - leaves the domain persistent on destination host physical machine

--undefinesource - undefines the domain on the source host physical machine

--suspend - leaves the domain paused on the destination host physical machine

--change-protection - enforces that no incompatible configuration changes will be made to the domain while the migration is underway; this flag is implicitly enabled when supported by the hypervisor, but can be explicitly used to reject the migration if the hypervisor lacks change protection support.

--unsafe - forces the migration to occur, ignoring all safety procedures.

--verbose - displays the progress of migration as it is occurring

--compressed - activates compression of memory pages that have to be transferred repeatedly during live migration.

--abort-on-error - cancels the migration if a soft error (for example I/O error) happens during the migration.

--domain name - sets the domain name, id or uuid.

--desturi uri - connection URI of the destination host as seen from the client (normal migration) or source (p2p migration).

--migrateuri uri - the migration URI, which can usually be omitted.

--graphicsuri uri - graphics URI to be used for seamless graphics migration.

--listen-address address - sets the listen address that the hypervisor on the destination side should bind to for incoming migration.

--timeout seconds - forces a guest virtual machine to suspend when the live migration counter exceeds N seconds. It can only be used with a live migration. Once the timeout is initiated, the migration continues on the suspended guest virtual machine.

--dname newname - is used for renaming the domain during migration, which also usually can be omitted

--xml filename - the filename indicated can be used to supply an alternative XML file for use on the destination to supply a larger set of changes to any host-specific portions of the domain XML, such as accounting for naming differences between source and destination in accessing underlying storage. This option is usually omitted.

In addtion the following commands may help as well:

virsh migrate-setmaxdowntime domain downtime - will set a maximum tolerable downtime for a domain which is being live-migrated to another host. The specified downtime is in milliseconds. The domain specified must be the same domain that is being migrated.

virsh migrate-compcache domain --size - will set and or get the size of the cache in bytes which is used for compressing repeatedly transferred memory pages during a live migration. When the --size is not used the command displays the current size of the compression cache. When --size is used, and specified in bytes, the hypervisor is asked to change compression to match the indicated size, following which the current size is displayed. The --size argument is supposed to be used while the domain is being live migrated as a reaction to the migration progress and increasing number of compression cache misses obtained from the domjobingfo.

virsh migrate-setspeed domain bandwidth - sets the migration bandwidth in Mib/sec for the specified domain which is being migrated to another host.

virsh migrate-getspeed domain - gets the maximum migration bandwidth that is available in Mib/sec for the specified domain.

Refer to Live Migration Limitations or the virsh man page for more information.

⁠17.5. Migrating with virt-manager

This section covers migrating a KVM guest virtual machine with virt-manager from one host physical machine to another.

Open virt-manager

Open virt-manager. Choose Applications → System Tools → Virtual Machine Manager from the main menu bar to launch virt-manager.

⁠

Figure 17.1. Virt-Manager main menu

Connect to the target host physical machine

Connect to the target host physical machine by clicking on the File menu, then click Add Connection.

⁠

Figure 17.2. Open Add Connection window

Add connection

The Add Connection window appears.

⁠

Figure 17.3. Adding a connection to the target host physical machine

Enter the following details:

Hypervisor: Select QEMU/KVM.

Method: Select the connection method.

Username: Enter the username for the remote host physical machine.

Hostname: Enter the hostname for the remote host physical machine.

Click the Connect button. An SSH connection is used in this example, so the specified user's password must be entered in the next step.

⁠

Figure 17.4. Enter password

Migrate guest virtual machines

Open the list of guests inside the source host physical machine (click the small triangle on the left of the host name) and right click on the guest that is to be migrated (guest1-rhel6-64 in this example) and click Migrate.

⁠

Figure 17.5. Choosing the guest to be migrated

In the New Host field, use the drop-down list to select the host physical machine you wish to migrate the guest virtual machine to and click Migrate.

⁠

Figure 17.6. Choosing the destination host physical machine and starting the migration process

A progress window will appear.

⁠

Figure 17.7. Progress window

virt-manager now displays the newly migrated guest virtual machine running in the destination host. The guest virtual machine that was running in the source host physical machine is now listed inthe Shutoff state.

⁠

Figure 17.8. Migrated guest virtual machine running in the destination host physical machine

Optional - View the storage details for the host physical machine

In the Edit menu, click Connection Details, the Connection Details window appears.

Click the Storage tab. The iSCSI target details for the destination host physical machine is shown. Note that the migrated guest virtual machine is listed as using the storage

⁠

Figure 17.9. Storage details

This host was defined by the following XML configuration:

⁠

​
​
​<pool type='iscsi'>
​    <name>iscsirhel6guest</name>
​    <source>
​        <host name='virtlab22.example.com.'/>
​        <device path='iqn.2001-05.com.iscsivendor:0-8a0906-fbab74a06-a700000017a4cc89-rhevh'/>
​    </source>
​    <target>
​        <path>/dev/disk/by-path</path>
​    </target>
​</pool>
​  ...
​

Figure 17.10. XML configuration for the destination host physical machine

⁠Chapter 18. Guest virtual machine device configuration

Red Hat Enterprise Linux 7 supports three classes of devices for guest virtual machines:

Emulated devices are purely virtual devices that mimic real hardware, allowing unmodified guest operating systems to work with them using their standard in-box drivers. Red Hat Enterprise Linux 7 supports up to 216 virtio devices.

Virtio devices are purely virtual devices designed to work optimally in a virtual machine. Virtio devices are similar to emulated devices, however, non-Linux virtual machines do not include the drivers they require by default. Virtualization management software like the Virtual Machine Manager (virt-manager) and the Red Hat Enterprise Virtualization Hypervisor install these drivers automatically for supported non-Linux guest operating systems. Red Hat Enterprise Linux 7 supports up to 700 scsi disks.

Assigned devices are physical devices that are exposed to the virtual machine. This method is also known as 'passthrough'. Device assignment allows virtual machines exclusive access to PCI devices for a range of tasks, and allows PCI devices to appear and behave as if they were physically attached to the guest operating system. Red Hat Enterprise Linux 7 supports up to 32 assigned devices per virtual machine.

Device assignment is supported on PCIe devices, including select graphics devices. Nvidia K-series Quadro, GRID, and Tesla graphics card GPU functions are now supported with device assignment in Red Hat Enterprise Linux 7. Parallel PCI devices may be supported as assigned devices, but they have severe limitations due to security and system configuration conflicts. Refer to the sections within this chapter for more details regarding specific series and versions that are supported.

Red Hat Enterprise Linux 7 supports PCI hotplug of devices exposed as single function slots to the virtual machine. Single function host devices and individual functions of multi-function host devices may be configured to enable this. Configurations exposing devices as multi-function PCI slots to the virtual machine are recommended only for non-hotplug applications.

For more information on specific devices and for limitations refer to Section 26.18, “Devices”.

Note

Platform support for interrupt remapping is required to fully isolate a guest with assigned devices from the host. Without such support, the host may be vulnerable to interrupt injection attacks from a malicious guest. In an environment where guests are trusted, the admin may opt-in to still allow PCI device assignment using the allow_unsafe_interrupts option to the vfio_iommu_type1 module. This may either be done persistently by adding a .conf file (e.g. local.conf) to /etc/modprobe.d containing the following:

options vfio_iommu_type1 allow_unsafe_interrupts=1

or dynamically using the sysfs entry to do the same:

# echo 1 > /sys/module/vfio_iommu_type1/parameters/allow_unsafe_interrupts

⁠18.1. PCI devices

PCI device assignment is only available on hardware platforms supporting either Intel VT-d or AMD IOMMU. These Intel VT-d or AMD IOMMU specifications must be enabled in BIOS for PCI device assignment to function.

⁠

Procedure 18.1. Preparing an Intel system for PCI device assignment

Enable the Intel VT-d specifications

The Intel VT-d specifications provide hardware support for directly assigning a physical device to a virtual machine. These specifications are required to use PCI device assignment with Red Hat Enterprise Linux.

The Intel VT-d specifications must be enabled in the BIOS. Some system manufacturers disable these specifications by default. The terms used to refer to these specifications can differ between manufacturers; consult your system manufacturer's documentation for the appropriate terms.

Activate Intel VT-d in the kernel

Activate Intel VT-d in the kernel by adding the intel_iommu=pt parameter to the end of the GRUB_CMDLINX_LINUX line, within the quotes, in the /etc/sysconfig/grub file.

Note

Instead of using the *_iommu=pt parameter for device assignment, which puts IOMMU into passthrough mode, it is also possible to use *_iommu=on. However, iommu=on should be used with caution, as it enables IOMMU for all devices, including those not used for device assignment by KVM, which may have a negative impact on guest performance.

The example below is a modified grub file with Intel VT-d activated.

GRUB_CMDLINE_LINUX="rd.lvm.lv=vg_VolGroup00/LogVol01
vconsole.font=latarcyrheb-sun16 rd.lvm.lv=vg_VolGroup_1/root
vconsole.keymap=us $([ -x /usr/sbin/rhcrashkernel-param ] && /usr/sbin/
rhcrashkernel-param || :) rhgb quiet intel_iommu=pt"

Regenerate config file

Regenerate /boot/grub2/grub.cfg by running:

grub2-mkconfig -o /boot/grub2/grub.cfg

Ready to use

Reboot the system to enable the changes. Your system is now capable of PCI device assignment.

⁠

Procedure 18.2. Preparing an AMD system for PCI device assignment

Enable the AMD IOMMU specifications

The AMD IOMMU specifications are required to use PCI device assignment in Red Hat Enterprise Linux. These specifications must be enabled in the BIOS. Some system manufacturers disable these specifications by default.

Enable IOMMU kernel support

Append amd_iommu=pt to the end of the GRUB_CMDLINX_LINUX line, within the quotes, in /etc/sysconfig/grub so that AMD IOMMU specifications are enabled at boot.

Regenerate config file

Regenerate /boot/grub2/grub.cfg by running:

grub2-mkconfig -o /boot/grub2/grub.cfg

Ready to use

Reboot the system to enable the changes. Your system is now capable of PCI device assignment.

Note

For further information on IOMMU, see Appendix D, Working with IOMMU Groups.

⁠18.1.1. Assigning a PCI device with virsh

These steps cover assigning a PCI device to a virtual machine on a KVM hypervisor.

This example uses a PCIe network controller with the PCI identifier code, pci_0000_01_00_0, and a fully virtualized guest machine named guest1-rhel7-64.

⁠

Procedure 18.3. Assigning a PCI device to a guest virtual machine with virsh

Identify the device

First, identify the PCI device designated for device assignment to the virtual machine. Use the lspci command to list the available PCI devices. You can refine the output of lspci with grep.

This example uses the Ethernet controller highlighted in the following output:

# lspci | grep Ethernet
00:19.0 Ethernet controller: Intel Corporation 82567LM-2 Gigabit Network Connection
01:00.0 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)
01:00.1 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)

This Ethernet controller is shown with the short identifier 00:19.0. We need to find out the full identifier used by virsh in order to assign this PCI device to a virtual machine.

To do so, use the virsh nodedev-list command to list all devices of a particular type (pci) that are attached to the host machine. Then look at the output for the string that maps to the short identifier of the device you wish to use.

This example shows the string that maps to the Ethernet controller with the short identifier 00:19.0. Note that the : and . characters are replaced with underscores in the full identifier.

# virsh nodedev-list --cap pci
pci_0000_00_00_0
pci_0000_00_01_0
pci_0000_00_03_0
pci_0000_00_07_0
pci_0000_00_10_0
pci_0000_00_10_1
pci_0000_00_14_0
pci_0000_00_14_1
pci_0000_00_14_2
pci_0000_00_14_3
pci_0000_00_19_0
pci_0000_00_1a_0
pci_0000_00_1a_1
pci_0000_00_1a_2
pci_0000_00_1a_7
pci_0000_00_1b_0
pci_0000_00_1c_0
pci_0000_00_1c_1
pci_0000_00_1c_4
pci_0000_00_1d_0
pci_0000_00_1d_1
pci_0000_00_1d_2
pci_0000_00_1d_7
pci_0000_00_1e_0
pci_0000_00_1f_0
pci_0000_00_1f_2
pci_0000_00_1f_3
pci_0000_01_00_0
pci_0000_01_00_1
pci_0000_02_00_0
pci_0000_02_00_1
pci_0000_06_00_0
pci_0000_07_02_0
pci_0000_07_03_0

Record the PCI device number that maps to the device you want to use; this is required in other steps.

Review device information

Information on the domain, bus, and function are available from output of the virsh nodedev-dumpxml command:

⁠

​
​
​# virsh nodedev-dumpxml pci_0000_00_19_0
​<device>
​  <name>pci_0000_00_19_0</name>
​  <parent>computer</parent>
​  <driver>
​    <name>e1000e</name>
​  </driver>
​  <capability type='pci'>
​    <domain>0</domain>
​    <bus>0</bus>
​    <slot>25</slot>
​    <function>0</function>
​    <product id='0x1502'>82579LM Gigabit Network Connection</product>
​    <vendor id='0x8086'>Intel Corporation</vendor>
​    <iommuGroup number='7'>
​      <address domain='0x0000' bus='0x00' slot='0x19' function='0x0'/>
​    </iommuGroup>
​  </capability>
​</device>

Figure 18.1. Dump contents

Note

An IOMMU group is determined based on the visibility and isolation of devices from the perspective of the IOMMU. Each IOMMU group may contain one or more devices. When multiple devices are present, all endpoints within the IOMMU group must be claimed for any device within the group to be assigned to a guest. This can be accomplished either by also assigning the extra endpoints to the guest or by detaching them from the host driver using virsh nodedev-detach. Devices contained within a single group may not be split between multiple guests or split between host and guest. Non-endpoint devices such as PCIe root ports, switch ports, and bridges should not be detached from the host drivers and will not interfere with assignment of endpoints.

Devices within an IOMMU group can be determined using the iommuGroup section of the virsh nodedev-dumpxml output. Each member of the group is provided via a separate "address" field. This information may also be found in sysfs using the following:

$ ls /sys/bus/pci/devices/0000:01:00.0/iommu_group/devices/

An example of the output from this would be:

0000:01:00.0  0000:01:00.1

To assign only 0000.01.00.0 to the guest, the unused endpoint should be detached from the host before starting the guest:

$ virsh nodedev-detach pci_0000_01_00_1

Determine required configuration details

Refer to the output from the virsh nodedev-dumpxml pci_0000_00_19_0 command for the values required for the configuration file.

The example device has the following values: bus = 0, slot = 25 and function = 0. The decimal configuration uses those three values:

bus='0'
slot='25'
function='0'

Add configuration details

Run virsh edit, specifying the virtual machine name, and add a device entry in the <source> section to assign the PCI device to the guest virtual machine.

# virsh edit guest1-rhel7-64

⁠

​
​<hostdev mode='subsystem' type='pci' managed='yes'>
​  <source>
​     <address domain='0' bus='0' slot='25' function='0'/>
​  </source>
​</hostdev>

Figure 18.2. Add PCI device

Alternately, run virsh attach-device, specifying the virtual machine name and the guest's XML file:

virsh attach-device guest1-rhel7-64 file.xml

Start the virtual machine

# virsh start guest1-rhel7-64

The PCI device should now be successfully assigned to the virtual machine, and accessible to the guest operating system.

⁠18.1.2. Assigning a PCI device with virt-manager

PCI devices can be added to guest virtual machines using the graphical virt-manager tool. The following procedure adds a Gigabit Ethernet controller to a guest virtual machine.

⁠

Procedure 18.4. Assigning a PCI device to a guest virtual machine using virt-manager

Open the hardware settings

Open the guest virtual machine and click the Add Hardware button to add a new device to the virtual machine.

⁠

Figure 18.3. The virtual machine hardware information window

Select a PCI device

Select PCI Host Device from the Hardware list on the left.

Select an unused PCI device. Note that selecting PCI devices presently in use by another guest causes errors. In this example, a spare 82576 network device is used. Click Finish to complete setup.

⁠

Figure 18.4. The Add new virtual hardware wizard

Add the new device

The setup is complete and the guest virtual machine now has direct access to the PCI device.

⁠

Figure 18.5. The virtual machine hardware information window

Note

If device assignment fails, there may be other endpoints in the same IOMMU group that are still attached to the host. There is no way to retrieve group information using virt-manager, but virsh commands can be used to analyze the bounds of the IOMMU group and if necessary sequester devices.

Refer to the Note in Section 18.1.1, “Assigning a PCI device with virsh” for more information on IOMMU groups and how to detach endpoint devices using virsh.

⁠18.1.3. PCI device assignment with virt-install

To use virt-install to assign a PCI device, use the --host-device parameter.

⁠

Procedure 18.5. Assigning a PCI device to a virtual machine with virt-install

Identify the device

Identify the PCI device designated for device assignment to the guest virtual machine.

# lspci | grep Ethernet
00:19.0 Ethernet controller: Intel Corporation 82567LM-2 Gigabit Network Connection
01:00.0 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)
01:00.1 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)

The virsh nodedev-list command lists all devices attached to the system, and identifies each PCI device with a string. To limit output to only PCI devices, run the following command:

# virsh nodedev-list --cap pci
pci_0000_00_00_0
pci_0000_00_01_0
pci_0000_00_03_0
pci_0000_00_07_0
pci_0000_00_10_0
pci_0000_00_10_1
pci_0000_00_14_0
pci_0000_00_14_1
pci_0000_00_14_2
pci_0000_00_14_3
pci_0000_00_19_0
pci_0000_00_1a_0
pci_0000_00_1a_1
pci_0000_00_1a_2
pci_0000_00_1a_7
pci_0000_00_1b_0
pci_0000_00_1c_0
pci_0000_00_1c_1
pci_0000_00_1c_4
pci_0000_00_1d_0
pci_0000_00_1d_1
pci_0000_00_1d_2
pci_0000_00_1d_7
pci_0000_00_1e_0
pci_0000_00_1f_0
pci_0000_00_1f_2
pci_0000_00_1f_3
pci_0000_01_00_0
pci_0000_01_00_1
pci_0000_02_00_0
pci_0000_02_00_1
pci_0000_06_00_0
pci_0000_07_02_0
pci_0000_07_03_0

Record the PCI device number; the number is needed in other steps.

Information on the domain, bus and function are available from output of the virsh nodedev-dumpxml command:

# virsh nodedev-dumpxml pci_0000_01_00_0

⁠

​
​
​<device>
​  <name>pci_0000_01_00_0</name>
​  <parent>pci_0000_00_01_0</parent>
​  <driver>
​    <name>igb</name>
​  </driver>
​  <capability type='pci'>
​    <domain>0</domain>
​    <bus>1</bus>
​    <slot>0</slot>
​    <function>0</function>
​    <product id='0x10c9'>82576 Gigabit Network Connection</product>
​    <vendor id='0x8086'>Intel Corporation</vendor>
​    <iommuGroup number='7'>
​      <address domain='0x0000' bus='0x00' slot='0x19' function='0x0'/>
​    </iommuGroup>
​  </capability>
​</device>

Figure 18.6. PCI device file contents

Note

If there are multiple endpoints in the IOMMU group and not all of them are assigned to the guest, you will need to manually detach the other endpoint(s) from the host by running the following command before you start the guest:

$ virsh nodedev-detach pci_0000_00_19_1

Refer to the Note in Section 18.1.1, “Assigning a PCI device with virsh” for more information on IOMMU groups.

Add the device

Use the PCI identifier output from the virsh nodedev command as the value for the --host-device parameter.

virt-install \
--name=guest1-rhel7-64 \
--disk path=/var/lib/libvirt/images/guest1-rhel7-64.img,size=8 \
--nonsparse --graphics spice \
--vcpus=2 --ram=2048 \
--location=http://example1.com/installation_tree/RHEL7.0-Server-x86_64/os \
--nonetworks \
--os-type=linux \
--os-variant=rhel7
--host-device=pci_0000_01_00_0

Complete the installation

Complete the guest installation. The PCI device should be attached to the guest.

⁠18.1.4. Detaching an assigned PCI device

When a host PCI device has been assigned to a guest machine, the host can no longer use the device. Read this section to learn how to detach the device from the guest with virsh or virt-manager so it is available for host use.

⁠

Procedure 18.6. Detaching a PCI device from a guest with virsh

Detach the device

Use the following command to detach the PCI device from the guest by removing it in the guest's XML file:

# virsh detach-device name_of_guest file.xml

Re-attach the device to the host (optional)

If the device is in managed mode, skip this step. The device will be returned to the host automatically.

If the device is not using managed mode, use the following command to re-attach the PCI device to the host machine:

# virsh nodedev-reattach device

For example, to re-attach the pci_0000_01_00_0 device to the host:

virsh nodedev-reattach pci_0000_01_00_0

The device is now available for host use.

⁠

Procedure 18.7. Detaching a PCI Device from a guest with virt-manager

Open the virtual hardware details screen

In virt-manager, double-click on the virtual machine that contains the device. Select the Show virtual hardware details button to display a list of virtual hardware.

⁠

Figure 18.7. The virtual hardware details button

Select and remove the device

Select the PCI device to be detached from the list of virtual devices in the left panel.

⁠

Figure 18.8. Selecting the PCI device to be detached

Click the Remove button to confirm. The device is now available for host use.

⁠18.1.5. Creating PCI bridges

Peripheral Component Interconnects (PCI) bridges are used to attach to devices such as network cards, modems and sound cards. Just like their physical counterparts, virtual devices can also be attached to a PCI Bridge. In the past, only 31 PCI devices could be added to any guest virtual machine. Now, when a 31st PCI device is added, a PCI bridge is automatically placed in the 31st slot moving the additional PCI device to the PCI bridge. Each PCI bridge has 31 slots for 31 additional devices, all of which can be bridges. In this manner, over 900 devices can be available for guest virtual machines. Note that this action cannot be performed when the guest virtual machine is running. You must add the PCI device on a guest virtual machine that is shutdown.

⁠18.1.5.1. PCI Bridge hotplug/unhotplug support

PCI Bridge hotplug/unhotplug is supported on the following device types:

virtio-net-pci

virtio-scsi-pci

e1000

rtl8139

virtio-serial-pci

virtio-balloon-pci

⁠18.1.6. PCI passthrough

A PCI network device (specified by the <source> element) is directly assigned to the guest using generic device passthrough, after first optionally setting the device's MAC address to the configured value, and associating the device with an 802.1Qbh capable switch using an optionally specified <virtualport> element (see the examples of virtualport given above for type='direct' network devices). Note that - due to limitations in standard single-port PCI ethernet card driver design - only SR-IOV (Single Root I/O Virtualization) virtual function (VF) devices can be assigned in this manner; to assign a standard single-port PCI or PCIe Ethernet card to a guest, use the traditional <hostdev> device definition.

To use VFIO device assignment rather than traditional/legacy KVM device assignment (VFIO is a new method of device assignment that is compatible with UEFI Secure Boot), a <type='hostdev'> interface can have an optional driver sub-element with a name attribute set to "vfio". To use legacy KVM device assignment you can set name to "kvm" (or simply omit the <driver> element, since <driver='kvm'> is currently the default).

Note that this "intelligent passthrough" of network devices is very similar to the functionality of a standard <hostdev> device, the difference being that this method allows specifying a MAC address and <virtualport> for the passed-through device. If these capabilities are not required, if you have a standard single-port PCI, PCIe, or USB network card that does not support SR-IOV (and hence would anyway lose the configured MAC address during reset after being assigned to the guest domain), or if you are using a version of libvirt older than 0.9.11, you should use standard <hostdev> to assign the device to the guest instead of <interface type='hostdev'/>.

⁠

​
​
​     <devices>
​    <interface type='hostdev'>
​      <driver name='vfio'/>
​      <source>
​        <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
​      </source>
​      <mac address='52:54:00:6d:90:02'>
​      <virtualport type='802.1Qbh'>
​        <parameters profileid='finance'/>
​      </virtualport>
​    </interface>
​  </devices>

Figure 18.9. XML example for PCI device assignment

⁠18.1.7. Configuring PCI assignment (passthrough) with SR-IOV devices

This section is for SR-IOV devices only. SR-IOV network cards provide multiple Virtual Functions (VFs) that can each be individually assigned to a guest virtual machines using PCI device assignment. Once assigned, each will behave as a full physical network device. This permits many guest virtual machines to gain the performance advantage of direct PCI device assignment, while only using a single slot on the host physical machine.

These VFs can be assigned to guest virtual machines in the traditional manner using the element <hostdev>, but as SR-IOV VF network devices do not have permanent unique MAC addresses, it causes issues where the guest virtual machine's network settings would have to be re-configured each time the host physical machine is rebooted. To remedy this, you would need to set the MAC address prior to assigning the VF to the host physical machine and you would need to set this each and every time the guest virtual machine boots. In order to assign this MAC address as well as other options, refert to the procedure described in Procedure 18.8, “Configuring MAC addresses, vLAN, and virtual ports for assigning PCI devices on SR-IOV”.

⁠

Procedure 18.8. Configuring MAC addresses, vLAN, and virtual ports for assigning PCI devices on SR-IOV

It is important to note that the <hostdev> element cannot be used for function-specific items like MAC address assignment, vLAN tag ID assignment, or virtual port assignment because the <mac>, <vlan>, and <virtualport> elements are not valid children for <hostdev>. As they are valid for <interface>, support for a new interface type was added (<interface type='hostdev'>). This new interface device type behaves as a hybrid of an <interface> and <hostdev>. Thus, before assigning the PCI device to the guest virtual machine, libvirt initializes the network-specific hardware/switch that is indicated (such as setting the MAC address, setting a vLAN tag, and/or associating with an 802.1Qbh switch) in the guest virtual machine's XML configuration file. For information on setting the vLAN tag, refer to Section 20.16, “Setting vLAN tags”.

Shutdown the guest virtual machine

Using virsh shutdown command (refer to Section 23.10.2, “Shutting down Red Hat Enterprise Linux 6 guests on a Red Hat Enterprise Linux 7 host”), shutdown the guest virtual machine named guestVM.

# virsh shutdown guestVM

Gather information

In order to use <interface type='hostdev'>, you must have an SR-IOV-capable network card, host physical machine hardware that supports either the Intel VT-d or AMD IOMMU extensions, and you must know the PCI address of the VF that you wish to assign.

Open the XML file for editing

Run the # virsh save-image-edit command to open the XML file for editing (refer to Section 23.9.11, “Editing the guest virtual machine configuration files” for more information). As you would want to restore the guest virtual machine to its former running state, the --running would be used in this case. The name of the configuration file in this example is guestVM.xml, as the name of the guest virtual machine is guestVM.

 # virsh save-image-edit guestVM.xml --running

The guestVM.xml opens in your default editor.

Edit the XML file

Update the configuration file (guestVM.xml) to have a <devices> entry similar to the following:

⁠

​
​
​ <devices>
​   ...
​   <interface type='hostdev' managed='yes'>
​     <source>
​       <address type='pci' domain='0x0' bus='0x00' slot='0x07' function='0x0'/> <!--these values can be decimal as well-->
​     </source>
​     <mac address='52:54:00:6d:90:02'/>                                         <!--sets the mac address-->
​     <virtualport type='802.1Qbh'>                                              <!--sets the virtual port for the 802.1Qbh switch-->
​       <parameters profileid='finance'/>
​     </virtualport>
​     <vlan>                                                                     <!--sets the vlan tag-->
​      <tag id='42'/>
​     </vlan>
​   </interface>
​   ...
​ </devices>
​

Figure 18.10. Sample domain XML for hostdev interface type

Note that if you do not provide a MAC address, one will be automatically generated, just as with any other type of interface device. Also, the <virtualport> element is only used if you are connecting to an 802.11Qgh hardware switch (802.11Qbg (a.k.a. "VEPA") switches are currently not supported.

Re-start the guest virtual machine

Run the virsh start command to restart the guest virtual machine you shutdown in the first step (example uses guestVM as the guest virtual machine's domain name). Refer to Section 23.9.1, “Starting a virtual machine” for more information.

 # virsh start guestVM

When the guest virtual machine starts, it sees the network device provided to it by the physical host machine's adapter, with the configured MAC address. This MAC address will remain unchanged across guest virtual machine and host physical machine reboots.

⁠18.1.8. Setting PCI device assignment from a pool of SR-IOV virtual functions

Hard coding the PCI addresses of a particular Virtual Functions (VFs) into a guest's configuration has two serious limitations:

The specified VF must be available any time the guest virtual machine is started, implying that the administrator must permanently assign each VF to a single guest virtual machine (or modify the configuration file for every guest virtual machine to specify a currently unused VF's PCI address each time every guest virtual machine is started).

If the guest vitual machine is moved to another host physical machine, that host physical machine must have exactly the same hardware in the same location on the PCI bus (or, again, the guest vitual machine configuration must be modified prior to start).

It is possible to avoid both of these problems by creating a libvirt network with a device pool containing all the VFs of an SR-IOV device. Once that is done you would configure the guest virtual machine to reference this network. Each time the guest is started, a single VF will be allocated from the pool and assigned to the guest virtual machine. When the guest virtual machine is stopped, the VF will be returned to the pool for use by another guest virtual machine.

⁠

Procedure 18.9. Creating a device pool

Shutdown the guest virtual machine

Using virsh shutdown command (refer to Section 23.10.2, “Shutting down Red Hat Enterprise Linux 6 guests on a Red Hat Enterprise Linux 7 host”), shutdown the guest virtual machine named guestVM.

# virsh shutdown guestVM

Create a configuration file

Using your editor of chocice create an XML file (named passthrough.xml, for example) in the /tmp directory. Make sure to replace pf dev='eth3' with the netdev name of your own SR-IOV device's PF

The following is an example network definition that will make available a pool of all VFs for the SR-IOV adapter with its physical function (PF) at "eth3' on the host physical machine:

⁠

​
​
​<network>
​   <name>passthrough</name> <!-- This is the name of the file you created -->
​   <forward mode='hostdev' managed='yes'>
​     <pf dev='myNetDevName'/>  <!-- Use the netdev name of your SR-IOV devices PF here -->
​   </forward>
​</network>
​
​

Figure 18.11. Sample network definition domain XML

Load the new XML file

Run the following command, replacing /tmp/passthrough.xml, with the name and location of your XML file you created in the previous step:

# virsh net-define /tmp/passthrough.xml

Restarting the guest

Run the following replacing passthrough.xml, with the name of your XML file you created in the previous step:

 # virsh net-autostart passthrough # virsh net-start passthrough

Re-start the guest virtual machine

Run the virsh start command to restart the guest virtual machine you shutdown in the first step (example uses guestVM as the guest virtual machine's domain name). Refer to Section 23.9.1, “Starting a virtual machine” for more information.

 # virsh start guestVM

Initiating passthrough for devices

Although only a single device is shown, libvirt will automatically derive the list of all VFs associated with that PF the first time a guest virtual machine is started with an interface definition in its domain XML like the following:

⁠

​
​
​<interface type='network'>
​   <source network='passthrough'>
​</interface>
​
​

Figure 18.12. Sample domain XML for interface network definition

Verification

You can verify this by running virsh net-dumpxml passthrough command after starting the first guest that uses the network; you will get output similar to the following:

⁠

​
​
​<network connections='1'>
​   <name>passthrough</name>
​   <uuid>a6b49429-d353-d7ad-3185-4451cc786437</uuid>
​   <forward mode='hostdev' managed='yes'>
​     <pf dev='eth3'/>
​     <address type='pci' domain='0x0000' bus='0x02' slot='0x10' function='0x1'/>
​     <address type='pci' domain='0x0000' bus='0x02' slot='0x10' function='0x3'/>
​     <address type='pci' domain='0x0000' bus='0x02' slot='0x10' function='0x5'/>
​     <address type='pci' domain='0x0000' bus='0x02' slot='0x10' function='0x7'/>
​     <address type='pci' domain='0x0000' bus='0x02' slot='0x11' function='0x1'/>
​     <address type='pci' domain='0x0000' bus='0x02' slot='0x11' function='0x3'/>
​     <address type='pci' domain='0x0000' bus='0x02' slot='0x11' function='0x5'/>
​   </forward>
​</network>
​
​

Figure 18.13. XML dump file passthrough contents

⁠18.2. USB devices

This section gives the commands required for handling USB devices.

⁠18.2.1. Assigning USB devices to guest virtual machines

Most devices such as web cameras, card readers, disk drives, keyboards, mice, etc are connected to a computer using a USB port and cable. There are two ways to pass such devices to a guest virtual machine:

Using USB passthrough - this requires the device to be physically connected to the host physical machine that is hosting the guest virtual machine. SPICE is not needed in this case. USB devices on the host can be passed to the guest via the command line or virt-manager. Refer to Section 22.3.2, “Attaching USB devices to a guest virtual machine” for virt manager directions. Note that the virt-manager directions are not suitable for hot plugging or hot unplugging devices. If you want to hot plug/or hot unplug a USB device, refer to Procedure 23.5, “Hotplugging USB devices for use by the guest virtual machine”.

Using USB re-direction - USB re-direction is best used in cases where there is a host physical machine that is running in a data center. The user connects to his/her guest virtual machine from a local machine or thin client. On this local machine there is a SPICE client. The user can attach any USB device to the thin client and the SPICE client will redirect the device to the host physical machine on the data center so it can be used by the guest virtual machine that is running on the thin client. For instructions via the virt-manager refer to Section 22.3.3, “USB redirection”.

⁠18.2.2. Setting a limit on USB device redirection

To filter out certain devices from redirection, pass the filter property to -device usb-redir. The filter property takes a string consisting of filter rules, the format for a rule is:

<class>:<vendor>:<product>:<version>:<allow>

Use the value -1 to designate it to accept any value for a particular field. You may use multiple rules on the same command line using | as a separator. Note that if a device matches none of the passed in rules, redirecting it will not be allowed!

⁠

Example 18.1. An example of limiting redirection with a windows guest virtual machine

Prepare a Windows 7 guest virtual machine.

Add the following code excerpt to the guest virtual machine's' domain xml file:

    <redirdev bus='usb' type='spicevmc'>
      <alias name='redir0'/>
      <address type='usb' bus='0' port='3'/>
    </redirdev>
    <redirfilter>
      <usbdev class='0x08' vendor='0x1234' product='0xBEEF' version='2.0' allow='yes'/>
      <usbdev class='-1' vendor='-1' product='-1' version='-1' allow='no'/>
    </redirfilter>

Start the guest virtual machine and confirm the setting changes by running the following:

#ps -ef | grep $guest_name

-device usb-redir,chardev=charredir0,id=redir0,/
filter=0x08:0x1234:0xBEEF:0x0200:1|-1:-1:-1:-1:0,bus=usb.0,port=3

Plug a USB device into a host physical machine, and use virt-manager to connect to the guest virtual machine.

Click USB device selection in the menu, which will produce the following message: "Some USB devices are blocked by host policy". Click OK to confirm and continue.

The filter takes effect.

To make sure that the filter captures properly check the USB device vendor and product, then make the following changes in the host physical machine's domain XML to allow for USB redirection.

   <redirfilter>
      <usbdev class='0x08' vendor='0x0951' product='0x1625' version='2.0' allow='yes'/>
      <usbdev allow='no'/>
    </redirfilter>

Restart the guest virtual machine, then use virt-viewer to connect to the guest virtual machine. The USB device will now redirect traffic to the guest virtual machine.

⁠18.3. Configuring device controllers

Depending on the guest virtual machine architecture, some device buses can appear more than once, with a group of virtual devices tied to a virtual controller. Normally, libvirt can automatically infer such controllers without requiring explicit XML markup, but in some cases it is better to explicitly set a virtual controller element.

⁠

​
​
​  ...
​  <devices>
​    <controller type='ide' index='0'/>
​    <controller type='virtio-serial' index='0' ports='16' vectors='4'/>
​    <controller type='virtio-serial' index='1'>
​      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
​    </controller>
​    ...
​  </devices>
​  ...

Figure 18.14. Domain XML example for virtual controllers

Each controller has a mandatory attribute <controller type>, which must be one of:

ide

fdc

scsi

sata

usb

ccid

virtio-serial

pci

The <controller> element has a mandatory attribute <controller index> which is the decimal integer describing in which order the bus controller is encountered (for use in controller attributes of <address> elements). When <controller type ='virtio-serial'> there are two additional optional attributes (named ports and vectors), which control how many devices can be connected through the controller.

When <controller type ='scsi'>, there is an optional attribute model model, which can have the following values:

auto

buslogic

ibmvscsi

lsilogic

lsisas1068

lsisas1078

virtio-scsi

vmpvscsi

When <controller type ='usb'>, there is an optional attribute model model, which can have the following values:

piix3-uhci

piix4-uhci

ehci

ich9-ehci1

ich9-uhci1

ich9-uhci2

ich9-uhci3

vt82c686b-uhci

pci-ohci

nec-xhci

Note that if the USB bus needs to be explicitly disabled for the guest virtual machine, <model='none'> may be used. .

For controllers that are themselves devices on a PCI or USB bus, an optional sub-element <address> can specify the exact relationship of the controller to its master bus, with semantics as shown in Section 18.4, “Setting addresses for devices”.

An optional sub-element <driver> can specify the driver specific options. Currently it only supports attribute queues, which specifies the number of queues for the controller. For best performance, it's recommended to specify a value matching the number of vCPUs.

USB companion controllers have an optional sub-element <master> to specify the exact relationship of the companion to its master controller. A companion controller is on the same bus as its master, so the companion index value should be equal.

An example XML which can be used is as follows:

⁠

​
​
​     ...
​  <devices>
​    <controller type='usb' index='0' model='ich9-ehci1'>
​      <address type='pci' domain='0' bus='0' slot='4' function='7'/>
​    </controller>
​    <controller type='usb' index='0' model='ich9-uhci1'>
​      <master startport='0'/>
​      <address type='pci' domain='0' bus='0' slot='4' function='0' multifunction='on'/>
​    </controller>
​    ...
​  </devices>
​  ...
​
​

Figure 18.15. Domain XML example for USB controllers

PCI controllers have an optional model attribute with the following possible values:

pci-root

pcie-root

pci-bridge

dmi-to-pci-bridge

For machine types which provide an implicit PCI bus, the pci-root controller with index='0' is auto-added and required to use PCI devices. pci-root has no address. PCI bridges are auto-added if there are too many devices to fit on the one bus provided by model='pci-root', or a PCI bus number greater than zero was specified. PCI bridges can also be specified manually, but their addresses should only refer to PCI buses provided by already specified PCI controllers. Leaving gaps in the PCI controller indexes might lead to an invalid configuration. The following XML example can be added to the <devices> section:

⁠

​
​
​  ...
​  <devices>
​    <controller type='pci' index='0' model='pci-root'/>
​    <controller type='pci' index='1' model='pci-bridge'>
​      <address type='pci' domain='0' bus='0' slot='5' function='0' multifunction='off'/>
​    </controller>
​  </devices>
​  ...

Figure 18.16. Domain XML example for PCI bridge

For machine types which provide an implicit PCI Express (PCIe) bus (for example, the machine types based on the Q35 chipset), the pcie-root controller with index='0' is auto-added to the domain's configuration. pcie-root has also no address, but provides 31 slots (numbered 1-31) and can only be used to attach PCIe devices. In order to connect standard PCI devices on a system which has a pcie-root controller, a pci controller with model='dmi-to-pci-bridge' is automatically added. A dmi-to-pci-bridge controller plugs into a PCIe slot (as provided by pcie-root), and itself provides 31 standard PCI slots (which are not hot-pluggable). In order to have hot-pluggable PCI slots in the guest system, a pci-bridge controller will also be automatically created and connected to one of the slots of the auto-created dmi-to-pci-bridge controller; all guest devices with PCI addresses that are auto-determined by libvirt will be placed on this pci-bridge device.

⁠

​
​
​     ...
​  <devices>
​    <controller type='pci' index='0' model='pcie-root'/>
​    <controller type='pci' index='1' model='dmi-to-pci-bridge'>
​      <address type='pci' domain='0' bus='0' slot='0xe' function='0'/>
​    </controller>
​    <controller type='pci' index='2' model='pci-bridge'>
​      <address type='pci' domain='0' bus='1' slot='1' function='0'/>
​    </controller>
​  </devices>
​  ...
​
​

Figure 18.17. Domain XML example for PCIe (PCI express)

The following XML configuration is used for USB 3.0 / xHCI emulation:

⁠

​
​
​     ...
​  <devices>
​    <controller type='usb' index='3' model='nec-xhci'>
​      <address type='pci' domain='0x0000' bus='0x00' slot='0x0f' function='0x0'/>
​    </controller>
​  </devices>
​    ...

Figure 18.18. Domain XML example for USB3/xHCI devices

⁠18.4. Setting addresses for devices

Many devices have an optional <address> sub-element which is used to describe where the device is placed on the virtual bus presented to the guest virtual machine. If an address (or any optional attribute within an address) is omitted on input, libvirt will generate an appropriate address; but an explicit address is required if more control over layout is required. See Figure 18.9, “XML example for PCI device assignment” for domain XML device examples including an <address> element.

Every address has a mandatory attribute type that describes which bus the device is on. The choice of which address to use for a given device is constrained in part by the device and the architecture of the guest virtual machine. For example, a <disk> device uses type='drive', while a <console> device would use type='pci' on i686 or x86_64 guest virtual machine architectures. Each address type has further optional attributes that control where on the bus the device will be placed as described in the table:

⁠

Table 18.1. Supported device address types

Address type
Description

type='pci'
PCI addresses have the following additional attributes:

domain (a 2-byte hex integer, not currently used by qemu)

bus (a hex value between 0 and 0xff, inclusive)

slot (a hex value between 0x0 and 0x1f, inclusive)

function (a value between 0 and 7, inclusive)

multifunction controls turning on the multifunction bit for a particular slot/function in the PCI control register By default it is set to 'off', but should be set to 'on' for function 0 of a slot that will have multiple functions used.

type='drive'
Drive addresses have the following additional attributes:

controller (a 2-digit controller number)

bus (a 2-digit bus number

target (a 2-digit bus number)

unit (a 2-digit unit number on the bus)

type='virtio-serial'
Each virtio-serial address has the following additional attributes:

controller (a 2-digit controller number)

bus (a 2-digit bus number)

slot (a 2-digit slot within the bus)

type='ccid'
A CCID address, for smart-cards, has the following additional attributes:

bus (a 2-digit bus number)

slot attribute (a 2-digit slot within the bus)

type='usb'
USB addresses have the following additional attributes:

bus (a hex value between 0 and 0xfff, inclusive)

port (a dotted notation of up to four octets, such as 1.2 or 2.1.3.1)

type='isa'
ISA addresses have the following additional attributes:

iobase

irq

⁠18.5. Random number generator device

virtio-rng is a virtual hardware random number generator device that can provide the guest with fresh entropy upon request. The driver feeds the data back to the guest virtual machine's OS.

On the host physical machine, the hardware rng interface creates a chardev at /dev/hwrng, which can be opened and then read to fetch entropy from the host physical machine. Coupled with the rngd daemon, the entropy from the host physical machine can be routed to the guest virtual machine's /dev/random, which is the primary source of randomness.

Using a random number generator is particularly useful when a device such as a keyboard, mouse and other inputs are not enough to generate sufficient entropy on the guest virtual machine.The virtual random number generator device allows the host physical machine to pass through entropy to guest virtual machine operating systems. This procedure can be done either via the command line or via virt-manager. For virt-manager instructions refer to Procedure 18.10, “Implementing virtio-rng via Virtualzation Manager” and for command line instructions, refer to Procedure 18.11, “Implementing virtio-rng via command line tools”.

⁠

Procedure 18.10. Implementing virtio-rng via Virtualzation Manager

Shutdown the guest virtual machine.

Select the guest virtual machine and from the Edit menu, select Virtual Machine Details, to open the Details window for the specified guest virtual machine.

Click the Add Hardware button.

In the Add New Virtual Hardware window, select RNG to open the Random Number Generator window.

⁠

Figure 18.19. Random Number Generator window

Enter the desired parameters and click Finish when done. The parameters are explained in virtio-rng elements.

⁠

Procedure 18.11. Implementing virtio-rng via command line tools

Shutdown the guest virtual machine.

Using virsh edit domain-name command, open the XML file for the desired guest virtual machine.

Edit the <devices> element to include the following:

⁠

​
​
​  ...
​  <devices>
​    <rng model='virtio'>
​      <rate period="2000" bytes="1234"/>
​      <backend model='random'>/dev/random</backend>
​      <!-- OR -->
​      <backend model='egd' type='udp'>
​        <source mode='bind' service='1234'>
​        <source mode='connect' host physical machine='1.2.3.4' service='1234'>
​      </backend>
​    </rng>
​  </devices>
​  ...

Figure 18.20. Random number generator device

The random number generator device allows the following attributes/elements:

⁠

virtio-rng elements

model - The required model attribute specifies what type of RNG device is provided. 'virtio'

<backend> - The <backend> element specifies the source of entropy to be used for the domain. The source model is configured using the model attribute. Supported source models include 'random' — /dev/random (default setting) or similar device as source and 'egd' which sets a EGD protocol backend.

backend type='random' - This <backend> type expects a non-blocking character device as input. Examples of such devices are /dev/random and /dev/urandom. The file name is specified as contents of the <backend> element. When no file name is specified the hypervisor default is used.

<backend type='egd'> - This backend connects to a source using the EGD protocol. The source is specified as a character device. Refer to character device host physical machine interface for more information.

⁠18.6. Assigning GPU devices

Red Hat Enterprise Linux 7 supports PCI device assignment of NVIDIA K-Series Quadro (model 2000 series or higher), GRID, and Tesla as non-VGA graphics devices. Currently up to two GPUs may be attached to the virtual machine in addition to one of the standard, emulated VGA interfaces. The emulated VGA is used for pre-boot and installation and the NVIDIA GPU takes over when the NVIDIA graphics drivers are loaded. Note that the NVIDIA Quadro 2000 is not supported, nor is the Quadro K420 card.

This procedure will, in short, identify the device from lspci, detach it from host physical machine and then attach it to the guest virtual machine.

⁠

Enable IOMMU support in the host physical machine kernel

For an Intel VT-d system this is done by adding the intel_iommu=pt parameter to the kernel command line. For an AMD-Vi system, the option is amd_iommu=pt. To enable this option you will need to edit or add the GRUB_CMDLINX_LINUX line to the /etc/sysconfig/grub configuration file as follows:

GRUB_CMDLINE_LINUX="rd.lvm.lv=vg_VolGroup00/LogVol01
vconsole.font=latarcyrheb-sun16 rd.lvm.lv=vg_VolGroup_1/root
vconsole.keymap=us $([ -x /usr/sbin/rhcrashkernel-param ]  &&
/usr/sbin/rhcrashkernel-param || :) rhgb quiet intel_iommu=pt"

Note

For further information on IOMMU, see Appendix D, Working with IOMMU Groups.

Regenerate the bootloader configuration

Regenerate the bootloader configuration using the grub2-mkconfig to include this option, by running the following command:

# grub2-mkconfig -o /etc/grub2.cfg

Note that if you are using a UEFI-based host, the target file will be /etc/grub2-efi.cfg.

Reboot the host physical machine

In order for this option to take effect, reboot the host physical machine with the following command:

# reboot

⁠

Procedure 18.12. Excluding the GPU device from binding to the host physical machine driver

For GPU assignment it is recommended to exclude the device from binding to host drivers as these drivers often do not support dynamic unbinding of the device.

Identify the PCI bus address

To identify the PCI bus address and IDs of the device, run the following lspci command. In this example, a VGA controller such as a Quadro or GRID card is used as follows:

# lspci -Dnn | grep VGA
0000:02:00.0 VGA compatible controller [0300]: NVIDIA Corporation GK106GL [Quadro K4000] [10de:11fa] (rev a1)

The resulting search reveals that the PCI bus address of this device is 0000:02:00.0 and the PCI IDs for the device are 10de:11fa.

Prevent the native host physical machine driver from using the GPU device

To prevent the native host physical machine driver from using the GPU device you can use a PCI ID with the pci-stub driver. To do this, append the following additional option to the GRUB_CMDLINX_LINUX configuration file located in /etc/sysconfig/grub as follows:

pci-stub.ids=10de:11fa

To add additional PCI IDs for pci-stub, separate them with a comma.

Regenerate the bootloader configuration

Regenerate the bootloader configuration using the grub2-mkconfig to include this option, by running the following command:

# grub2-mkconfig -o /etc/grub2.cfg

Note that if you are using a UEFI-based host, the target file will be /etc/grub2-efi.cfg.

Reboot the host physical machine

In order for this option to take effect, reboot the host physical machine with the following command:

# reboot

The virsh commands can be used to further evaluate the device, however in order to use virsh with the devices you need to convert the PCI bus address to libvirt compatible format by appending pci_ and converting delimiters to underscores. In this example the libvirt address of PCI device 0000:02:00.0 becomes pci_0000_02_00_0. The nodedev-dumpxml option provides additional information for the device as shown:

# virsh nodedev-dumpxml pci_0000_02_00_0

⁠

​
​
​<device>
​  <name>pci_0000_02_00_0</name>
​  <path>/sys/devices/pci0000:00/0000:00:03.0/0000:02:00.0</path>
​  <parent>pci_0000_00_03_0</parent>
​  <driver>
​    <name>pci-stub</name>
​  </driver>
​  <capability type='pci'>
​    <domain>0</domain>
​    <bus>2</bus>
​    <slot>0</slot>
​    <function>0</function>
​    <product id='0x11fa'>GK106GL [Quadro K4000]</product>
​    <vendor id='0x10de'>NVIDIA Corporation</vendor>
​	<!-- pay attention to this part -->
​    <iommuGroup number='13'>
​      <address domain='0x0000' bus='0x02' slot='0x00' function='0x0'/>
​      <address domain='0x0000' bus='0x02' slot='0x00' function='0x1'/>
​    </iommuGroup>
​    <pci-express>
​      <link validity='cap' port='0' speed='8' width='16'/>
​      <link validity='sta' speed='2.5' width='16'/>
​    </pci-express>
​  </capability>
​</device>
​

Figure 18.21. XML file adaptation for GPU - Example

Particularly important in this output is the <iommuGroup> element. The iommuGroup indicates the set of devices which are considered isolated from other devices due to IOMMU capabilities and PCI bus topologies. All of the endpoint devices within the iommuGroup (ie. devices that are not PCIe root ports, bridges, or switch ports) need to be unbound from the native host drivers in order to be assigned to a guest. In the example above, the group is composed of the GPU device (0000:02:00.0) as well as the companion audio device (0000:02:00.1). For more information, refer to Appendix D, Working with IOMMU Groups.

Note

Assignment of Nvidia audio functions is not supported due to hardware issues with legacy interrupt support. In order to assign the GPU to a guest, the audio function must first be detached from native host drivers. This can either be done by using lspci to find the PCI IDs for the device and appending it to the pci-stub.ids option or dynamically using the nodedev-detach option of virsh. For example:

# virsh nodedev-detach pci_0000_02_00_1
Device pci_0000_02_00_1 detached

The GPU audio function is generally not useful without the GPU itself, so it’s generally recommended to use the pci-stub.ids option instead.

The GPU can be attached to the VM using virt-manager or using virsh, either by directly editing the VM XML ( virsh edit [domain]) or attaching the GPU to the domain with virsh attach-device. If you are using the virsh attach-device command, an XML fragment first needs to be created for the device, such as the following:

⁠

​
​
​<hostdev mode='subsystem' type='pci' managed='yes'>
​  <driver name='vfio'/>
​  <source>
​    <address domain='0x0000' bus='0x02' slot='0x00' function='0x0'/>
​  </source>
​</hostdev>

Figure 18.22. XML file for attaching GPU - Example

Save this to a file and run virsh attach-device [domain] [file] --persistent to include the XML in the VM configuration. Note that the assigned GPU is added in addition to the existing emulated graphics device in the guest virtual machine. The assigned GPU is handled as a secondary graphics device in the VM. Assignment as a primary graphics device is not supported and emulated graphics devices in the VM's XML should not be removed.

Note

When using an assigned Nvidia GPU in the guest, only the Nvidia drivers are supported. Other drivers may not work and may generate errors. For a Red Hat Enterprise Linux 7 guest, the nouveau driver can be blacklisted using the option modprobe.blacklist=nouveau on the kernel command line during install. For information on other guest virtual machines refer to the operating system's specific documentation.

When configuring Xorg for use with an assigned GPU in a KVM guest, the BusID option must be added to xorg.conf to specify the guest address of the GPU. For example, within the guest determine the PCI bus address of the GPU (this will be different than the host address):

# lspci | grep VGA
00:02.0 VGA compatible controller: Device 1234:1111
00:09.0 VGA compatible controller: NVIDIA Corporation GK106GL [Quadro K4000] (rev a1)

In this example the address is 00:09.0. The file /etc/X11/xorg.conf is then modified to add the highlighted entry below.

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BusID          "PCI:0:9:0"
EndSection

Depending on the guest operating system, with the Nvidia drivers loaded, the guest may support using both the emulated graphics and assigned graphics simultaneously or may disable the emulated graphics. Note that access to the assigned graphics framebuffer is not provided by tools such as virt-manager. If the assigned GPU is not connected to a physical display, guest-based remoting solutions may be necessary to access the GPU desktop. As with all PCI device assignment, migration of a guest with an assigned GPU is not supported and each GPU is owned exclusively by a single guest. Depending on the guest operating system, hotplug support of GPUs may be available.

⁠Chapter 19. SR-IOV

Developed by the PCI-SIG (PCI Special Interest Group), the Single Root I/O Virtualization (SR-IOV) specification is a standard for a type of PCI device assignment that can share a single device to multiple virtual machines. SR-IOV improves device performance for virtual machines.

Note

Virtual machines that use the Xeon E3-1200 series chip set, do not support SR-IOV. More information can be found on Intel's website or in this article.

⁠

Figure 19.1. How SR-IOV works

SR-IOV enables a Single Root Function (for example, a single Ethernet port), to appear as multiple, separate, physical devices. A physical device with SR-IOV capabilities can be configured to appear in the PCI configuration space as multiple functions. Each device has its own configuration space complete with Base Address Registers (BARs).

SR-IOV uses two PCI functions:

Physical Functions (PFs) are full PCIe devices that include the SR-IOV capabilities. Physical Functions are discovered, managed, and configured as normal PCI devices. Physical Functions configure and manage the SR-IOV functionality by assigning Virtual Functions.

Virtual Functions (VFs) are simple PCIe functions that only process I/O. Each Virtual Function is derived from a Physical Function. The number of Virtual Functions a device may have is limited by the device hardware. A single Ethernet port, the Physical Device, may map to many Virtual Functions that can be shared to virtual machines.

The hypervisor can map one or more Virtual Functions to a virtual machine. The Virtual Function's configuration space is then mapped to the configuration space presented to the guest.

Each Virtual Function can only be mapped to a single guest at a time, as Virtual Functions require real hardware resources. A virtual machine can have multiple Virtual Functions. A Virtual Function appears as a network card in the same way as a normal network card would appear to an operating system.

The SR-IOV drivers are implemented in the kernel. The core implementation is contained in the PCI subsystem, but there must also be driver support for both the Physical Function (PF) and Virtual Function (VF) devices. An SR-IOV capable device can allocate VFs from a PF. The VFs appear as PCI devices which are backed on the physical PCI device by resources such as queues and register sets.

⁠19.1. Advantages of SR-IOV

SR-IOV devices can share a single physical port with multiple virtual machines.

Virtual Functions have near-native performance and provide better performance than para-virtualized drivers and emulated access. Virtual Functions provide data protection between virtual machines on the same physical server as the data is managed and controlled by the hardware.

These features allow for increased virtual machine density on hosts within a data center.

SR-IOV is better able to utilize the bandwidth of devices with multiple guests.

⁠19.2. Using SR-IOV

This section covers the use of PCI passthrough to assign a Virtual Function of an SR-IOV capable multiport network card to a virtual machine as a network device.

SR-IOV Virtual Functions (VFs) can be assigned to virtual machines by adding a device entry in <hostdev> with the virsh edit or virsh attach-device command. However, this can be problematic because unlike a regular network device, an SR-IOV VF network device does not have a permanent unique MAC address, and is assigned a new MAC address each time the host is rebooted. Because of this, even if the guest is assigned the same VF after a reboot, when the host is rebooted the guest determines its network adapter to have a new MAC address. As a result, the guest believes there is new hardware connected each time, and will usually require re-configuration of the guest's network settings.

libvirt-0.9.10 and newer contains the <interface type='hostdev'> interface device. Using this interface device, libvirt will first perform any network-specific hardware/switch initialization indicated (such as setting the MAC address, VLAN tag, or 802.1Qbh virtualport parameters), then perform the PCI device assignment to the guest.

Using the <interface type='hostdev'> interface device requires:

an SR-IOV-capable network card,

host hardware that supports either the Intel VT-d or the AMD IOMMU extensions, and

the PCI address of the VF to be assigned.

Important

Assignment of an SR-IOV device to a virtual machine requires that the host hardware supports the Intel VT-d or the AMD IOMMU specification.

To attach an SR-IOV network device on an Intel or an AMD system, follow this procedure:

⁠

Procedure 19.1. Attach an SR-IOV network device on an Intel or AMD system

Enable Intel VT-d or the AMD IOMMU specifications in the BIOS and kernel

On an Intel system, enable Intel VT-d in the BIOS if it is not enabled already. Refer to Procedure 18.1, “Preparing an Intel system for PCI device assignment” for procedural help on enabling Intel VT-d in the BIOS and kernel.

Skip this step if Intel VT-d is already enabled and working.

On an AMD system, enable the AMD IOMMU specifications in the BIOS if they are not enabled already. Refer to Procedure 18.2, “Preparing an AMD system for PCI device assignment” for procedural help on enabling IOMMU in the BIOS.

Verify support

Verify if the PCI device with SR-IOV capabilities is detected. This example lists an Intel 82576 network interface card which supports SR-IOV. Use the lspci command to verify whether the device was detected.

# lspci
03:00.0 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)
03:00.1 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)

Note that the output has been modified to remove all other devices.

Start the SR-IOV kernel modules

If the device is supported the driver kernel module should be loaded automatically by the kernel. Optional parameters can be passed to the module using the modprobe command. The Intel 82576 network interface card uses the igb driver kernel module.

# modprobe igb [<option>=<VAL1>,<VAL2>,]
# lsmod |grep igb
igb    87592  0
dca    6708    1 igb

Activate Virtual Functions

The max_vfs parameter of the igb module allocates the maximum number of Virtual Functions. The max_vfs parameter causes the driver to spawn, up to the value of the parameter in, Virtual Functions. For this particular card the valid range is 0 to 7.

Remove the module to change the variable.

# modprobe -r igb

Restart the module with the max_vfs set to 7 or any number of Virtual Functions up to the maximum supported by your device.

# modprobe igb max_vfs=7

Make the Virtual Functions persistent

To make the Virtual Functions persistent across reboots, add the following to the /etc/rc.d/rc.local file. Specify the number of VFs desired (in this example, 2), up to the limit supported by the network interface card, and replace enp14s0f0 with the PF network device name(s):

# echo 2 > /sys/class/net/enp14s0f0/device/sriov_numvfs

This will ensure the feature is enabled at boot-time.

Note

If the /etc/rc.d/rc.local file does not already exist on your system, first create the file, then make it executable with this command:

# chmod +x /etc/rc.d/rc.local

Inspect the new Virtual Functions

Using the lspci command, list the newly added Virtual Functions attached to the Intel 82576 network device. (Alternatively, use grep to search for Virtual Function, to search for devices that support Virtual Functions.)

# lspci | grep 82576
0b:00.0 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)
0b:00.1 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)
0b:10.0 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:10.1 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:10.2 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:10.3 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:10.4 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:10.5 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:10.6 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:10.7 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:11.0 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:11.1 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:11.2 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:11.3 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:11.4 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)
0b:11.5 Ethernet controller: Intel Corporation 82576 Virtual Function (rev 01)

The identifier for the PCI device is found with the -n parameter of the lspci command. The Physical Functions correspond to 0b:00.0 and 0b:00.1. All Virtual Functions have Virtual Function in the description.

Verify devices exist with virsh

The libvirt service must recognize the device before adding a device to a virtual machine. libvirt uses a similar notation to the lspci output. All punctuation characters, ; and ., in lspci output are changed to underscores (_).

Use the virsh nodedev-list command and the grep command to filter the Intel 82576 network device from the list of available host devices. 0b is the filter for the Intel 82576 network devices in this example. This may vary for your system and may result in additional devices.

# virsh nodedev-list | grep 0b
pci_0000_0b_00_0
pci_0000_0b_00_1
pci_0000_0b_10_0
pci_0000_0b_10_1
pci_0000_0b_10_2
pci_0000_0b_10_3
pci_0000_0b_10_4
pci_0000_0b_10_5
pci_0000_0b_10_6
pci_0000_0b_11_7
pci_0000_0b_11_1
pci_0000_0b_11_2
pci_0000_0b_11_3
pci_0000_0b_11_4
pci_0000_0b_11_5

The serial numbers for the Virtual Functions and Physical Functions should be in the list.

Get device details with virsh

The pci_0000_0b_00_0 is one of the Physical Functions and pci_0000_0b_10_0 is the first corresponding Virtual Function for that Physical Function. Use the virsh nodedev-dumpxml command to get advanced output for both devices.

# virsh nodedev-dumpxml pci_0000_0b_00_0
<device>
   <name>pci_0000_0b_00_0</name>
   <parent>pci_0000_00_01_0</parent>
   <driver>
      <name>igb</name>
   </driver>
   <capability type='pci'>
      <domain>0</domain>
      <bus>11</bus>
      <slot>0</slot>
      <function>0</function>
      <product id='0x10c9'>Intel Corporation</product>
      <vendor id='0x8086'>82576 Gigabit Network Connection</vendor>
   </capability>
</device>

# virsh nodedev-dumpxml pci_0000_0b_10_0
<device>
   <name>pci_0000_0b_10_0</name>
   <parent>pci_0000_00_01_0</parent>
   <driver>
      <name>igbvf</name>
   </driver>
   <capability type='pci'>
      <domain>0</domain>
      <bus>11</bus>
      <slot>16</slot>
      <function>0</function>
      <product id='0x10ca'>Intel Corporation</product>
      <vendor id='0x8086'>82576 Virtual Function</vendor>
   </capability>
</device>

This example adds the Virtual Function pci_0000_0b_10_0 to the virtual machine in Step 9. Note the bus, slot and function parameters of the Virtual Function: these are required for adding the device.

Copy these parameters into a temporary XML file, such as /tmp/new-interface.xml for example.

   <interface type='hostdev' managed='yes'>
     <source>
       <address type='pci' domain='0' bus='11' slot='16' function='0'/>
     </source>
   </interface>

Note

If you do not specify a MAC address, one will be automatically generated. The <virtualport> element is only used when connecting to an 802.11Qbh hardware switch. The <vlan> element will transparently put the guest's device on the VLAN tagged 42.

When the virtual machine starts, it should see a network device of the type provided by the physical adapter, with the configured MAC address. This MAC address will remain unchanged across host and guest reboots.

The following <interface> example shows the syntax for the optional <mac address>, <virtualport>, and <vlan> elements. In practice, use either the <vlan> or <virtualport> element, not both simultaneously as shown in the example:

...
 <devices>
   ...
   <interface type='hostdev' managed='yes'>
     <source>
       <address type='pci' domain='0' bus='11' slot='16' function='0'/>
     </source>
     <mac address='52:54:00:6d:90:02'>
     <vlan>
        <tag id='42'/>
     </vlan>
     <virtualport type='802.1Qbh'>
       <parameters profileid='finance'/>
     </virtualport>
   </interface>
   ...
 </devices>

⁠

Add the Virtual Function to the virtual machine

Add the Virtual Function to the virtual machine using the following command with the temporary file created in the previous step. This attaches the new device immediately and saves it for subsequent guest restarts.

virsh attach-device MyGuest /tmp/new-interface.xml --live --config

Specifying the --live option with virsh attach-device attaches the new device to the running guest. Using the --config option ensures the new device is available after future guest restarts.

Note

The --live option is only accepted when the guest is running. virsh will return an error if the --live option is used on a non-running guest.

The virtual machine detects a new network interface card. This new card is the Virtual Function of the SR-IOV device.

⁠19.3. Troubleshooting SR-IOV

This section contains solutions for problems which may affect SR-IOV. If you need additional help, refer to Appendix A, Troubleshooting as well as Section 18.1.8, “Setting PCI device assignment from a pool of SR-IOV virtual functions”.

⁠Error starting the guest

When starting a configured virtual machine, an error occurs as follows:

# virsh start test
error: Failed to start domain test
error: Requested operation is not valid: PCI device 0000:03:10.1 is in use by domain rhel7

This error is often caused by a device that is already assigned to another guest or to the host itself.

⁠Error migrating, saving, or dumping the guest

Attempts to migrate and dump the virtual machine cause an error similar to the following:

# virsh dump rhel7/tmp/rhel7.dump

error: Failed to core dump domain rhel7 to /tmp/rhel7.dump
error: internal error: unable to execute QEMU command 'migrate': State blocked by non-migratable device '0000:00:03.0/vfio-pci'

Because device assignment uses hardware on the specific host where the virtual machine was started, guest migration and save are not supported when device assignment is in use. Currently, the same limitation also applies to core-dumping a guest; this may change in the future. It is important to note that QEMU does not currently support migrate, save, and dump operations on guest virtual machines with PCI devices attached. Currently it only can support these actions with USB devices. Work is currently being done to improve this in the future.

⁠Chapter 20. Virtual Networking

This chapter introduces the concepts needed to create, start, stop, remove, and modify virtual networks with libvirt.

Additional information can be found in the libvirt reference chapter

⁠20.1. Virtual network switches

Libvirt virtual networking uses the concept of a virtual network switch. A virtual network switch is a software construct that operates on a host physical machine server, to which virtual machines (guests) connect. The network traffic for a guest is directed through this switch:

⁠

Figure 20.1. Virtual network switch with two guests

Linux host physical machine servers represent a virtual network switch as a network interface. When the libvirtd daemon (libvirtd) is first installed and started, the default network interface representing the virtual network switch is virbr0.

⁠

Figure 20.2. Linux host physical machine with an interface to a virtual network switch

This virbr0 interface can be viewed with the ifconfig and ip commands like any other interface:

$ ifconfig virbr0
 virbr0    Link encap:Ethernet  HWaddr 1B:C4:94:CF:FD:17
           inet addr:192.168.122.1  Bcast:192.168.122.255  Mask:255.255.255.0
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           RX packets:0 errors:0 dropped:0 overruns:0 frame:0
           TX packets:11 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:0
           RX bytes:0 (0.0 b)  TX bytes:3097 (3.0 KiB)

 $ ip addr show virbr0
 3: virbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN
     link/ether 1b:c4:94:cf:fd:17 brd ff:ff:ff:ff:ff:ff
     inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0

⁠20.2. Bridge Mode

When using Bridge mode, all of the guest virtual machines appear within the same subnet as the host physical machine. All other physical machines on the same physical network are aware of the virtual machines, and can access the virtual machines. Bridging operates on Layer 2 of the OSI networking model.

It is possible to use multiple physical interfaces on the hypervisor by joining them together with a bond. The bond is then added to a bridge and then guest virtual machines are added onto the bridge as well. However, the bonding driver has several modes of operation, and only a few of these modes work with a bridge where virtual guest machines are in use.

Warning

The only bonding modes that should be used with a guest virtual machine are Mode 1, Mode 2, and Mode 4. Under no circumstances should Modes 0, 3, 5, or 6 be used. It should also be noted that mii-monitoring should be used to monitor bonding modes as arp-monitoring does not work.

For more information on bonding modes, refer to the knowledge base article on bonding modes, or The Red Hat Enterprise Linux 7 Deployment Guide.

⁠20.3. Network Address Translation

By default, virtual network switches operate in NAT mode. They use IP masquerading rather than SNAT (Source-NAT) or DNAT (Destination-NAT). IP masquerading enables connected guests to use the host physical machine IP address for communication to any external network. By default, computers that are placed externally to the host physical machine cannot communicate to the guests inside when the virtual network switch is operating in NAT mode, as shown in the following diagram:

⁠

Figure 20.3. Virtual network switch using NAT with two guests

Warning

Virtual network switches use NAT configured by iptables rules. Editing these rules while the switch is running is not recommended, as incorrect rules may result in the switch being unable to communicate.

If the switch is not running, you can set th public IP range for foward mode NAT in order to create a port masquerading range by running:

# iptables -j SNAT --to-source [start]-[end]

⁠20.4. DNS and DHCP

IP information can be assigned to guests via DHCP. A pool of addresses can be assigned to a virtual network switch for this purpose. Libvirt uses the dnsmasq program for this. An instance of dnsmasq is automatically configured and started by libvirt for each virtual network switch that needs it.

⁠

Figure 20.4. Virtual network switch running dnsmasq

⁠20.5. Routed mode

When using Routed mode, the virtual switch connects to the physical LAN connected to the host physical machine, passing traffic back and forth without the use of NAT. The virtual switch can examine all traffic and use the information contained within the network packets to make routing decisions. When using this mode, all of the virtual machines are in their own subnet, routed through a virtual switch. This situation is not always ideal as no other host physical machines on the physical network are aware of the virtual machines without manual physical router configuration, and cannot access the virtual machines. Routed mode operates at Layer 3 of the OSI networking model.

⁠

Figure 20.5. Virtual network switch in routed mode

⁠20.6. Isolated mode

When using Isolated mode, guests connected to the virtual switch can communicate with each other, and with the host physical machine, but their traffic will not pass outside of the host physical machine, nor can they receive traffic from outside the host physical machine. Using dnsmasq in this mode is required for basic functionality such as DHCP. However, even if this network is isolated from any physical network, DNS names are still resolved. Therefore a situation can arise when DNS names resolve but ICMP echo request (ping) commands fail.

⁠

Figure 20.6. Virtual network switch in isolated mode

⁠20.7. The default configuration

When the libvirtd daemon (libvirtd) is first installed, it contains an initial virtual network switch configuration in NAT mode. This configuration is used so that installed guests can communicate to the external network, through the host physical machine. The following image demonstrates this default configuration for libvirtd:

⁠

Figure 20.7. Default libvirt network configuration

Note

A virtual network can be restricted to a specific physical interface. This may be useful on a physical system that has several interfaces (for example, eth0, eth1 and eth2). This is only useful in routed and NAT modes, and can be defined in the dev=<interface> option, or in virt-manager when creating a new virtual network.

⁠20.8. Examples of common scenarios

This section demonstrates different virtual networking modes and provides some example scenarios.

⁠20.8.1. Bridged mode

Bridged mode operates on Layer 2 of the OSI model. When used, all of the guest virtual machines will appear on the same subnet as the host physical machine. The most common use cases for bridged mode include:

Deploying guest virtual machines in an existing network alongside host physical machines making the difference between virtual and physical machines transparent to the end user.

Deploying guest virtual machines without making any changes to existing physical network configuration settings.

Deploying guest virtual machines which must be easily accessible to an existing physical network. Placing guest virtual machines on a physical network where they must access services within an existing broadcast domain, such as DHCP.

Connecting guest virtual machines to an exsting network where VLANs are used.

⁠20.8.2. Routed mode

⁠

DMZ

Consider a network where one or more nodes are placed in a controlled sub-network for security reasons. The deployment of a special sub-network such as this is a common practice, and the sub-network is known as a DMZ. Refer to the following diagram for more details on this layout:

⁠

Figure 20.8. Sample DMZ configuration

Host physical machines in a DMZ typically provide services to WAN (external) host physical machines as well as LAN (internal) host physical machines. As this requires them to be accessible from multiple locations, and considering that these locations are controlled and operated in different ways based on their security and trust level, routed mode is the best configuration for this environment.

⁠

Virtual Server hosting

Consider a virtual server hosting company that has several host physical machines, each with two physical network connections. One interface is used for management and accounting, the other is for the virtual machines to connect through. Each guest has its own public IP address, but the host physical machines use private IP address as management of the guests can only be performed by internal administrators. Refer to the following diagram to understand this scenario:

⁠

Figure 20.9. Virtual server hosting sample configuration

When the host physical machine has a public IP address and the virtual machines have static public IP addresses, bridged networking cannot be used, as the provider only accepts packets from the MAC address of the public host physical machine. The following diagram demonstrates this:

⁠

Figure 20.10. Virtual server using static IP addresses

⁠20.8.3. NAT mode

NAT (Network Address Translation) mode is the default mode. It can be used for testing when there is no need for direct network visibility.

⁠20.8.4. Isolated mode

Isolated mode allows virtual machines to communicate with each other only. They are unable to interact with the physical network.

⁠20.9. Managing a virtual network

To configure a virtual network on your system:

From the Edit menu, select Connection Details.

⁠

Figure 20.11. Selecting a host physical machine's details

This will open the Connection Details menu. Click the Virtual Networks tab.

⁠

Figure 20.12. Virtual network configuration

All available virtual networks are listed on the left-hand box of the menu. You can edit the configuration of a virtual network by selecting it from this box and editing as you see fit.

⁠20.10. Creating a virtual network

To create a virtual network on your system using the Virtual Manager (virt-manager):

⁠

Open the Virtual Networks tab from within the Connection Details menu. Click the Add Network button, identified by a plus sign (+) icon. For more information, refer to Section 20.9, “Managing a virtual network”.

⁠

Figure 20.13. Virtual network configuration

This will open the Create a new virtual network window. Click Forward to continue.

⁠

Figure 20.14. Creating a new virtual network

Enter an appropriate name for your virtual network and click Forward.

⁠

Figure 20.15. Naming your virtual network

Enter an IPv4 address space for your virtual network and click Forward.

⁠

Figure 20.16. Choosing an IPv4 address space

Define the DHCP range for your virtual network by specifying a Start and End range of IP addresses. Click Forward to continue.

⁠

Figure 20.17. Selecting the DHCP range

Select how the virtual network should connect to the physical network.

⁠

Figure 20.18. Connecting to physical network

If you select Forwarding to physical network, choose whether the Destination should be Any physical device or a specific physical device. Also select whether the Mode should be NAT or Routed.

Click Forward to continue.

You are now ready to create the network. Check the configuration of your network and click Finish.

⁠

Figure 20.19. Ready to create network

The new virtual network is now available in the Virtual Networks tab of the Connection Details window.

⁠20.11. Attaching a virtual network to a guest

To attach a virtual network to a guest:

In the Virtual Machine Manager window, highlight the guest that will have the network assigned.

⁠

Figure 20.20. Selecting a virtual machine to display

From the Virtual Machine Manager Edit menu, select Virtual Machine Details.

⁠

Figure 20.21. Displaying the virtual machine details

Click the Add Hardware button on the Virtual Machine Details window.

⁠

Figure 20.22. The Virtual Machine Details window

In the Add new virtual hardware window, select Network from the left pane, and select your network name (network1 in this example) from the Host device menu and click Finish.

⁠

Figure 20.23. Select your network from the Add new virtual hardware window

The new network is now displayed as a virtual network interface that will be presented to the guest upon launch.

⁠

Figure 20.24. New network shown in guest hardware list

⁠20.12. Directly attaching to physical interface

The instructions provided in this chapter will assist in the direct attachment of the virtual machine's NIC to the given physical interface of the host physical machine. If you want to use an assignment method (known as passthrough), refer to Chapter 18, Guest virtual machine device configuration and Section 23.18, “Interface Commands”. This setup requires the Linux macvtap driver to be available. There are four modes that you can choose for the operation mode of the macvtap device, with 'vepa' being the default mode. Their behavior is as follows:

⁠

Physical interface delivery modes

⁠vepa

All VMs' packets are sent to the external bridge. Packets whose destination is a VM on the same host physical machine as where the packet originates from are sent back to the host physical machine by the VEPA capable bridge (today's bridges are typically not VEPA capable).

⁠bridge

Packets whose destination is on the same host physical machine as where they originate from are directly delivered to the target macvtap device. Both origin and destination devices need to be in bridge mode for direct delivery. If either one of them is in vepa mode, a VEPA capable bridge is required.

⁠private

All packets are sent to the external bridge and will only be delivered to a target VM on the same host physical machine if they are sent through an external router or gateway and that device sends them back to the host physical machine. This procedure is followed if either the source or destination device is in private mode.

⁠passthrough

This feature attaches a virtual function of a SRIOV capable NIC directly to a VM without losing the migration capability. All packets are sent to the VF/IF of the configured network device. Depending on the capabilities of the device additional prerequisites or limitations may apply; for example, on Linux this requires kernel 2.6.38 or newer.

Each of the four modes is configured by changing the domain xml file. Once this file is opened, change the mode setting as shown:

  <devices>
    ...
    <interface type='direct'>
      <source dev='eth0' mode='vepa'/>
    </interface>
  </devices>

The network access of direct attached guest virtual machines can be managed by the hardware switch to which the physical interface of the host physical machine is connected to.

The interface can have additional parameters as shown below, if the switch is conforming to the IEEE 802.1Qbg standard. The parameters of the virtualport element are documented in more detail in the IEEE 802.1Qbg standard. The values are network specific and should be provided by the network administrator. In 802.1Qbg terms, the Virtual Station Interface (VSI) represents the virtual interface of a virtual machine.

Note that IEEE 802.1Qbg requires a non-zero value for the VLAN ID. Also if the switch is conforming to the IEEE 802.1Qbh standard, the values are network specific and should be provided by the network administrator.

⁠

Virtual Station Interface types

⁠managerid

The VSI Manager ID identifies the database containing the VSI type and instance definitions. This is an integer value and the value 0 is reserved.

⁠typeid

The VSI Type ID identifies a VSI type characterizing the network access. VSI types are typically managed by network administrator. This is an integer value.

⁠typeidversion

The VSI Type Version allows multiple versions of a VSI Type. This is an integer value.

⁠instanceid

The VSI Instance ID Identifier is generated when a VSI instance (i.e. a virtual interface of a virtual machine) is created. This is a globally unique identifier.

⁠profileid

The profile ID contains the name of the port profile that is to be applied onto this interface. This name is resolved by the port profile database into the network parameters from the port profile, and those network parameters will be applied to this interface.

Each of the four types is configured by changing the domain xml file. Once this file is opened, change the mode setting as shown:

  <devices>
    ...
    <interface type='direct'>
      <source dev='eth0.2' mode='vepa'/>
      <virtualport type="802.1Qbg">
        <parameters managerid="11" typeid="1193047" typeidversion="2" instanceid="09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f"/>
      </virtualport>
    </interface>
  </devices>

The profile ID is shown here:

  <devices>
    ...
    <interface type='direct'>
      <source dev='eth0' mode='private'/>
      <virtualport type='802.1Qbh'>
        <parameters profileid='finance'/>
      </virtualport>
    </interface>
  </devices>
  ...

⁠20.13. Dynamically changing a host physical machine or a network bridge that is attached to a virtual NIC

This section demonstrates how to move the vNIC of a guest virtual machine from one bridge to another while the guest virtual machine is running without compromising the guest virtual machine

Prepare guest virtual machine with a configuration similar to the following:

<interface type='bridge'>
      <mac address='52:54:00:4a:c9:5e'/>
      <source bridge='virbr0'/>
      <model type='virtio'/>
</interface>

Prepare an XML file for interface update:

# cat br1.xml

<interface type='bridge'>
      <mac address='52:54:00:4a:c9:5e'/>
      <source bridge='virbr1'/>
      <model type='virtio'/>
</interface>

Start the guest virtual machine, confirm the guest virtual machine's network functionality, and check that the guest virtual machine's vnetX is connected to the bridge you indicated.

# brctl show
bridge name     bridge id               STP enabled     interfaces
virbr0          8000.5254007da9f2       yes                  virbr0-nic

vnet0
virbr1          8000.525400682996       yes                  virbr1-nic

Update the guest virtual machine's network with the new interface parameters with the following command:

# virsh update-device test1 br1.xml

Device updated successfully

On the guest virtual machine, run service network restart. The guest virtual machine gets a new IP address for virbr1. Check the guest virtual machine's vnet0 is connected to the new bridge(virbr1)

# brctl show
bridge name     bridge id               STP enabled     interfaces
virbr0          8000.5254007da9f2       yes             virbr0-nic
virbr1          8000.525400682996       yes             virbr1-nic     vnet0

⁠20.14. Applying network filtering

This section provides an introduction to libvirt's network filters, their goals, concepts and XML format.

⁠20.14.1. Introduction

The goal of the network filtering, is to enable administrators of a virtualized system to configure and enforce network traffic filtering rules on virtual machines and manage the parameters of network traffic that virtual machines are allowed to send or receive. The network traffic filtering rules are applied on the host physical machine when a virtual machine is started. Since the filtering rules cannot be circumvented from within the virtual machine, it makes them mandatory from the point of view of a virtual machine user.

From the point of view of the guest virtual machine, the network filtering system allows each virtual machine's network traffic filtering rules to be configured individually on a per interface basis. These rules are applied on the host physical machine when the virtual machine is started and can be modified while the virtual machine is running. The latter can be achieved by modifying the XML description of a network filter.

Multiple virtual machines can make use of the same generic network filter. When such a filter is modified, the network traffic filtering rules of all running virtual machines that reference this filter are updated. The machines that are not running will update on start.

As previously mentioned, applying network traffic filtering rules can be done on individual network interfaces that are configured for certain types of network configurations. Supported network types include:

network

ethernet -- must be used in bridging mode

bridge

⁠

Example 20.1. An example of network filtering

The interface XML is used to reference a top-level filter. In the following example, the interface description references the filter clean-traffic.

   <devices>
    <interface type='bridge'>
      <mac address='00:16:3e:5d:c7:9e'/>
      <filterref filter='clean-traffic'/>
    </interface>
  </devices>

Network filters are written in XML and may either contain: references to other filters, rules for traffic filtering, or hold a combination of both. The above referenced filter clean-traffic is a filter that only contains references to other filters and no actual filtering rules. Since references to other filters can be used, a tree of filters can be built. The clean-traffic filter can be viewed using the command: # virsh nwfilter-dumpxml clean-traffic.

As previously mentioned, a single network filter can be referenced by multiple virtual machines. Since interfaces will typically have individual parameters associated with their respective traffic filtering rules, the rules described in a filter's XML can be generalized using variables. In this case, the variable name is used in the filter XML and the name and value are provided at the place where the filter is referenced.

⁠

Example 20.2. Description extended

In the following example, the interface description has been extended with the parameter IP and a dotted IP address as a value.

  <devices>
    <interface type='bridge'>
      <mac address='00:16:3e:5d:c7:9e'/>
      <filterref filter='clean-traffic'>
        <parameter name='IP' value='10.0.0.1'/>
      </filterref>
    </interface>
  </devices>

In this particular example, the clean-traffic network traffic filter will be represented with the IP address parameter 10.0.0.1 and as per the rule dictates that all traffic from this interface will always be using 10.0.0.1 as the source IP address, which is one of the purpose of this particular filter.

⁠20.14.2. Filtering chains

Filtering rules are organized in filter chains. These chains can be thought of as having a tree structure with packet filtering rules as entries in individual chains (branches).

Packets start their filter evaluation in the root chain and can then continue their evaluation in other chains, return from those chains back into the root chain or be dropped or accepted by a filtering rule in one of the traversed chains.

Libvirt's network filtering system automatically creates individual root chains for every virtual machine's network interface on which the user chooses to activate traffic filtering. The user may write filtering rules that are either directly instantiated in the root chain or may create protocol-specific filtering chains for efficient evaluation of protocol-specific rules.

The following chains exist:

root

mac

stp (spanning tree protocol)

vlan

arp and rarp

ipv4

ipv6

Multiple chains evaluating the mac, stp, vlan, arp, rarp, ipv4, or ipv6 protocol can be created using the protocol name only as a prefix in the chain's name.

⁠

Example 20.3. ARP traffic filtering

This example allows chains with names arp-xyz or arp-test to be specified and have their ARP protocol packets evaluated in those chains.

The following filter XML shows an example of filtering ARP traffic in the arp chain.

<filter name='no-arp-spoofing' chain='arp' priority='-500'>
  <uuid>f88f1932-debf-4aa1-9fbe-f10d3aa4bc95</uuid>
  <rule action='drop' direction='out' priority='300'>
    <mac match='no' srcmacaddr='$MAC'/>
  </rule>
  <rule action='drop' direction='out' priority='350'>
    <arp match='no' arpsrcmacaddr='$MAC'/>
  </rule>
  <rule action='drop' direction='out' priority='400'>
    <arp match='no' arpsrcipaddr='$IP'/>
  </rule>
  <rule action='drop' direction='in' priority='450'>
    <arp opcode='Reply'/>
    <arp match='no' arpdstmacaddr='$MAC'/>
  </rule>
  <rule action='drop' direction='in' priority='500'>
    <arp match='no' arpdstipaddr='$IP'/>
  </rule>
  <rule action='accept' direction='inout' priority='600'>
    <arp opcode='Request'/>
  </rule>
  <rule action='accept' direction='inout' priority='650'>
    <arp opcode='Reply'/>
  </rule>
  <rule action='drop' direction='inout' priority='1000'/>
</filter>

The consequence of putting ARP-specific rules in the arp chain, rather than for example in the root chain, is that packets protocols other than ARP do not need to be evaluated by ARP protocol-specific rules. This improves the efficiency of the traffic filtering. However, one must then pay attention to only putting filtering rules for the given protocol into the chain since other rules will not be evaluated. For example, an IPv4 rule will not be evaluated in the ARP chain since IPv4 protocol packets will not traverse the ARP chain.

⁠20.14.3. Filtering chain priorities

As previously mentioned, when creating a filtering rule, all chains are connected to the root chain. The order in which those chains are accessed is influenced by the priority of the chain. The following table shows the chains that can be assigned a priority and their default priorities.

⁠

Table 20.1. Filtering chain default priorities values

Chain (prefix) | Default priority
stp | -810
mac | -800
vlan | -750
ipv4 | -700
ipv6 | -600
arp | -500
rarp | -400

Note

A chain with a lower priority value is accessed before one with a higher value.

The chains listed in Table 20.1, “Filtering chain default priorities values” can be also be assigned custom priorities by writing a value in the range [-1000 to 1000] into the priority (XML) attribute in the filter node. Section 20.14.2, “Filtering chains”filter shows the default priority of -500 for arp chains, for example.

⁠20.14.4. Usage of variables in filters

There are two variables that have been reserved for usage by the network traffic filtering subsystem: MAC and IP.

MAC is designated for the MAC address of the network interface. A filtering rule that references this variable will automatically be replaced with the MAC address of the interface. This works without the user having to explicitly provide the MAC parameter. Even though it is possible to specify the MAC parameter similar to the IP parameter above, it is discouraged since libvirt knows what MAC address an interface will be using.

The parameter IP represents the IP address that the operating system inside the virtual machine is expected to use on the given interface. The IP parameter is special in so far as the libvirt daemon will try to determine the IP address (and thus the IP parameter's value) that is being used on an interface if the parameter is not explicitly provided but referenced. For current limitations on IP address detection, consult the section on limitations Section 20.14.12, “Limitations” on how to use this feature and what to expect when using it. The XML file shown in Section 20.14.2, “Filtering chains” contains the filter no-arp-spoofing, which is an example of using a network filter XML to reference the MAC and IP variables.

Note that referenced variables are always prefixed with the character $. The format of the value of a variable must be of the type expected by the filter attribute identified in the XML. In the above example, the IP parameter must hold a legal IP address in standard format. Failure to provide the correct structure will result in the filter variable not being replaced with a value and will prevent a virtual machine from starting or will prevent an interface from attaching when hotplugging is being used. Some of the types that are expected for each XML attribute are shown in the example Example 20.4, “Sample variable types”.

⁠

Example 20.4. Sample variable types

As variables can contain lists of elements, (the variable IP can contain multiple IP addresses that are valid on a particular interface, for example), the notation for providing multiple elements for the IP variable is:

  <devices>
    <interface type='bridge'>
      <mac address='00:16:3e:5d:c7:9e'/>
      <filterref filter='clean-traffic'>
        <parameter name='IP' value='10.0.0.1'/>
        <parameter name='IP' value='10.0.0.2'/>
        <parameter name='IP' value='10.0.0.3'/>
      </filterref>
    </interface>
  </devices>

This XML file creates filters to enable multiple IP addresses per interface. Each of the IP addresses will result in a separate filtering rule. Therefore using the XML above and the the following rule, three individual filtering rules (one for each IP address) will be created:

  <rule action='accept' direction='in' priority='500'>
    <tcp srpipaddr='$IP'/>
  </rule>

As it is possible to access individual elements of a variable holding a list of elements, a filtering rule like the following accesses the 2nd element of the variable DSTPORTS.

  <rule action='accept' direction='in' priority='500'>
    <udp dstportstart='$DSTPORTS[1]'/>
  </rule>

⁠

Example 20.5. Using a variety of variables

As it is possible to create filtering rules that represent all of the permissible rules from different lists using the notation $VARIABLE[@<iterator id="x">]. The following rule allows a virtual machine to receive traffic on a set of ports, which are specified in DSTPORTS, from the set of source IP address specified in SRCIPADDRESSES. The rule generates all combinations of elements of the variable DSTPORTS with those of SRCIPADDRESSES by using two independent iterators to access their elements.

  <rule action='accept' direction='in' priority='500'>
    <ip srcipaddr='$SRCIPADDRESSES[@1]' dstportstart='$DSTPORTS[@2]'/>
  </rule>

Assign concrete values to SRCIPADDRESSES and DSTPORTS as shown:

  SRCIPADDRESSES = [ 10.0.0.1, 11.1.2.3 ]
  DSTPORTS = [ 80, 8080 ]

Assigning values to the variables using $SRCIPADDRESSES[@1] and $DSTPORTS[@2] would then result in all variants of addresses and ports being created as shown:

10.0.0.1, 80

10.0.0.1, 8080

11.1.2.3, 80

11.1.2.3, 8080

Accessing the same variables using a single iterator, for example by using the notation $SRCIPADDRESSES[@1] and $DSTPORTS[@1], would result in parallel access to both lists and result in the following combination:

10.0.0.1, 80

11.1.2.3, 8080

Note

$VARIABLE is short-hand for $VARIABLE[@0]. The former notation always assumes the role of iterator with iterator id="0" added as shown in the opening paragraph at the top of this section.

⁠20.14.5. Automatic IP address detection and DHCP snooping

⁠20.14.5.1. Introduction

The detection of IP addresses used on a virtual machine's interface is automatically activated if the variable IP is referenced but no value has been assigned to it. The variable CTRL_IP_LEARNING can be used to specify the IP address learning method to use. Valid values include: any, dhcp, or none.

The value any instructs libvirt to use any packet to determine the address in use by a virtual machine, which is the default setting if the variable TRL_IP_LEARNING is not set. This method will only detect a single IP address per interface. Once a guest virtual machine's IP address has been detected, its IP network traffic will be locked to that address, if for example, IP address spoofing is prevented by one of its filters. In that case, the user of the VM will not be able to change the IP address on the interface inside the guest virtual machine, which would be considered IP address spoofing. When a guest virtual machine is migrated to another host physical machine or resumed after a suspend operation, the first packet sent by the guest virtual machine will again determine the IP address that the guest virtual machine can use on a particular interface.

The value of dhcp instructs libvirt to only honor DHCP server-assigned addresses with valid leases. This method supports the detection and usage of multiple IP address per interface. When a guest virtual machine resumes after a suspend operation, any valid IP address leases are applied to its filters. Otherwise the guest virtual machine is expected to use DHCP to obtain a new IP addresses. When a guest virtual machine migrates to another physical host physical machine, the guest virtual machine is required to re-run the DHCP protocol.

If CTRL_IP_LEARNING is set to none, libvirt does not do IP address learning and referencing IP without assigning it an explicit value is an error.

⁠20.14.5.2. DHCP snooping

CTRL_IP_LEARNING=dhcp (DHCP snooping) provides additional anti-spoofing security, especially when combined with a filter allowing only trusted DHCP servers to assign IP addresses. To enable this, set the variable DHCPSERVER to the IP address of a valid DHCP server and provide filters that use this variable to filter incoming DHCP responses.

When DHCP snooping is enabled and the DHCP lease expires, the guest virtual machine will no longer be able to use the IP address until it acquires a new, valid lease from a DHCP server. If the guest virtual machine is migrated, it must get a new valid DHCP lease to use an IP address (e.g., by bringing the VM interface down and up again).

Note

Automatic DHCP detection listens to the DHCP traffic the guest virtual machine exchanges with the DHCP server of the infrastructure. To avoid denial-of-service attacks on libvirt, the evaluation of those packets is rate-limited, meaning that a guest virtual machine sending an excessive number of DHCP packets per second on an interface will not have all of those packets evaluated and thus filters may not get adapted. Normal DHCP client behavior is assumed to send a low number of DHCP packets per second. Further, it is important to setup appropriate filters on all guest virtual machines in the infrastructure to avoid them being able to send DHCP packets. Therefore guest virtual machines must either be prevented from sending UDP and TCP traffic from port 67 to port 68 or the DHCPSERVER variable should be used on all guest virtual machines to restrict DHCP server messages to only be allowed to originate from trusted DHCP servers. At the same time anti-spoofing prevention must be enabled on all guest virtual machines in the subnet.

⁠

Example 20.6. Activating IPs for DHCP snooping

The following XML provides an example for the activation of IP address learning using the DHCP snooping method:

    <interface type='bridge'>
      <source bridge='virbr0'/>
      <filterref filter='clean-traffic'>
        <parameter name='CTRL_IP_LEARNING' value='dhcp'/>
      </filterref>
    </interface>

⁠20.14.6. Reserved Variables

Table 20.2, “Reserved variables” shows the variables that are considered reserved and are used by libvirt:

⁠

Table 20.2. Reserved variables

Variable Name | Definition
MAC | The MAC address of the interface
IP | The list of IP addresses in use by an interface
IPV6 | Not currently implemented: the list of IPV6 addresses in use by an interface
DHCPSERVER | The list of IP addresses of trusted DHCP servers
DHCPSERVERV6 | Not currently implemented: The list of IPv6 addresses of trusted DHCP servers
CTRL_IP_LEARNING | The choice of the IP address detection mode

⁠20.14.7. Element and attribute overview

The root element required for all network filters is named <filter> with two possible attributes. The name attribute provides a unique name of the given filter. The chain attribute is optional but allows certain filters to be better organized for more efficient processing by the firewall subsystem of the underlying host physical machine. Currently the system only supports the following chains: root, ipv4, ipv6, arp and rarp.

⁠20.14.8. References to other filters

Any filter may hold references to other filters. Individual filters may be referenced multiple times in a filter tree but references between filters must not introduce loops.

⁠

Example 20.7. An Example of a clean traffic filter

The following shows the XML of the clean-traffic network filter referencing several other filters.

<filter name='clean-traffic'>
  <uuid>6ef53069-ba34-94a0-d33d-17751b9b8cb1</uuid>
  <filterref filter='no-mac-spoofing'/>
  <filterref filter='no-ip-spoofing'/>
  <filterref filter='allow-incoming-ipv4'/>
  <filterref filter='no-arp-spoofing'/>
  <filterref filter='no-other-l2-traffic'/>
  <filterref filter='qemu-announce-self'/>
</filter>

To reference another filter, the XML node <filterref> needs to be provided inside a filter node. This node must have the attribute filter whose value contains the name of the filter to be referenced.

New network filters can be defined at any time and may contain references to network filters that are not known to libvirt, yet. However, once a virtual machine is started or a network interface referencing a filter is to be hotplugged, all network filters in the filter tree must be available. Otherwise the virtual machine will not start or the network interface cannot be attached.

⁠20.14.9. Filter rules

The following XML shows a simple example of a network traffic filter implementing a rule to drop traffic if the IP address (provided through the value of the variable IP) in an outgoing IP packet is not the expected one, thus preventing IP address spoofing by the VM.

⁠

Example 20.8. Example of network traffic filtering

<filter name='no-ip-spoofing' chain='ipv4'>
  <uuid>fce8ae33-e69e-83bf-262e-30786c1f8072</uuid>
  <rule action='drop' direction='out' priority='500'>
    <ip match='no' srcipaddr='$IP'/>
  </rule>
</filter>

The traffic filtering rule starts with the rule node. This node may contain up to three of the following attributes:

action is mandatory can have the following values:

drop (matching the rule silently discards the packet with no further analysis)

reject (matching the rule generates an ICMP reject message with no further analysis)

accept (matching the rule accepts the packet with no further analysis)

return (matching the rule passes this filter, but returns control to the calling filter for further analysis)

continue (matching the rule goes on to the next rule for further analysis)

direction is mandatory can have the following values:

in for incomming traffic

out for outgoing traffic

inout for incoming and outgoing traffic

priority is optional. The priority of the rule controls the order in which the rule will be instantiated relative to other rules. Rules with lower values will be instantiated before rules with higher values. Valid values are in the range of -1000 to 1000. If this attribute is not provided, priority 500 will be assigned by default. Note that filtering rules in the root chain are sorted with filters connected to the root chain following their priorities. This allows to interleave filtering rules with access to filter chains. Refer to Section 20.14.3, “Filtering chain priorities” for more information.

statematch is optional. Possible values are '0' or 'false' to turn the underlying connection state matching off. The default setting is 'true' or 1

For more information see Section 20.14.11, “Advanced Filter Configuration Topics”.

The above example Example 20.7, “An Example of a clean traffic filter” indicates that the traffic of type ip will be associated with the chain ipv4 and the rule will have priority=500. If for example another filter is referenced whose traffic of type ip is also associated with the chain ipv4 then that filter's rules will be ordered relative to the priority=500 of the shown rule.

A rule may contain a single rule for filtering of traffic. The above example shows that traffic of type ip is to be filtered.

⁠20.14.10. Supported protocols

The following sections list and give some details about the protocols that are supported by the network filtering subsystem. This type of traffic rule is provided in the rule node as a nested node. Depending on the traffic type a rule is filtering, the attributes are different. The above example showed the single attribute srcipaddr that is valid inside the ip traffic filtering node. The following sections show what attributes are valid and what type of data they are expecting. The following datatypes are available:

UINT8 : 8 bit integer; range 0-255

UINT16: 16 bit integer; range 0-65535

MAC_ADDR: MAC address in dotted decimal format, i.e., 00:11:22:33:44:55

MAC_MASK: MAC address mask in MAC address format, i.e., FF:FF:FF:FC:00:00

IP_ADDR: IP address in dotted decimal format, i.e., 10.1.2.3

IP_MASK: IP address mask in either dotted decimal format (255.255.248.0) or CIDR mask (0-32)

IPV6_ADDR: IPv6 address in numbers format, i.e., FFFF::1

IPV6_MASK: IPv6 mask in numbers format (FFFF:FFFF:FC00::) or CIDR mask (0-128)

STRING: A string

BOOLEAN: 'true', 'yes', '1' or 'false', 'no', '0'

IPSETFLAGS: The source and destination flags of the ipset described by up to 6 'src' or 'dst' elements selecting features from either the source or destination part of the packet header; example: src,src,dst. The number of 'selectors' to provide here depends on the type of ipset that is referenced

Every attribute except for those of type IP_MASK or IPV6_MASK can be negated using the match attribute with value no. Multiple negated attributes may be grouped together. The following XML fragment shows such an example using abstract attributes.

[...]
  <rule action='drop' direction='in'>
    <protocol match='no' attribute1='value1' attribute2='value2'/>
    <protocol attribute3='value3'/>
  </rule>
[...]

Rules behave evaluate the rule as well as look at it logically within the boundaries of the given protocol attributes. Thus, if a single attribute's value does not match the one given in the rule, the whole rule will be skipped during the evaluation process. Therefore, in the above example incoming traffic will only be dropped if: the protocol property attribute1 does not match both value1 and the protocol property attribute2 does not match value2 and the protocol property attribute3 matches value3.

⁠20.14.10.1. MAC (Ethernet)

Protocol ID: mac

Rules of this type should go into the root chain.

⁠

Table 20.3. MAC protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcmacmask | MAC_MASK | Mask applied to MAC address of sender
dstmacaddr | MAC_ADDR | MAC address of destination
dstmacmask | MAC_MASK | Mask applied to MAC address of destination
protocolid | UINT16 (0x600-0xffff), STRING | Layer 3 protocol ID. Valid strings include [arp, rarp, ipv4, ipv6]
comment | STRING | text string up to 256 characters

The filter can be written as such:

[...]
<mac match='no' srcmacaddr='$MAC'/>
[...]

⁠20.14.10.2. VLAN (802.1Q)

Protocol ID: vlan

Rules of this type should go either into the root or vlan chain.

⁠

Table 20.4. VLAN protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcmacmask | MAC_MASK | Mask applied to MAC address of sender
dstmacaddr | MAC_ADDR | MAC address of destination
dstmacmask | MAC_MASK | Mask applied to MAC address of destination
vlan-id | UINT16 (0x0-0xfff, 0 - 4095) | VLAN ID
encap-protocol | UINT16 (0x03c-0xfff), String | Encapsulated layer 3 protocol ID, valid strings are arp, ipv4, ipv6
comment | STRING | text string up to 256 characters

⁠20.14.10.3. STP (Spanning Tree Protocol)

Protocol ID: stp

Rules of this type should go either into the root or stp chain.

⁠

Table 20.5. STP protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcmacmask | MAC_MASK | Mask applied to MAC address of sender
type | UINT8 | Bridge Protocol Data Unit (BPDU) type
flags | UINT8 | BPDU flagdstmacmask
root-priority | UINT16 | Root priority range start
root-priority-hi | UINT16 (0x0-0xfff, 0 - 4095) | Root priority range end
root-address | MAC _ADDRESS | root MAC Address
root-address-mask | MAC _MASK | root MAC Address mask
roor-cost | UINT32 | Root path cost (range start)
root-cost-hi | UINT32 | Root path cost range end
sender-priority-hi | UINT16 | Sender prioriry range end
sender-address | MAC_ADDRESS | BPDU sender MAC address
sender-address-mask | MAC_MASK | BPDU sender MAC address mask
port | UINT16 | Port identifier (range start)
port_hi | UINT16 | Port identifier range end
msg-age | UINT16 | Message age timer (range start)
msg-age-hi | UINT16 | Message age timer range end
max-age-hi | UINT16 | Maximum age time range end
hello-time | UINT16 | Hello time timer (range start)
hello-time-hi | UINT16 | Hello time timer range end
forward-delay | UINT16 | Forward delay (range start)
forward-delay-hi | UINT16 | Forward delay range end
comment | STRING | text string up to 256 characters

⁠20.14.10.4. ARP/RARP

Protocol ID: arp or rarp

Rules of this type should either go into the root or arp/rarp chain.

⁠

Table 20.6. ARP and RARP protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcmacmask | MAC_MASK | Mask applied to MAC address of sender
dstmacaddr | MAC_ADDR | MAC address of destination
dstmacmask | MAC_MASK | Mask applied to MAC address of destination
hwtype | UINT16 | Hardware type
protocoltype | UINT16 | Protocol type
opcode | UINT16, STRING | Opcode valid strings are: Request, Reply, Request_Reverse, Reply_Reverse, DRARP_Request, DRARP_Reply, DRARP_Error, InARP_Request, ARP_NAK
arpsrcmacaddr | MAC_ADDR | Source MAC address in ARP/RARP packet
arpdstmacaddr | MAC _ADDR | Destination MAC address in ARP/RARP packet
arpsrcipaddr | IP_ADDR | Source IP address in ARP/RARP packet
arpdstipaddr | IP_ADDR | Destination IP address in ARP/RARP packet
gratututous | BOOLEAN | Boolean indiating whether to check for a gratuitous ARP packet
comment | STRING | text string up to 256 characters

⁠20.14.10.5. IPv4

Protocol ID: ip

Rules of this type should either go into the root or ipv4 chain.

⁠

Table 20.7. IPv4 protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcmacmask | MAC_MASK | Mask applied to MAC address of sender
dstmacaddr | MAC_ADDR | MAC address of destination
dstmacmask | MAC_MASK | Mask applied to MAC address of destination
srcipaddr | IP_ADDR | Source IP address
srcipmask | IP_MASK | Mask applied to source IP address
dstipaddr | IP_ADDR | Destination IP address
dstipmask | IP_MASK | Mask applied to destination IP address
protocol | UINT8, STRING | Layer 4 protocol identifier. Valid strings for protocol are: tcp, udp, udplite, esp, ah, icmp, igmp, sctp
srcportstart | UINT16 | Start of range of valid source ports; requires protocol
srcportend | UINT16 | End of range of valid source ports; requires protocol
dstportstart | UNIT16 | Start of range of valid destination ports; requires protocol
dstportend | UNIT16 | End of range of valid destination ports; requires protocol
comment | STRING | text string up to 256 characters

⁠20.14.10.6. IPv6

Protocol ID: ipv6

Rules of this type should either go into the root or ipv6 chain.

⁠

Table 20.8. IPv6 protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcmacmask | MAC_MASK | Mask applied to MAC address of sender
dstmacaddr | MAC_ADDR | MAC address of destination
dstmacmask | MAC_MASK | Mask applied to MAC address of destination
srcipaddr | IP_ADDR | Source IP address
srcipmask | IP_MASK | Mask applied to source IP address
dstipaddr | IP_ADDR | Destination IP address
dstipmask | IP_MASK | Mask applied to destination IP address
protocol | UINT8, STRING | Layer 4 protocol identifier. Valid strings for protocol are: tcp, udp, udplite, esp, ah, icmpv6, sctp
scrportstart | UNIT16 | Start of range of valid source ports; requires protocol
srcportend | UINT16 | End of range of valid source ports; requires protocol
dstportstart | UNIT16 | Start of range of valid destination ports; requires protocol
dstportend | UNIT16 | End of range of valid destination ports; requires protocol
comment | STRING | text string up to 256 characters

⁠20.14.10.7. TCP/UDP/SCTP

Protocol ID: tcp, udp, sctp

The chain parameter is ignored for this type of traffic and should either be omitted or set to root. .

⁠

Table 20.9. TCP/UDP/SCTP protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcipaddr | IP_ADDR | Source IP address
srcipmask | IP_MASK | Mask applied to source IP address
dstipaddr | IP_ADDR | Destination IP address
dstipmask | IP_MASK | Mask applied to destination IP address
scripto | IP_ADDR | Start of range of source IP address
srcipfrom | IP_ADDR | End of range of source IP address
dstipfrom | IP_ADDR | Start of range of destination IP address
dstipto | IP_ADDR | End of range of destination IP address
scrportstart | UNIT16 | Start of range of valid source ports; requires protocol
srcportend | UINT16 | End of range of valid source ports; requires protocol
dstportstart | UNIT16 | Start of range of valid destination ports; requires protocol
dstportend | UNIT16 | End of range of valid destination ports; requires protocol
comment | STRING | text string up to 256 characters
state | STRING | comma separated list of NEW,ESTABLISHED,RELATED,INVALID or NONE
flags | STRING | TCP-only: format of mask/flags with mask and flags each being a comma separated list of SYN,ACK,URG,PSH,FIN,RST or NONE or ALL
ipset | STRING | The name of an IPSet managed outside of libvirt
ipsetflags | IPSETFLAGS | flags for the IPSet; requires ipset attribute

⁠20.14.10.8. ICMP

Protocol ID: icmp

Note: The chain parameter is ignored for this type of traffic and should either be omitted or set to root.

⁠

Table 20.10. ICMP protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcmacmask | MAC_MASK | Mask applied to the MAC address of the sender
dstmacaddr | MAD_ADDR | MAC address of the destination
dstmacmask | MAC_MASK | Mask applied to the MAC address of the destination
srcipaddr | IP_ADDR | Source IP address
srcipmask | IP_MASK | Mask applied to source IP address
dstipaddr | IP_ADDR | Destination IP address
dstipmask | IP_MASK | Mask applied to destination IP address
srcipfrom | IP_ADDR | start of range of source IP address
scripto | IP_ADDR | end of range of source IP address
dstipfrom | IP_ADDR | Start of range of destination IP address
dstipto | IP_ADDR | End of range of destination IP address
type | UNIT16 | ICMP type
code | UNIT16 | ICMP code
comment | STRING | text string up to 256 characters
state | STRING | comma separated list of NEW,ESTABLISHED,RELATED,INVALID or NONE
ipset | STRING | The name of an IPSet managed outside of libvirt
ipsetflags | IPSETFLAGS | flags for the IPSet; requires ipset attribute

⁠20.14.10.9. IGMP, ESP, AH, UDPLITE, 'ALL'

Protocol ID: igmp, esp, ah, udplite, all

The chain parameter is ignored for this type of traffic and should either be omitted or set to root.

⁠

Table 20.11. IGMP, ESP, AH, UDPLITE, 'ALL'

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcmacmask | MAC_MASK | Mask applied to the MAC address of the sender
dstmacaddr | MAD_ADDR | MAC address of the destination
dstmacmask | MAC_MASK | Mask applied to the MAC address of the destination
srcipaddr | IP_ADDR | Source IP address
srcipmask | IP_MASK | Mask applied to source IP address
dstipaddr | IP_ADDR | Destination IP address
dstipmask | IP_MASK | Mask applied to destination IP address
srcipfrom | IP_ADDR | start of range of source IP address
scripto | IP_ADDR | end of range of source IP address
dstipfrom | IP_ADDR | Start of range of destination IP address
dstipto | IP_ADDR | End of range of destination IP address
comment | STRING | text string up to 256 characters
state | STRING | comma separated list of NEW,ESTABLISHED,RELATED,INVALID or NONE
ipset | STRING | The name of an IPSet managed outside of libvirt
ipsetflags | IPSETFLAGS | flags for the IPSet; requires ipset attribute

⁠20.14.10.10. TCP/UDP/SCTP over IPV6

Protocol ID: tcp-ipv6, udp-ipv6, sctp-ipv6

The chain parameter is ignored for this type of traffic and should either be omitted or set to root.

⁠

Table 20.12. TCP, UDP, SCTP over IPv6 protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcipaddr | IP_ADDR | Source IP address
srcipmask | IP_MASK | Mask applied to source IP address
dstipaddr | IP_ADDR | Destination IP address
dstipmask | IP_MASK | Mask applied to destination IP address
srcipfrom | IP_ADDR | start of range of source IP address
scripto | IP_ADDR | end of range of source IP address
dstipfrom | IP_ADDR | Start of range of destination IP address
dstipto | IP_ADDR | End of range of destination IP address
srcportstart | UINT16 | Start of range of valid source ports
srcportend | UINT16 | End of range of valid source ports
dstportstart | UINT16 | Start of range of valid destination ports
dstportend | UINT16 | End of range of valid destination ports
comment | STRING | text string up to 256 characters
state | STRING | comma separated list of NEW,ESTABLISHED,RELATED,INVALID or NONE
ipset | STRING | The name of an IPSet managed outside of libvirt
ipsetflags | IPSETFLAGS | flags for the IPSet; requires ipset attribute

⁠20.14.10.11. ICMPv6

Protocol ID: icmpv6

The chain parameter is ignored for this type of traffic and should either be omitted or set to root.

⁠

Table 20.13. ICMPv6 protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcipaddr | IP_ADDR | Source IP address
srcipmask | IP_MASK | Mask applied to source IP address
dstipaddr | IP_ADDR | Destination IP address
dstipmask | IP_MASK | Mask applied to destination IP address
srcipfrom | IP_ADDR | start of range of source IP address
scripto | IP_ADDR | end of range of source IP address
dstipfrom | IP_ADDR | Start of range of destination IP address
dstipto | IP_ADDR | End of range of destination IP address
type | UINT16 | ICMPv6 type
code | UINT16 | ICMPv6 code
comment | STRING | text string up to 256 characters
state | STRING | comma separated list of NEW,ESTABLISHED,RELATED,INVALID or NONE
ipset | STRING | The name of an IPSet managed outside of libvirt
ipsetflags | IPSETFLAGS | flags for the IPSet; requires ipset attribute

⁠20.14.10.12. IGMP, ESP, AH, UDPLITE, 'ALL' over IPv6

Protocol ID: igmp-ipv6, esp-ipv6, ah-ipv6, udplite-ipv6, all-ipv6

The chain parameter is ignored for this type of traffic and should either be omitted or set to root.

⁠

Table 20.14. IGMP, ESP, AH, UDPLITE, 'ALL' over IPv protocol types

Attribute Name | Datatype | Definition
srcmacaddr | MAC_ADDR | MAC address of sender
srcipaddr | IP_ADDR | Source IP address
srcipmask | IP_MASK | Mask applied to source IP address
dstipaddr | IP_ADDR | Destination IP address
dstipmask | IP_MASK | Mask applied to destination IP address
srcipfrom | IP_ADDR | start of range of source IP address
scripto | IP_ADDR | end of range of source IP address
dstipfrom | IP_ADDR | Start of range of destination IP address
dstipto | IP_ADDR | End of range of destination IP address
comment | STRING | text string up to 256 characters
state | STRING | comma separated list of NEW,ESTABLISHED,RELATED,INVALID or NONE
ipset | STRING | The name of an IPSet managed outside of libvirt
ipsetflags | IPSETFLAGS | flags for the IPSet; requires ipset attribute

⁠20.14.11. Advanced Filter Configuration Topics

The following sections discuss advanced filter configuration topics.

⁠20.14.11.1. Connection tracking

The network filtering subsystem (on Linux) makes use of the connection tracking support of IP tables. This helps in enforcing the direction of the network traffic (state match) as well as counting and limiting the number of simultaneous connections towards a guest virtual machine. As an example, if a guest virtual machine has TCP port 8080 open as a server, clients may connect to the guest virtual machine on port 8080. Connection tracking and enforcement of the direction and then prevents the guest virtual machine from initiating a connection from (TCP client) port 8080 to the host physical machine back to a remote host physical machine. More importantly, tracking helps to prevent remote attackers from establishing a connection back to a guest virtual machine. For example, if the user inside the guest virtual machine established a connection to port 80 on an attacker site, then the attacker will not be able to initiate a connection from TCP port 80 back towards the guest virtual machine. By default the connection state match that enables connection tracking and then enforcement of the direction of traffic is turned on.

⁠

Example 20.9. XML example for turning off connections to the TCP port

The following shows an example XML fragment where this feature has been turned off for incoming connections to TCP port 12345.

   [...]
    <rule direction='in' action='accept' statematch='false'>
      <cp dstportstart='12345'/>
    </rule>
   [...]

This now allows incoming traffic to TCP port 12345, but would also enable the initiation from (client) TCP port 12345 within the VM, which may or may not be desirable.

⁠20.14.11.2. Limiting Number of Connections

To limit the number of connections a guest virtual machine may establish, a rule must be provided that sets a limit of connections for a given type of traffic. If for example a VM is supposed to be allowed to only ping one other IP address at a time and is supposed to have only one active incoming ssh connection at a time.

⁠

Example 20.10. XML sample file that sets limits to connections

The following XML fragment can be used to limit connections

  [...]
  <rule action='drop' direction='in' priority='400'>
    <tcp connlimit-above='1'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <tcp dstportstart='22'/>
  </rule>
  <rule action='drop' direction='out' priority='400'>
    <icmp connlimit-above='1'/>
  </rule>
  <rule action='accept' direction='out' priority='500'>
    <icmp/>
  </rule>
  <rule action='accept' direction='out' priority='500'>
    <udp dstportstart='53'/>
  </rule>
  <rule action='drop' direction='inout' priority='1000'>
    <all/>
  </rule>
  [...]

Note

Limitation rules must be listed in the XML prior to the rules for accepting traffic. According to the XML file in Example 20.10, “XML sample file that sets limits to connections”, an additional rule for allowing DNS traffic sent to port 22 go out the guest virtual machine, has been added to avoid ssh sessions not getting established for reasons related to DNS lookup failures by the ssh daemon. Leaving this rule out may result in the ssh client hanging unexpectedly as it tries to connect. Additional caution should be used in regards to handling timeouts related to tracking of traffic. An ICMP ping that the user may have terminated inside the guest virtual machine may have a long timeout in the host physical machine's connection tracking system and will therefore not allow another ICMP ping to go through.

The best solution is to tune the timeout in the host physical machine's sysfs with the following command:# echo 3 > /proc/sys/net/netfilter/nf_conntrack_icmp_timeout. This command sets the ICMP connection tracking timeout to 3 seconds. The effect of this is that once one ping is terminated, another one can start after 3 seconds.

If for any reason the guest virtual machine has not properly closed its TCP connection, the connection to be held open for a longer period of time, especially if the TCP timeout value was set for a large amount of time on the host physical machine. In addition, any idle connection may result in a time out in the connection tracking system which can be re-activated once packets are exchanged.

However, if the limit is set too low, newly initiated connections may force an idle connection into TCP backoff. Therefore, the limit of connections should be set rather high so that fluctuations in new TCP connections don't cause odd traffic behavior in relation to idle connections.

⁠20.14.11.3. Command line tools

virsh has been extended with life-cycle support for network filters. All commands related to the network filtering subsystem start with the prefix nwfilter. The following commands are available:

nwfilter-list : lists UUIDs and names of all network filters

nwfilter-define : defines a new network filter or updates an existing one (must supply a name)

nwfilter-undefine : deletes a specified network filter (must supply a name). Do not delete a network filter currently in use.

nwfilter-dumpxml : displays a specified network filter (must supply a name)

nwfilter-edit : edits a specified network filter (must supply a name)

⁠20.14.11.4. Pre-existing network filters

The following is a list of example network filters that are automatically installed with libvirt:

⁠

Table 20.15. ICMPv6 protocol types

Command Name | Description
no-arp-spoofing | Prevents a guest virtual machine from spoofing ARP traffic; this filter only allows ARP request and reply messages and enforces that those packets contain the MAC and IP addresses of the guest virtual machine.
allow-dhcp | Allows a guest virtual machine to request an IP address via DHCP (from any DHCP server)
allow-dhcp-server | Allows a guest virtual machine to request an IP address from a specified DHCP server. The dotted decimal IP address of the DHCP server must be provided in a reference to this filter. The name of the variable must be DHCPSERVER.
no-ip-spoofing | Prevents a guest virtual machine from sending IP packets with a source IP address different from the one inside the packet.
no-ip-multicast | Prevents a guest virtual machine from sending IP multicast packets.
clean-traffic | Prevents MAC, IP and ARP spoofing. This filter references several other filters as building blocks.

These filters are only building blocks and require a combination with other filters to provide useful network traffic filtering. The most used one in the above list is the clean-traffic filter. This filter itself can for example be combined with the no-ip-multicast filter to prevent virtual machines from sending IP multicast traffic on top of the prevention of packet spoofing.

⁠20.14.11.5. Writing your own filters

Since libvirt only provides a couple of example networking filters, you may consider writing your own. When planning on doing so there are a couple of things you may need to know regarding the network filtering subsystem and how it works internally. Certainly you also have to know and understand the protocols very well that you want to be filtering on so that no further traffic than what you want can pass and that in fact the traffic you want to allow does pass.

The network filtering subsystem is currently only available on Linux host physical machines and only works for QEMU and KVM type of virtual machines. On Linux, it builds upon the support for ebtables, iptables and ip6tables and makes use of their features. Considering the list found in Section 20.14.10, “Supported protocols” the following protocols can be implemented using ebtables:

mac

stp (spanning tree protocol)

vlan (802.1Q)

arp, rarp

ipv4

ipv6

Any protocol that runs over IPv4 is supported using iptables, those over IPv6 are implemented using ip6tables.

Using a Linux host physical machine, all traffic filtering rules created by libvirt's network filtering subsystem first passes through the filtering support implemented by ebtables and only afterwards through iptables or ip6tables filters. If a filter tree has rules with the protocols including: mac, stp, vlan arp, rarp, ipv4, or ipv6; the ebtable rules and values listed will automatically be used first.

Multiple chains for the same protocol can be created. The name of the chain must have a prefix of one of the previously enumerated protocols. To create an additional chain for handling of ARP traffic, a chain with name arp-test, can for example be specified.

As an example, it is possible to filter on UDP traffic by source and destination ports using the ip protocol filter and specifying attributes for the protocol, source and destination IP addresses and ports of UDP packets that are to be accepted. This allows early filtering of UDP traffic with ebtables. However, once an IP or IPv6 packet, such as a UDP packet, has passed the ebtables layer and there is at least one rule in a filter tree that instantiates iptables or ip6tables rules, a rule to let the UDP packet pass will also be necessary to be provided for those filtering layers. This can be achieved with a rule containing an appropriate udp or udp-ipv6 traffic filtering node.

⁠

Example 20.11. Creating a custom filter

Suppose a filter is needed to fulfill the following list of requirements:

prevents a VM's interface from MAC, IP and ARP spoofing

opens only TCP ports 22 and 80 of a VM's interface

allows the VM to send ping traffic from an interface but not let the VM be pinged on the interface

allows the VM to do DNS lookups (UDP towards port 53)

The requirement to prevent spoofing is fulfilled by the existing clean-traffic network filter, thus the way to do this is to reference it from a custom filter.

To enable traffic for TCP ports 22 and 80, two rules are added to enable this type of traffic. To allow the guest virtual machine to send ping traffic a rule is added for ICMP traffic. For simplicity reasons, general ICMP traffic will be allowed to be initiated from the guest virtual machine, and will not be specified to ICMP echo request and response messages. All other traffic will be prevented to reach or be initiated by the guest virtual machine. To do this a rule will be added that drops all other traffic. Assuming the guest virtual machine is called test and the interface to associate our filter with is called eth0, a filter is created named test-eth0.

The result of these considerations is the following network filter XML:

<filter name='test-eth0'>
  <!- - This rule references the clean traffic filter to prevent MAC, IP and ARP spoofing. By not providing an IP address parameter, libvirt will detect the IP address the guest virtual machine is using. - ->
  <filterref filter='clean-traffic'/>

  <!- - This rule enables TCP ports 22 (ssh) and 80 (http) to be reachable - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='22'/>
  </rule>

  <rule action='accept' direction='in'>
    <tcp dstportstart='80'/>
  </rule>

  <!- - This rule enables general ICMP traffic to be initiated by the guest virtual machine including ping traffic - ->
  <rule action='accept' direction='out'>
    <icmp/>
  </rule>>

  <!- - This rule enables outgoing DNS lookups using UDP - ->
  <rule action='accept' direction='out'>
    <udp dstportstart='53'/>
  </rule>

  <!- - This rule drops all other traffic - ->
  <rule action='drop' direction='inout'>
    <all/>
  </rule>

</filter>

⁠20.14.11.6. Sample custom filter

Although one of the rules in the above XML contains the IP address of the guest virtual machine as either a source or a destination address, the filtering of the traffic works correctly. The reason is that whereas the rule's evaluation occurs internally on a per-interface basis, the rules are additionally evaluated based on which (tap) interface has sent or will receive the packet, rather than what their source or destination IP address may be.

⁠

Example 20.12. Sample XML for network interface descriptions

An XML fragment for a possible network interface description inside the domain XML of the test guest virtual machine could then look like this:

   [...]
    <interface type='bridge'>
      <source bridge='mybridge'/>
      <filterref filter='test-eth0'/>
    </interface>
   [...]

To more strictly control the ICMP traffic and enforce that only ICMP echo requests can be sent from the guest virtual machine and only ICMP echo responses be received by the guest virtual machine, the above ICMP rule can be replaced with the following two rules:

  <!- - enable outgoing ICMP echo requests- ->
  <rule action='accept' direction='out'>
    <icmp type='8'/>
  </rule>

  <!- - enable incoming ICMP echo replies- ->
  <rule action='accept' direction='in'>
    <icmp type='0'/>
  </rule>

⁠

Example 20.13. Second example custom filter

This example demonstrates how to build a similar filter as in the example above, but extends the list of requirements with an ftp server located inside the guest virtual machine. The requirements for this filter are:

prevents a guest virtual machine's interface from MAC, IP, and ARP spoofing

opens only TCP ports 22 and 80 in a guest virtual machine's interface

allows the guest virtual machine to send ping traffic from an interface but does not allow the guest virtual machine to be pinged on the interface

allows the guest virtual machine to do DNS lookups (UDP towards port 53)

enables the ftp server (in active mode) so it can run inside the guest virtual machine

The additional requirement of allowing an FTP server to be run inside the guest virtual machine maps into the requirement of allowing port 21 to be reachable for FTP control traffic as well as enabling the guest virtual machine to establish an outgoing TCP connection originating from the guest virtual machine's TCP port 20 back to the FTP client (FTP active mode). There are several ways of how this filter can be written and two possible solutions are included in this example.

The first solution makes use of the state attribute of the TCP protocol that provides a hook into the connection tracking framework of the Linux host physical machine. For the guest virtual machine-initiated FTP data connection (FTP active mode) the RELATED state is used to enable detection that the guest virtual machine-initiated FTP data connection is a consequence of ( or 'has a relationship with' ) an existing FTP control connection, thereby allowing it to pass packets through the firewall. The RELATED state, however, is only valid for the very first packet of the outgoing TCP connection for the FTP data path. Afterwards, the state is ESTABLISHED, which then applies equally to the incoming and outgoing direction. All this is related to the FTP data traffic originating from TCP port 20 of the guest virtual machine. This then leads to the following solution:

<filter name='test-eth0'>
  <!- - This filter (eth0) references the clean traffic filter to prevent MAC, IP, and ARP spoofing. By not providing an IP address parameter, libvirt will detect the IP address the guest virtual machine is using. - ->
  <filterref filter='clean-traffic'/>

  <!- - This rule enables TCP port 21 (FTP-control) to be reachable - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='21'/>
  </rule>

  <!- - This rule enables TCP port 20 for guest virtual machine-initiated FTP data connection related to an existing FTP control connection - ->
  <rule action='accept' direction='out'>
    <tcp srcportstart='20' state='RELATED,ESTABLISHED'/>
  </rule>

  <!- - This rule accepts all packets from a client on the FTP data connection - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='20' state='ESTABLISHED'/>
  </rule>

  <!- - This rule enables TCP port 22 (SSH) to be reachable - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='22'/>
  </rule>

  <!- -This rule enables TCP port 80 (HTTP) to be reachable - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='80'/>
  </rule>

  <!- - This rule enables general ICMP traffic to be initiated by the guest virtual machine, including ping traffic - ->
  <rule action='accept' direction='out'>
    <icmp/>
  </rule>

  <!- - This rule enables outgoing DNS lookups using UDP - ->
  <rule action='accept' direction='out'>
    <udp dstportstart='53'/>
  </rule>

  <!- - This rule drops all other traffic - ->
  <rule action='drop' direction='inout'>
    <all/>
  </rule>

</filter>

Before trying out a filter using the RELATED state, you have to make sure that the appropriate connection tracking module has been loaded into the host physical machine's kernel. Depending on the version of the kernel, you must run either one of the following two commands before the FTP connection with the guest virtual machine is established:

#modprobe nf_conntrack_ftp - where available OR

#modprobe ip_conntrack_ftp if above is not available

If protocols other than FTP are used in conjunction with the RELATED state, their corresponding module must be loaded. Modules are available for the protocols: ftp, tftp, irc, sip, sctp, and amanda.

The second solution makes use of the state flags of connections more than the previous solution did. This solution takes advantage of the fact that the NEW state of a connection is valid when the very first packet of a traffic flow is detected. Subsequently, if the very first packet of a flow is accepted, the flow becomes a connection and thus enters into the ESTABLISHED state. Therefore a general rule can be written for allowing packets of ESTABLISHED connections to reach the guest virtual machine or be sent by the guest virtual machine. This is done writing specific rules for the very first packets identified by the NEW state and dictates the ports that the data is acceptable. All packets meant for ports that are not explicitly accepted are dropped, thus not reaching an ESTABLISHED state. Any subsequent packets sent from that port are dropped as well.

<filter name='test-eth0'>
  <!- - This filter references the clean traffic filter to prevent MAC, IP and ARP spoofing. By not providing and IP address parameter, libvirt will detect the IP address the VM is using. - ->
  <filterref filter='clean-traffic'/>

  <!- - This rule allows the packets of all previously accepted connections to reach the guest virtual machine - ->
  <rule action='accept' direction='in'>
    <all state='ESTABLISHED'/>
  </rule>

  <!- - This rule allows the packets of all previously accepted and related connections be sent from the guest virtual machine - ->
  <rule action='accept' direction='out'>
    <all state='ESTABLISHED,RELATED'/>
  </rule>

  <!- - This rule enables traffic towards port 21 (FTP) and port 22 (SSH)- ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='21' dstportend='22' state='NEW'/>
  </rule>

  <!- - This rule enables traffic towards port 80 (HTTP) - ->
  <rule action='accept' direction='in'>
    <tcp dstportstart='80' state='NEW'/>
  </rule>

  <!- - This rule enables general ICMP traffic to be initiated by the guest virtual machine, including ping traffic - ->
  <rule action='accept' direction='out'>
    <icmp state='NEW'/>
  </rule>

  <!- - This rule enables outgoing DNS lookups using UDP - ->
  <rule action='accept' direction='out'>
    <udp dstportstart='53' state='NEW'/>
  </rule>

  <!- - This rule drops all other traffic - ->
  <rule action='drop' direction='inout'>
    <all/>
  </rule>

</filter>

⁠20.14.12. Limitations

The following is a list of the currently known limitations of the network filtering subsystem.

VM migration is only supported if the whole filter tree that is referenced by a guest virtual machine's top level filter is also available on the target host physical machine. The network filter clean-traffic for example should be available on all libvirt installations and thus enable migration of guest virtual machines that reference this filter. To assure version compatibility is not a problem make sure you are using the most current version of libvirt by updating the package regularly.

Migration must occur between libvirt insallations of version 0.8.1 or later in order not to lose the network traffic filters associated with an interface.

VLAN (802.1Q) packets, if sent by a guest virtual machine, cannot be filtered with rules for protocol IDs arp, rarp, ipv4 and ipv6. They can only be filtered with protocol IDs, MAC and VLAN. Therefore, the example filter clean-traffic Example 20.1, “An example of network filtering” will not work as expected.

⁠20.15. Creating Tunnels

This section will demonstrate how to implement different tunneling scenarios.

⁠20.15.1. Creating Multicast tunnels

A multicast group is setup to represent a virtual network. Any guest virtual machines whose network devices are in the same multicast group can talk to each other even across host physical machines. This mode is also available to unprivileged users. There is no default DNS or DHCP support and no outgoing network access. To provide outgoing network access, one of the guest virtual machines should have a second NIC which is connected to one of the first four network types thus providing appropriate routing. The multicast protocol is compatible the guest virtual machine user mode. Note that the source address that you provide must be from the address used fot the multicast address block.

To create a multicast tunnel place the following XML details into the <devices> element:

⁠

​
​
​  ...
​  <devices>
​    <interface type='mcast'>
​      <mac address='52:54:00:6d:90:01'>
​      <source address='230.0.0.1' port='5558'/>
​    </interface>
​  </devices>
​  ...
​
​

Figure 20.25. Multicast tunnel domain XMl example

⁠20.15.2. Creating TCP tunnels

A TCP client/server architecture provides a virtual network. In this configuration, one guest virtual machine provides the server end of the network while all other guest virtual machines are configured as clients. All network traffic is routed between the guest virtual machine clients via the guest virtual machine server. This mode is also available for unprivileged users. Note that this mode does not provide default DNS or DHCP support nor does it provide outgoing network access. To provide outgoing network access, one of the guest virtual machines should have a second NIC which is connected to one of the first four network types thus providing appropriate routing.

To create a TCP tunnel place the following XML details into the <devices> element:

⁠

​
​
​       ...
​  <devices>
​    <interface type='server'>
​      <mac address='52:54:00:22:c9:42'>
​      <source address='192.168.0.1' port='5558'/>
​    </interface>
​    ...
​    <interface type='client'>
​      <mac address='52:54:00:8b:c9:51'>
​      <source address='192.168.0.1' port='5558'/>
​    </interface>
​  </devices>
​  ...
​
​

Figure 20.26. TCP tunnel domain XMl example

⁠20.16. Setting vLAN tags

virtual local area network (vLAN) tags are added using the virsh net-edit command. This tag can also be used with PCI device assignment with SR-IOV devices. For more information, refer to Section 18.1.7, “Configuring PCI assignment (passthrough) with SR-IOV devices”.

⁠

​
​
​<network>
​  <name>ovs-net</name>
​  <forward mode='bridge'/>
​  <bridge name='ovsbr0'/>
​  <virtualport type='openvswitch'>
​    <parameters interfaceid='09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f'/>
​  </virtualport>
​  <vlan trunk='yes'>
​    <tag id='42' nativeMode='untagged'/>
​    <tag id='47'/>
​  </vlan>
​  <portgroup name='dontpanic'>
​    <vlan>
​      <tag id='42'/>
​    </vlan>
​  </portgroup>
​</network>

Figure 20.27. vSetting VLAN tag (on supported network types only)

If (and only if) the network type supports vlan tagging transparent to the guest, an optional <vlan> element can specify one or more vlan tags to apply to the traffic of all guests using this network. (openvswitch and type='hostdev' SR-IOV networks do support transparent vlan tagging of guest traffic; everything else, including standard linux bridges and libvirt's own virtual networks, do not support it. 802.1Qbh (vn-link) and 802.1Qbg (VEPA) switches provide their own way (outside of libvirt) to tag guest traffic onto specific vlans.) As expected, the tag attribute specifies which vlan tag to use. If a network has more than one <vlan> element defined, it is assumed that the user wants to do VLAN trunking using all the specified tags. In the case that vlan trunking with a single tag is desired, the optional attribute trunk='yes' can be added to the vlan element.

For network connections using openvswitch it is possible to configure the 'native-tagged' and 'native-untagged' vlan modes. This uses the optional nativeMode attribute on the <tag> element: nativeMode may be set to 'tagged' or 'untagged'. The id attribute of the element sets the native vlan.

<vlan> elements can also be specified in a <portgroup> element, as well as directly in a domain's <interface> element. In the case that a vlan tag is specified in multiple locations, the setting in <interface> takes precedence, followed by the setting in the <portgroup> selected by the interface config. The <vlan> in <network> will be selected only if none is given in <portgroup> or <interface>.

⁠20.17. Applying QoS to your virtual network

Quality of Service (QoS) refers to the resource control systems that guarantees an optimal experience for all users on a network, making sure that there is no delay, jitter, or packet loss. QoS can be application specific or user / group specific. Refer to Section 26.18.9.14, “Quality of service (QoS)” for more information.

⁠Chapter 21. Remote management of guests

This section explains how to remotely manage your guests using ssh or TLS and SSL. More information on SSH can be found in the Red Hat Enterprise Linux Deployment Guide

⁠21.1. Remote management with SSH

The ssh package provides an encrypted network protocol which can securely send management functions to remote virtualization servers. The method described uses the libvirt management connection securely tunneled over an SSH connection to manage the remote machines. All the authentication is done using SSH public key cryptography and passwords or passphrases gathered by your local SSH agent. In addition the VNC console for each guest is tunneled over SSH.

Be aware of the issues with using SSH for remotely managing your virtual machines, including:

you require root log in access to the remote machine for managing virtual machines,

the initial connection setup process may be slow,

there is no standard or trivial way to revoke a user's key on all hosts or guests, and

ssh does not scale well with larger numbers of remote machines.

Note

Red Hat Enterprise Virtualization enables remote management of large numbers of virtual machines. Refer to the Red Hat Enterprise Virtualization documentation for further details.

The following packages are required for ssh access:

openssh

openssh-askpass

openssh-clients

openssh-server

⁠

Configuring password less or password managed SSH access for virt-manager

The following instructions assume you are starting from scratch and do not already have SSH keys set up. If you have SSH keys set up and copied to the other systems you can skip this procedure.

Important

SSH keys are user dependent and may only be used by their owners. A key's owner is the one who generated it. Keys may not be shared.

virt-manager must be run by the user who owns the keys to connect to the remote host. That means, if the remote systems are managed by a non-root user virt-manager must be run in unprivileged mode. If the remote systems are managed by the local root user then the SSH keys must be owned and created by root.

You cannot manage the local host as an unprivileged user with virt-manager.

Optional: Changing user

Change user, if required. This example uses the local root user for remotely managing the other hosts and the local host.

$ su -

Generating the SSH key pair

Generate a public key pair on the machine virt-manager is used. This example uses the default key location, in the ~/.ssh/ directory.

# ssh-keygen -t rsa

Copying the keys to the remote hosts

Remote login without a password, or with a pass-phrase, requires an SSH key to be distributed to the systems being managed. Use the ssh-copy-id command to copy the key to root user at the system address provided (in the example, root@host2.example.com).

# ssh-copy-id -i ~/.ssh/id_rsa.pub root@host2.example.com
root@host2.example.com's password:

Now try logging into the machine, with the ssh root@host2.example.com command and check in the .ssh/authorized_keys file to make sure unexpected keys have not been added.

Repeat for other systems, as required.

Optional: Add the passphrase to the ssh-agent

The instructions below describe how to add a passphrase to an existing ssh-agent. It will fail to run if the ssh-agent is not running. To avoid errors or conflicts make sure that your SSH parameters are set correctly. Refer to the Red Hat Enterprise Linux Deployment Guide for more information.

Add the pass-phrase for the SSH key to the ssh-agent, if required. On the local host, use the following command to add the pass-phrase (if there was one) to enable password-less login.

# ssh-add ~/.ssh/id_rsa

The SSH key is added to the remote system.

⁠

The libvirt daemon (libvirtd)

The libvirt daemon provides an interface for managing virtual machines. You must have the libvirtd daemon installed and running on every remote host that needs managing.

$ ssh root@somehost
# systemctl enable libvirtd.service
# systemctl start libvirtd

After libvirtd and SSH are configured you should be able to remotely access and manage your virtual machines. You should also be able to access your guests with VNC at this point.

⁠

Accessing remote hosts with virt-manager

Remote hosts can be managed with the virt-manager GUI tool. SSH keys must belong to the user executing virt-manager for password-less login to work.

Start virt-manager.

Open the File->Add Connection menu.

⁠

Figure 21.1. Add connection menu

Use the drop down menu to select hypervisor type, and click the Connect to remote host check box to open the Connection Method (in this case Remote tunnel over SSH), and enter the desired User name and Hostname, then click Connect.

⁠21.2. Remote management over TLS and SSL

You can manage virtual machines using TLS and SSL. TLS and SSL provides greater scalability but is more complicated than ssh (refer to Section 21.1, “Remote management with SSH”). TLS and SSL is the same technology used by web browsers for secure connections. The libvirt management connection opens a TCP port for incoming connections, which is securely encrypted and authenticated based on x509 certificates. The procedures that follow provide instructions on creating and deploying authentication certificates for TLS and SSL management.

⁠

Procedure 21.1. Creating a certificate authority (CA) key for TLS management

Before you begin, confirm that certtool is installed. If not:

# yum install certtool

Generate a private key, using the following command:

# certtool --generate-privkey > cakey.pem

Once the key generates, the next step is to create a signature file so the key can be self-signed. To do this, create a file with signature details and name it ca.info. This file should contain the following:

# vim ca.info

cn = Name of your organization
ca
cert_signing_key

Generate the self-signed key with the following command:

# certtool --generate-self-signed --load-privkey cakey.pem --template ca.info --outfile cacert.pem

Once the file generates, the ca.info file may be deleted using the rm command. The file that results from the generation process is named cacert.pem. This file is the public key (certificate). The loaded file cakey.pem is the private key. This file should not be kept in a shared space. Keep this key private.

Install the cacert.pem Certificate Authority Certificate file on all clients and servers in the /etc/pki/CA/cacert.pem directory to let them know that the certificate issued by your CA can be trusted. To view the contents of this file, run:

# certtool -i --infile cacert.pem

This is all that is required to set up your CA. Keep the CA's private key safe as you will need it in order to issue certificates for your clients and servers.

⁠

Procedure 21.2. Issuing a server certificate

This procedure demonstrates how to issue a certificate with the X.509 CommonName (CN)field set to the hostname of the server. The CN must match the hostname which clients will be using to connect to the server. In this example, clients will be connecting to the server using the URI: qemu://mycommonname/system, so the CN field should be identical, ie mycommoname.

Create a private key for the server.

# certtool --generate-privkey > serverkey.pem

Generate a signature for the CA's private key by first creating a template file called server.info . Make sure that the CN is set to be the same as the server's hostname:

organization = Name of your organization
cn = mycommonname
tls_www_server
encryption_key
signing_key

Create the certificate with the following command:

# certtool --generate-certificate --load-privkey serverkey.pem --load-ca-certificate cacert.pem --load-ca-privkey cakey.pem \ --template server.info --outfile servercert.pem

This results in two files being generated:

serverkey.pem - The server's private key

servercert.pem - The server's public key

Make sure to keep the location of the private key secret. To view the contents of the file, perform the following command:

# certtool -i --inifile servercert.pem

When opening this file the CN= parameter should be the same as the CN that you set earlier. For example, mycommonname.

Install the two files in the following locations:

serverkey.pem - the server's private key. Place this file in the following location: /etc/pki/libvirt/private/serverkey.pem

servercert.pem - the server's certificate. Install it in the following location on the server: /etc/pki/libvirt/servercert.pem

⁠

Procedure 21.3. Issuing a client certificate

For every client (ie. any program linked with libvirt, such as virt-manager), you need to issue a certificate with the X.509 Distinguished Name (DN) set to a suitable name. This needs to be decided on a corporate level.

For example purposes the following information will be used:

C=USA,ST=North Carolina,L=Raleigh,O=Red Hat,CN=name_of_client

This process is quite similar to Procedure 21.2, “Issuing a server certificate”, with the following exceptions noted.

Make a private key with the following command:

# certtool --generate-privkey > clientkey.pem

Generate a signature for the CA's private key by first creating a template file called client.info . The file should contain the following (fields should be customized to reflect your region/location):

country = USA
state = North Carolina
locality = Raleigh
organization = Red Hat
cn = client1
tls_www_client
encryption_key
signing_key

Sign the certificate with the following command:

# certtool --generate-certificate --load-privkey clientkey.pem --load-ca-certificate cacert.pem \ --load-ca-privkey cakey.pem --template client.info --outfile clientcert.pem

Install the certificates on the client machine:

# cp clientkey.pem /etc/pki/libvirt/private/clientkey.pem
# cp clientcert.pem /etc/pki/libvirt/clientcert.pem

⁠21.3. Transport modes

For remote management, libvirt supports the following transport modes:

⁠

Transport Layer Security (TLS)

Transport Layer Security TLS 1.0 (SSL 3.1) authenticated and encrypted TCP/IP socket, usually listening on a public port number. To use this you will need to generate client and server certificates. The standard port is 16514.

⁠

UNIX sockets

UNIX domain sockets are only accessible on the local machine. Sockets are not encrypted, and use UNIX permissions or SELinux for authentication. The standard socket names are /var/run/libvirt/libvirt-sock and /var/run/libvirt/libvirt-sock-ro (for read-only connections).

⁠

SSH

Transported over a Secure Shell protocol (SSH) connection. Requires Netcat (the nc package) installed. The libvirt daemon (libvirtd) must be running on the remote machine. Port 22 must be open for SSH access. You should use some sort of SSH key management (for example, the ssh-agent utility) or you will be prompted for a password.

⁠

ext

The ext parameter is used for any external program which can make a connection to the remote machine by means outside the scope of libvirt. This parameter is unsupported.

⁠

TCP

Unencrypted TCP/IP socket. Not recommended for production use, this is normally disabled, but an administrator can enable it for testing or use over a trusted network. The default port is 16509.

The default transport, if no other is specified, is TLS.

⁠

Remote URIs

A Uniform Resource Identifier (URI) is used by virsh and libvirt to connect to a remote host. URIs can also be used with the --connect parameter for the virsh command to execute single commands or migrations on remote hosts. Remote URIs are formed by taking ordinary local URIs and adding a hostname and/or transport name. As a special case, using a URI scheme of 'remote', will tell the remote libvirtd server to probe for the optimal hypervisor driver. This is equivalent to passing a NULL URI for a local connection

libvirt URIs take the general form (content in square brackets, "[]", represents optional functions):

driver[+transport]://[username@][hostname][:port]/path[?extraparameters]

Note that if the hypervisor(driver) is QEMU, the path is mandatory. If it is XEN, it is optional.

The following are examples of valid remote URIs:

qemu://hostname/

xen://hostname/

xen+ssh://hostname/

The transport method or the hostname must be provided to target an external location. For more information refer to http://libvirt.org/guide/html/Application_Development_Guide-Architecture-Remote_URIs.html.

⁠

Examples of remote management parameters

Connect to a remote KVM host named host2, using SSH transport and the SSH username virtuser.The connect command for each is connect [<name>] [--readonly], where <name> is a valid URI as explained here. For more information about the virsh connect command refer to Section 23.7, “Connecting to the hypervisor with virsh connect”

qemu+ssh://virtuser@host2/

Connect to a remote KVM hypervisor on the host named host2 using TLS.

qemu://host2/

⁠

Testing examples

Connect to the local KVM hypervisor with a non-standard UNIX socket. The full path to the UNIX socket is supplied explicitly in this case.

qemu+unix:///system?socket=/opt/libvirt/run/libvirt/libvirt-sock

Connect to the libvirt daemon with an non-encrypted TCP/IP connection to the server with the IP address 10.1.1.10 on port 5000. This uses the test driver with default settings.

test+tcp://10.1.1.10:5000/default

⁠

Extra URI parameters

Extra parameters can be appended to remote URIs. The table below Table 21.1, “Extra URI parameters” covers the recognized parameters. All other parameters are ignored. Note that parameter values must be URI-escaped (that is, a question mark (?) is appended before the parameter and special characters are converted into the URI format).

⁠

Table 21.1. Extra URI parameters

Name
Transport mode
Description
Example usage

name
all modes
The name passed to the remote virConnectOpen function. The name is normally formed by removing transport, hostname, port number, username, and extra parameters from the remote URI, but in certain very complex cases it may be better to supply the name explicitly.
name=qemu:///system

command
ssh and ext
The external command. For ext transport this is required. For ssh the default is ssh. The PATH is searched for the command.
command=/opt/openssh/bin/ssh

socket
unix and ssh
The path to the UNIX domain socket, which overrides the default. For ssh transport, this is passed to the remote netcat command (see netcat).
socket=/opt/libvirt/run/libvirt/libvirt-sock

netcat
ssh

The netcat command can be used to connect to remote systems. The default netcat parameter uses the nc command. For SSH transport, libvirt constructs an SSH command using the form below:

command -p port [-l username] hostname

netcat -U socket

The port, username and hostname parameters can be specified as part of the remote URI. The command, netcat and socket come from other extra parameters.

netcat=/opt/netcat/bin/nc

no_verify
tls
If set to a non-zero value, this disables client checks of the server's certificate. Note that to disable server checks of the client's certificate or IP address you must change the libvirtd configuration.
no_verify=1

no_tty
ssh
If set to a non-zero value, this stops ssh from asking for a password if it cannot log in to the remote machine automatically . Use this when you do not have access to a terminal .
no_tty=1

⁠21.4. Configuring a VNC Server

To configure a VNC server, use the Remote Desktop application in System > Preferences. Alternatively, you can run the vino-preferences command.

Use the following step set up a dedicated VNC server session:

If needed, Create and then Edit the ~/.vnc/xstartup file to start a GNOME session whenever vncserver is started. The first time you run the vncserver script it will ask you for a password you want to use for your VNC session. For more information on vnc server files refer to the Red Hat Enterprise Linux Installation Guide.

⁠Chapter 22. Managing guests with the Virtual Machine Manager (virt-manager)

This section describes the Virtual Machine Manager (virt-manager) windows, dialog boxes, and various GUI controls.

virt-manager provides a graphical view of hypervisors and guests on your host system and on remote host systems. virt-manager can perform virtualization management tasks, including:

defining and creating guests,

assigning memory,

assigning virtual CPUs,

monitoring operational performance,

saving and restoring, pausing and resuming, and shutting down and starting guests,

links to the textual and graphical consoles, and

live and offline migrations.

⁠22.1. Starting virt-manager

To start virt-manager session open the Applications menu, then the System Tools menu and select Virtual Machine Manager (virt-manager).

The virt-manager main window appears.

⁠

Figure 22.1. Starting virt-manager

Alternatively, virt-manager can be started remotely using ssh as demonstrated in the following command:

ssh -X host's address
[remotehost]# virt-manager

Using ssh to manage virtual machines and hosts is discussed further in Section 21.1, “Remote management with SSH”.

⁠22.2. The Virtual Machine Manager main window

This main window displays all the running guests and resources used by guests. Select a guest by double clicking the guest's name.

⁠

Figure 22.2. Virtual Machine Manager main window

⁠22.3. The virtual hardware details window

The virtual hardware details window displays information about the virtual hardware configured for the guest. Virtual hardware resources can be added, removed and modified in this window. To access the virtual hardware details window, click on the icon in the toolbar.

⁠

Figure 22.3. The virtual hardware details icon

Clicking the icon displays the virtual hardware details window.

⁠

Figure 22.4. The virtual hardware details window

⁠22.3.1. Applying boot options to guest virtual machines

Using virt-manager you can select how the guest virtual machine will act on boot. The boot options will not take effect until the guest virtual machine reboots. You can either power down the virtual machine before making any changes, or you can reboot the machine afterwards. If you do not do either of these options, the changes will happen the next time the guest reboots.

⁠

Procedure 22.1. Configuring boot options

From the Virtual Machine Manager Edit menu, select Virtual Machine Details.

From the side panel, select Boot Options and then complete any or all of the following optional steps:

To indicate that this guest virtual machine should start each time the host physical machine boots, select the Autostart check box.

To indicate the order in which guest virtual machine should boot, click the Enable boot menu check box. After this is checked, you can then check the devices you want to boot from and using the arrow keys change the order that the guest virtual machine will use when booting.

If you want to boot directly from the Linux kernel, expand the Direct kernel boot menu. Fill in the Kernel path, Initrd path, and the Kernel arguments that you want to use.

Click Apply.

⁠

Figure 22.5. Configuring boot options

⁠22.3.2. Attaching USB devices to a guest virtual machine

Note

In order to attach the USB device to the guest virtual machine, you first must attach it to the host physical machine and confirm that the device is working. If the guest is running, you need to shut it down before proceeding.

⁠

Procedure 22.2. Attaching USB devices using Virt-Manager

Open the guest virtual machine's Virtual Machine Details screen.

Click Add Hardware

⁠

Figure 22.6. Add Hardware Button

In the Add New Virtual Hardware popup, select USB Host Device, select the device you want to attach from the list and Click Finish.

⁠

Figure 22.7. Add USB Device

To use the USB device in the guest virtual machine, start the guest virtual machine.

⁠22.3.3. USB redirection

USB re-direction is best used in cases where there is a host physical machine that is running in a data center. The user connects to his/her guest virtual machine from a local machine or thin client. On this local machine there is a SPICE client. The user can attach any USB device to the thin client and the SPICE client will redirect the device to the host physical machine on the data center so it can be used by the VM that is running on the thin client.

⁠

Procedure 22.3. Redirecting USB devices

Open the guest virtual machine's Virtual Machine Details screen.

Click Add Hardware

⁠

Figure 22.8. Add Hardware Button

In the Add New Virtual Hardware popup, select USB Redirection. Make sure to select Spice channel the Type drop-down menu and click Finish.

⁠

Figure 22.9. Add New Virtual Hardware window

A pop-up menu opens with a list of devices to choose from. Select a device by clicking the check box and click OK.

⁠

Figure 22.10. Select a USB device

⁠22.4. Virtual Machine graphical console

This window displays a guest's graphical console. Guests can use several different protocols to export their graphical frame buffers: virt-manager supports VNC and SPICE. If your virtual machine is set to require authentication, the Virtual Machine graphical console prompts you for a password before the display appears.

⁠

Figure 22.11. Graphical console window

Note

VNC is considered insecure by many security experts, however, several changes have been made to enable the secure usage of VNC for virtualization on Red Hat enterprise Linux. The guest machines only listen to the local host's loopback address (127.0.0.1). This ensures only those with shell privileges on the host can access virt-manager and the virtual machine through VNC. Although virt-manager is configured to listen to other public network interfaces and alternative methods can be configured, it is not recommended.

Remote administration can be performed by tunneling over SSH which encrypts the traffic. Although VNC can be configured to access remotely without tunneling over SSH, for security reasons, it is not recommended. To remotely administer the guest follow the instructions in: Chapter 21, Remote management of guests. TLS can provide enterprise level security for managing guest and host systems.

Your local desktop can intercept key combinations (for example, Ctrl+Alt+F1) to prevent them from being sent to the guest machine. You can use the Send key menu option to send these sequences. From the guest machine window, click the Send key menu and select the key sequence to send. In addition, from this menu you can also capture the screen output.

SPICE is an alternative to VNC available for Red Hat Enterprise Linux.

⁠22.5. Adding a remote connection

This procedure covers how to set up a connection to a remote system using virt-manager.

To create a new connection open the File menu and select the Add Connection... menu item.

The Add Connection wizard appears. Select the hypervisor. For Red Hat Enterprise Linux 7 systems select QEMU/KVM. Select Local for the local system or one of the remote connection options and click Connect. This example uses Remote tunnel over SSH which works on default installations. For more information on configuring remote connections refer to Chapter 21, Remote management of guests

⁠

Figure 22.12. Add Connection

Enter the root password for the selected host when prompted.

A remote host is now connected and appears in the main virt-manager window.

⁠

Figure 22.13. Remote host in the main virt-manager window

⁠22.6. Displaying guest details

You can use the Virtual Machine Monitor to view activity information for any virtual machines on your system.

To view a virtual system's details:

In the Virtual Machine Manager main window, highlight the virtual machine that you want to view.

⁠

Figure 22.14. Selecting a virtual machine to display

From the Virtual Machine Manager Edit menu, select Virtual Machine Details.

⁠

Figure 22.15. Displaying the virtual machine details

When the Virtual Machine details window opens, there may be a console displayed. Should this happen, click View and then select Details. The Overview window opens first by default. To go back to this window, select Overview from the navigation pane on the left hand side.

The Overview view shows a summary of configuration details for the guest.

⁠

Figure 22.16. Displaying guest details overview

Select Performance from the navigation pane on the left hand side.

The Performance view shows a summary of guest performance, including CPU and Memory usage.

⁠

Figure 22.17. Displaying guest performance details

Select Processor from the navigation pane on the left hand side. The Processor view allows you to view or change the current processor allocation.

⁠

Figure 22.18. Processor allocation panel

Select Memory from the navigation pane on the left hand side. The Memory view allows you to view or change the current memory allocation.

⁠

Figure 22.19. Displaying memory allocation

Each virtual disk attached to the virtual machine is displayed in the navigation pane. Click on a virtual disk to modify or remove it.

⁠

Figure 22.20. Displaying disk configuration

Each virtual network interface attached to the virtual machine is displayed in the navigation pane. Click on a virtual network interface to modify or remove it.

⁠

Figure 22.21. Displaying network configuration

⁠22.7. Performance monitoring

Performance monitoring preferences can be modified with virt-manager's preferences window.

To configure performance monitoring:

From the Edit menu, select Preferences.

⁠

Figure 22.22. Modifying guest preferences

The Preferences window appears.

From the Stats tab specify the time in seconds or stats polling options.

⁠

Figure 22.23. Configuring performance monitoring

⁠22.8. Displaying CPU usage for guests

To view the CPU usage for all guests on your system:

From the View menu, select Graph, then the Guest CPU Usage check box.

⁠

Figure 22.24. Enabling guest CPU usage statistics graphing

The Virtual Machine Manager shows a graph of CPU usage for all virtual machines on your system.

⁠

Figure 22.25. Guest CPU usage graph

⁠22.9. Displaying CPU usage for hosts

To view the CPU usage for all hosts on your system:

From the View menu, select Graph, then the Host CPU Usage check box.

⁠

Figure 22.26. Enabling host CPU usage statistics graphing

The Virtual Machine Manager shows a graph of host CPU usage on your system.

⁠

Figure 22.27. Host CPU usage graph

⁠22.10. Displaying Disk I/O

To view the disk I/O for all virtual machines on your system:

Make sure that the Disk I/O statistics collection is enabled. To do this, from the Edit menu, select Preferences and click the Stats tab.

Select the Disk I/O checkbox.

⁠

Figure 22.28. Enabling Disk I/O

To enable the Disk I/O display, from the View menu, select Graph, then the Disk I/O check box.

⁠

Figure 22.29. Selecting Disk I/O

The Virtual Machine Manager shows a graph of Disk I/O for all virtual machines on your system.

⁠

Figure 22.30. Displaying Disk I/O

⁠22.11. Displaying Network I/O

To view the network I/O for all virtual machines on your system:

Make sure that the Network I/O statistics collection is enabled. To do this, from the Edit menu, select Preferences and click the Statstab.

Select the Network I/O checkbox.

⁠

Figure 22.31. Enabling Network I/O

To display the Network I/O statistics, from the View menu, select Graph, then the Network I/O check box.

⁠

Figure 22.32. Selecting Network I/O

The Virtual Machine Manager shows a graph of Network I/O for all virtual machines on your system.

⁠

Figure 22.33. Displaying Network I/O

⁠Chapter 23. Managing guest virtual machines with virsh

virsh is a command line interface tool for managing guest virtual machines. It can be used to create, deploy, and manage guest virtual machines. The virsh command-line tool is built on the libvirt management API and operates as an alternative to the qemu-kvm command and the graphical virt-manager application. The virsh program can be used in read-only mode by unprivileged users or, with root access, full administration mode. The virsh program is ideal for scripting virtualization administration. The virsh package is installed with yum as part of the libvirt-client package. For installation instructions refer to libvirt-client. The remaining sections of this chapter cover the virsh command set in a logical order based on usage.

Note

Note that when using the help or when reading the MAN pages, the term 'domain' will be used instead of the term guest virtual machine. This is the term used by libvirt. In cases where the screen output is displayed and the word 'domain' is used, it will not be switched to guest or guest virtual machine. In all examples, the guest virtual machine 'guest1' will be used. You should replace this with the name of your guest virtual machine in all cases.

⁠23.1. Guest virtual machine states

During the life cycle of a virtual machine, libvirt will classify the VM as any of the following states:

Undefined - This is a guest virtual machine that has not been defined or created. As such libvirt is unaware of any guest in this state and will not report about guest virtual machines in this state.

Defined / Stopped - This is a guest virtual machine which is defined, but is not running. Only persistent guests can be in this state. When a transient guest virtual machine is put into this state it ceases to exist.

Running - The guest virtual machine in this state has been defined and is currently working. This state can be used with both persistent and transient guest virtual machines.

Paused - The guest virtual machine's execution on the hypervisor has been suspended, or its state has been temporarily stored until it is resumed. Guest virtual machines in this state are not aware they have been suspended and do not notice that time has passed when they are resumed.

Saved - This state is similar to the paused state, however the guest virtual machine's configuration is saved to persistent storage. Any guest virtual machine in this state is not aware it is paused and does not notice that time has passed once it has been restored.

⁠23.2. Running the virsh program

There are two ways to run virsh. One way is line by line and the other is inside an interactive terminal. The commands presented in this guide are shown in a line by line format. The same commands may be used in the interactive terminal, but the prefix 'virsh' is not needed, and the interactive terminal automatically registers the user as root. To enter the interactive terminal run $ virsh and click enter. The following prompt appears: virsh #.

⁠

Example 23.1. Launching the virsh interactive terminal

In the following output, you can see the virsh welcome screen for the interactive terminal.

[~]$ virsh
Welcome to virsh, the virtualization interactive terminal.

Type:  'help' for help with commands
       'quit' to quit

virsh #

⁠23.3. Interactive mode commands

The following commands are best used in the virsh interactive mode, or interactive terminal as explained in Example 23.1, “Launching the virsh interactive terminal”. They can be used in batch mode, but may behave differently as indicated.

⁠23.3.1. Quitting and exiting from the virsh interactive terminal

The virsh quit command and the virsh exit command will close the interactive terminal. For example:

virsh # exit

virsh # quit

The quit and exit commands are mainly useful in the interactive terminal. Note that these commands may also be used in bash mode from the command line as well. In that case, the command virsh exit exits automatically.

⁠23.3.2. Changing and displaying directories in a virtual machine

The virsh cd command works just like the standard bash cd command. To change to a different directory in virsh, either enter cd within the interactive terminal or virsh cd from a command line as root. Likewise the command pwd or virsh pwd, functions just like the bash command pwd, and displays the working directory you are currently in. This is particularly helpful when inside the guest.

⁠23.4. Displaying the virsh version

The virsh version command displays the current libvirt version and displays information about the local virsh client. For example:

$ virsh version
Compiled against library: libvirt 1.2.8
Using library: libvirt 1.2.8
Using API: QEMU 1.2.8
Running hypervisor: QEMU 1.5.3

The virsh version --daemon is useful for getting information about the libvirtd version and package information, including information about the libvirt daemon that is running on the host.

$ virsh version --daemon
Compiled against library: libvirt 1.2.8
Using library: libvirt 1.2.8
Using API: QEMU 1.2.8
Running hypervisor: QEMU 1.5.3
Running against daemon: 1.2.8

⁠23.5. Getting help

The virsh help command can be used with or without options. When used without options, all commands are listed, one per line. When used with an option, it is grouped into categories, displaying the keyword for each group.

⁠

Example 23.2. virsh help group example

To display the commands that are only for a specific option, you need to give the keyword for that group as an option. In the following output you can see the help information for the general search for all commands which contain the keyword 'pool'.

$ virsh help pool

 Storage Pool (help keyword 'pool'):
    find-storage-pool-sources-as   find potential storage pool sources
    find-storage-pool-sources      discover potential storage pool sources
    pool-autostart                 autostart a pool
    pool-build                     build a pool
    pool-create-as                 create a pool from a set of args
    pool-create                    create a pool from an XML file
    pool-define-as                 define a pool from a set of args
    pool-define                    define (but don't start) a pool from an XML file
    pool-delete                    delete a pool
    pool-destroy                   destroy (stop) a pool
    pool-dumpxml                   pool information in XML
    pool-edit                      edit XML configuration for a storage pool
    pool-info                      storage pool information
    pool-list                      list pools
    pool-name                      convert a pool UUID to pool name
    pool-refresh                   refresh a pool
    pool-start                     start a (previously defined) inactive pool
    pool-undefine                  undefine an inactive pool
    pool-uuid                      convert a pool name to pool UUID

⁠

Example 23.3. virsh help example with command option

Using the same command with a command option, gives the help information on that one specific command. In the following output you can see the help information for the virsh vol-path command.

$ virsh help vol-path
  NAME
    vol-path - returns the volume path for a given volume name or key

  SYNOPSIS
    vol-path <vol> [--pool <string>]

  OPTIONS
    [--vol] <string>  volume name or key
    --pool <string>  pool name or uuid

⁠23.6. Sending commands with echo

The virsh echo [--shell][--xml] <args> command displays the specified argument in the specified format. The formats you can use are --shell and --xml. Each argument queried is displayed separated by a space. Tthe --shell option generates output that is formatted in single quotes where needed, so it is suitable for copying and pasting into the bash mode as a command. If the --xml argument is used, the output is formatted for use in an XML file, which can then be saved or used for guest's configuration.

⁠23.7. Connecting to the hypervisor with virsh connect

The virsh connect [<name>] [--readonly] command begins a local hypervisor session using virsh. After the first time you run this command it will run automatically each time the virsh shell runs. The hypervisor connection URI specifies how to connect to the hypervisor. The most commonly used URIs are:

qemu:///system - connects locally as the root user to the daemon supervising guest virtual machines on the KVM hypervisor.

xen:///session - connects locally as a user to the user's set of guest local machines using the KVM hypervisor.

lxc:/// - connects to a local Linux container.

xen:/// - connects to the local Xen hypervisor.

The command can be run as follows, where [name] is the machine name (hostname) or URL (the output of the virsh uri command) of the hypervisor as shown:

$ virsh uri
qemu:///session

For example, to establish a session to connect to my set of guest virtual machines, with me as the local user:

$ virsh connect qemu:///session

To initiate a read-only connection, append the above command with --readonly. For more information on URIs, refer to Remote URIs. If you are unsure of the URI, the virsh uri command will display it:

⁠23.8. Displaying information about guest virtual machine

The virsh list command will list all guest virtual machines which fit the search parameter requested. Note that the information you enter in this command does not in any way change the guest configuration setting.

There are many parameters you can use to filter the list you receive. These options are available in the MAN page, by running man virsh or by running the virsh list --help command.

⁠

Example 23.4. How to list all locally connected virtual machines

In this example, you will see a list of all the virtual machines that have been created. Note that if a virtual machine was installed using virt-manager and you did not complete the installation process, the guest virtual machine will not be displayed. The output you will see has 3 columns in a table. Each guest virtual machine is listed with its ID, name, and state. There are five states that a guest can have in libvirt. For more information, refer to Section 23.1, “Guest virtual machine states”.

# virsh list --all

Id              Name                      State
------------------------------------------------
14		guest1			running
22		guest2			paused
35		guest3			stopped

In addition, the following commands can also be used to display basic information about the hypervisor:

$ virsh hostname - displays the hypervisor's hostname.

$ virsh sysinfo - displays the XML representation of the hypervisor's system information, if available.

⁠23.9. Guest virtual machine basic commands

The guest virtual machine name is required for almost every command you will run. libvirt refers to this name as a domain or domain name. It is required for most of these commands as they manipulate the specified virtual machine directly. The name may be given as a short integer (0,1,2...), a text string name, or a full UUID. For the sake of simplicity, all virtual machines used in this section will have the domain set as 'guest1'.

⁠23.9.1. Starting a virtual machine

The virsh start <domain> [--console] [--paused] [--autodestroy] [--bypass-cache] [--force-boot] [--pass-fds <string>] command starts a inactive virtual machine that was already defined but whose state is inactive since its last managed save state or a fresh boot. The command can take the following arguments:

--console - will boot the guest virtual machine in console mode

--paused - If this is supported by the driver it will boot the guest virtual machine and then put it into a paused state

--autodestroy - the guest virtual machine is automatically destroyed when virsh disconnects

--bypass-cache - used if the guest virtual machine is in the managedsave

--force-boot - discards any managedsave options and causes a fresh boot to occur

--pass-fds <string> - is a list of additional arguments separated by commas, which are passed onto the guest virtual machine.

⁠23.9.2. Configuring a virtual machine to be started automatically at boot

The $ virsh autostart [--disable] domain command will automatically start the guest virtual machine when the host machine boots. Adding the --disable argument to this command will cause the guest not to start automatically when the host physical machine boots. For example:

# virsh autostart guest1

⁠23.9.3. Suspending a guest virtual machine

The virsh suspend <domain> command suspends a guest virtual machine. For example:

# virsh suspend guest1

When a guest virtual machine is in a suspended state, it consumes system RAM but not processor resources. Disk and network I/O does not occur while the guest virtual machine is suspended. This operation is immediate and the guest virtual machine can be restarted with the resume (Section 23.9.7, “Resuming a guest virtual machine”) option.

⁠23.9.4. Suspending a running guest virtual machine

The virsh dompmsuspend <domain> [--duration <number>] [--target <string>] command will take a running guest virtual machine and suspended it so it can be placed into one of three possible states (S3, S4, or a hybrid of the two).

This command can take the following arguments:

--duration - sets the duration for the state change in seconds

--target - can be either mem (suspend to RAM (S3))disk (suspend to disk (S4)), or hybrid (hybrid suspend)

For example:

# virsh dompmsuspend guest1 --duration 100 --target mem

In this example, the guest1 virtual machine is suspended and placed into a mem (suspend to RAM (S3) state for 100 seconds.

⁠23.9.5. Waking up a guest virtual machine from pmsuspend state

The dompmwakeup <domain> command will inject a wake-up alert to a guest that was suspended using the virsh dompmsuspend command, rather than waiting for the duration time set in the command to expire. This operation will fail if the guest virtual machine is shutdown. For example:

# dompmwakeup guest1

⁠23.9.6. Undefining a domain

The virsh undefine <domain> [--managed-save] [<storage>] [--remove-all-storage] [--wipe-storage] [--snapshots-metadata] [--nvram] command undefines an inactive guest virtual machine. Once undefined, the configuration is removed. If the command is run on a running guest virtual machine it will convert the running guest virtual machine into a transient guest virtual machine without stopping it.

This command can take the following arguments:

--managed-save - this argument guarantees that any managed save image is also cleaned up. Without using this argument, attempts to undefine a guest virtual machine with a managed save will fail.

--snapshots-metadata - this argument guarantees that any snapshots (as shown with snapshot-list) are also cleaned up when undefining an inactive guest virtual machine. Note that any attempts to undefine an inactive guest virtual machine with snapshot metadata will fail. If this argument is used and the guest virtual machine is active, it is ignored.

--storage - using this argument requires a comma separated list of volume target names or source paths of storage volumes to be removed along with the undefined domain. This action will undefine the storage volume before it is removed. Note that this can only be done with inactive guest virtual machines. Note too that this will only work with storage volumes that are managed by libvirt.

--remove-all-storage - in addition to undefining the guest virtual machine, all associated storage volumes are deleted.

--wipe-storage - in addition to deleting the storage volume, the contents are wiped.

To undefine the guest virtual machine guest1 and remove all associated storage volumes for example:

# virsh undefine guest1 --remove-all-storage

⁠23.9.7. Resuming a guest virtual machine

The virsh resume <domain> command restores a suspended guest virtual machine. For example to restore the guest1 virtual machine:

# virsh resume guest1

This operation is immediate. The guest virtual machine parameters and configuration files are preserved when suspend and resume operations are performed. Note that this action will not resume a guest virtual machine that has been undefined.

⁠23.9.8. Saving a guest virtual machine

The virsh save [--bypass-cache] <domain> <file> [--xml <string>] [--running] [--paused] [--verbose] command saves the current state of a guest virtual machine to a specified file. This stops the guest virtual machine you specify and saves the data to a file, which may take some time given the amount of memory in use by your guest virtual machine. You can restore the state of the guest virtual machine with the virsh restore (Section 23.9.12, “Restoring a guest virtual machine”) command. Save is similar to pause, but instead of just pausing a guest virtual machine the present state of the guest virtual machine is saved as well. You can revert back to this saved state should you need to. If you want to restore the guest virtual machine directly from the XML file that you saved, run the the virsh restore command. You can monitor the process with the virsh domjobinfo command and cancel it with the virsh domjobabort command.

The virsh save command can take the following arguments:

--bypass-cache - causes the restore to avoid the file system cache but note that using this flag may slow down the restore operation.

--xml - this argument must be used with an XML file name. Although this argument is usually omitted, it can be used to supply an alternative XML file for use on a restored guest virtual machine with changes only in the host-specific portions of the domain XML. For example, it can be used to account for the file naming differences in underlying storage due to disk snapshots taken after the guest was saved.

--running - overrides the state recorded in the save image to start the guest virtual machine as running.

--paused- overrides the state recorded in the save image to start the guest virtual machine as paused.

--verbose - displays the progress of the save.

For example, to save the guest1 running configuration state:

# virsh save guest1 guest1-config.xml --running

⁠23.9.9. Updating the domain XML file that will be used for restoring a guest virtual machine

The virsh save-image-define <file> [--xml <string>] [--running] [--paused] command will update the domain XML file that will be used when the specified file is later used during the virsh restore command. The --xml argument must be an XML file name containing the alternative XML with changes only in the host physical machine specific portions of the domain XML. For example, it can be used to account for the file naming differences resulting from creating disk snapshots of underlying storage after the guest was saved. The save image records if the guest virtual machine should be restored to a running or paused state. Using the arguments --running or --paused dictates the state that is to be used. For example to update the domain XML file for guest1's running state run the following command:

# virsh save-image-define guest1-config.xml --running

⁠23.9.10. Extracting the domain XML file

save-image-dumpxml file --security-info command will extract the domain XML file that was in effect at the time the saved state file (used in the virsh save command) was referenced. Using the --security-info argument includes security sensitive information in the file.

⁠23.9.11. Editing the guest virtual machine configuration files

save-image-edit <file> [--running] [--paused] command edits the XML configuration file that was created by the virsh save command. Refer to Section 23.9.8, “Saving a guest virtual machine” for information on the virsh save command.

Note that the save image records whether the guest virtual machine should be restored to a --running or --paused state. Without using these arguments the state is determined by the file itself. By selecting --running or --paused you can overwrite the state that virsh restore should use. For example, to edit the guest1 file running configuration file:

# virsh save-image-edit guest1-config.xml --running

⁠23.9.12. Restoring a guest virtual machine

The virsh restore <file> [--bypass-cache] [--xml <string>] [--running] [--paused] command restores a guest virtual machine previously saved with the virsh save command. Refer to Section 23.9.8, “Saving a guest virtual machine” for information on the virsh save command. The restore action restarts the saved guest virtual machine, which may take some time. The guest virtual machine's name and UUID are preserved but are allocated for a new UUID.

The virsh restore command can take the following arguments:

--bypass-cache - causes the restore to avoid the file system cache but note that using this flag may slow down the restore operation.

--xml - this argument must be used with an XML file name. Although this argument is usually omitted, it can be used to supply an alternative XML file for use on a restored guest virtual machine with changes only in the host-specific portions of the domain XML. For example, it can be used to account for the file naming differences in underlying storage due to disk snapshots taken after the guest was saved.

--running - overrides the state recorded in the save image to start the guest virtual machine as running.

--paused- overrides the state recorded in the save image to start the guest virtual machine as paused.

For example, to restore the guest1 guest virtual machine and its running configuration file:

# virsh restore guest1-config.xml --running

⁠23.10. Shutting down, rebooting, and forcing a shutdown of a guest virtual machine

⁠23.10.1. Shutting down a guest virtual machine

The virsh shutdown <domain> [--mode <string>] command shuts down a guest virtual machine. You can control the behavior of how the guest virtual machine reboots by modifying the <on_shutdown> parameter in the guest virtual machine's configuration file.

The virsh shutdown command command can take the following optional argument:

--mode chooses the shutdown mode. This can be either acpi, agent, initctl, signal, or paravirt

For example, to shutdown the guest1 virtual machine in acpi mode:

# virsh shutdown guest1 --mode acpi

⁠23.10.2. Shutting down Red Hat Enterprise Linux 6 guests on a Red Hat Enterprise Linux 7 host

Installing Red Hat Enterprise Linux 6 guest virtual machines with the Minimal installation option does not install the acpid (acpi daemon). Red Hat Enterprise Linux 7 no longer requires this package, as it has been taken over by systemd. However, Red Hat Enterprise Linux 6 guest virtual machines running on a Red Hat Enterprise Linux 7 host still require it.

Without the acpid package, the Red Hat Enterprise Linux 6 guest virtual machine does not shut down when the virsh shutdown command is executed. The virsh shutdown command is designed to gracefully shut down guest virtual machines.

Using the virsh shutdown command is easier and safer for system administration. Without graceful shut down with the virsh shutdown command a system administrator must log into a guest virtual machine manually or send the Ctrl-Alt-Del key combination to each guest virtual machine.

Note

Other virtualized operating systems may be affected by this issue. The virsh shutdown command requires that the guest virtual machine operating system is configured to handle ACPI shut down requests. Many operating systems require additional configurations on the guest virtual machine operating system to accept ACPI shut down requests.

⁠

Procedure 23.1. Workaround for Red Hat Enterprise Linux 6 guests

Install the acpid package

The acpid service listens and processes ACPI requests.

Log into the guest virtual machine and install the acpid package on the guest virtual machine:

# yum install acpid

Enable the acpid service on the guest

Set the acpid service to start during the guest virtual machine boot sequence and start the service:

# enable acpid service
# service acpid start

Prepare guest domain xml

Edit the domain XML file to include the following element. Replace the virtio serial port with org.qemu.guest_agent.0 and use your guest's name instead of the one shown. In this example, the guest is guest1. Remember to save the file.

⁠

​
​
​<channel type='unix'>
​   <source mode='bind' path='/var/lib/libvirt/qemu/guest1.agent'/>
​   <target type='virtio' name='org.qemu.guest_agent.0'/>
​</channel>
​
​

Figure 23.1. Guest XML replacement

Install the QEMU guest agent

Install the QEMU guest agent (QEMU-GA) and start the service as directed in the Red Hat Enterprise Linux 6 Virtualization Administration Guide..

Shutdown the guest

List the known guest virtual machines so you can retrieve the name of the one you want to shutdown.

# virsh list --all
   Id Name              State
----------------------------------
  14 guest1             running

Shut down the guest virtual machine.

# virsh shutdown guest1

guest virtual machine guest1 is being shutdown

Wait a few seconds for the guest virtual machine to shut down. Verify it is shutdown.

# virsh list --all
 Id Name                 State
----------------------------------
 14 guest1               shut off

Start the guest virtual machine named guest1, with the XML file you edited.

# virsh start guest1

Shut down the acpi in the guest1 guest virtual machine.

# virsh shutdown --mode acpi guest1

List all the guest virtual machines again, guest1 should still be on the list, and it should indicate it is shut off.

# virsh list --all
   Id Name                 State
----------------------------------
   14 guest1               shut off

Start the guest virtual machine named guest1, with the XML file you edited.

# virsh start guest1

Shut down the guest1 guest virtual machine guest agent.

# virsh shutdown --mode agent guest1

List the guest virtual machines. guest1 should still be on the list, and it should indicate it is shut off.

# virsh list --all
   Id Name                 State
----------------------------------
   guest1                shut off

The guest virtual machine will shut down using the virsh shutdown command for the consecutive shutdowns, without using the workaround described above.

In addition to the method described above, a guest can be automatically shutdown, by stopping the libvirt-guest service. Refer to Section 23.10.3, “Manipulating the libvirt-guests configuration settings” for more information on this method.

⁠23.10.3. Manipulating the libvirt-guests configuration settings

The libvirt-guests service has parameter settings that can be configured to assure that the guest is shutdown properly. It is a package that is a part of the libvirt installation and is installed by default. This service automatically saves guests to the disk when the host shuts down, and restores them to their pre-shutdown state when the host reboots. By default, this setting is set to suspend the guest. If you want the guest to be gracefully shutdown, you will need to change one of the parameters of the libvirt-guests configuration file.

⁠

Procedure 23.2. Changing the libvirt-guests service parameters to allow for the graceful shutdown of guests

The procedure described here allows for the graceful shutdown of guest virtual machines when the host physical machine is stuck, powered off, or needs to be restarted.

Open the configuration file

The configuration file is located in /etc/sysconfig/libvirt-guests. Edit the file, remove the comment mark (#) and change the ON_SHUTDOWN=suspend to ON_SHUTDOWN=shutdown. Remember to save the change.

$ vi /etc/sysconfig/libvirt-guests

# URIs to check for running guests
# example: URIS='default xen:/// vbox+tcp://host/system lxc:///'
#URIS=default

# action taken on host boot
# - start   all guests which were running on shutdown are started on boot
#           regardless on their autostart settings
# - ignore  libvirt-guests init script won't start any guest on boot, however,
#           guests marked as autostart will still be automatically started by
#           libvirtd
#ON_BOOT=start

# Number of seconds to wait between each guest start. Set to 0 to allow
# parallel startup.
#START_DELAY=0

# action taken on host shutdown
# - suspend   all running guests are suspended using virsh managedsave
# - shutdown  all running guests are asked to shutdown. Please be careful with
#             this settings since there is no way to distinguish between a
#             guest which is stuck or ignores shutdown requests and a guest
#             which just needs a long time to shutdown. When setting
#             ON_SHUTDOWN=shutdown, you must also set SHUTDOWN_TIMEOUT to a
#             value suitable for your guests.
ON_SHUTDOWN=shutdown

# If set to non-zero, shutdown will suspend guests concurrently. Number of
# guests on shutdown at any time will not exceed number set in this variable.
#PARALLEL_SHUTDOWN=0

# Number of seconds we're willing to wait for a guest to shut down. If parallel
# shutdown is enabled, this timeout applies as a timeout for shutting down all
# guests on a single URI defined in the variable URIS. If this is 0, then there
# is no time out (use with caution, as guests might not respond to a shutdown
# request). The default value is 300 seconds (5 minutes).
#SHUTDOWN_TIMEOUT=300

# If non-zero, try to bypass the file system cache when saving and
# restoring guests, even though this may give slower operation for
# some file systems.
#BYPASS_CACHE=0

URIS - checks the specified connections for a running guest. The Default setting functions in the same manner as virsh does when no explicit URI is set In addition, one can explicitly set the URI from /etc/libvirt/libvirt.conf. It should be noted that when using the libvirt configuration file default setting, no probing will be used.

ON_BOOT - specifies the action to be done to / on the guests when the host boots. The start option starts all guests that were running prior to shutdown regardless on their autostart settings. The ignore option will not start the formally running guest on boot, however, any guest marked as autostart will still be automatically started by libvirtd.

The START_DELAY - sets a delay interval in between starting up the guests. This time period is set in seconds. Use the 0 time setting to make sure there is no delay and that all guests are started simultaneously.

ON_SHUTDOWN - specifies the action taken when a host shuts down. Options that can be set include: suspend which suspends all running guests using virsh managedsave and shutdown which shuts down all running guests. It is best to be careful with using the shutdown option as there is no way to distinguish between a guest which is stuck or ignores shutdown requests and a guest that just needs a longer time to shutdown. When setting the ON_SHUTDOWN=shutdown, you must also set SHUTDOWN_TIMEOUT to a value suitable for the guests.

PARALLEL_SHUTDOWN Dictates that the number of guests on shutdown at any time will not exceed number set in this variable and the guests will be suspended concurrently. If set to 0, then guests are not shutdown concurrently.

Number of seconds to wait for a guest to shut down. If SHUTDOWN_TIMEOUT is enabled, this timeout applies as a timeout for shutting down all guests on a single URI defined in the variable URIS. If SHUTDOWN_TIMEOUT is set to 0, then there is no time out (use with caution, as guests might not respond to a shutdown request). The default value is 300 seconds (5 minutes).

BYPASS_CACHE can have 2 values, 0 to disable and 1 to enable. If enabled it will by-pass the file system cache when guests are restored. Note that setting this may effect performance and may cause slower operation for some file systems.

Start libvirt-guests service

If you have not started the service, start the libvirt-guests service. Do not restart the service as this will cause all running guest virtual machines to shutdown.

⁠23.10.4. Rebooting a guest virtual machine

Reboot a guest virtual machine using virsh reboot command. Remember that this action will return once it has executed the reboot, but there may be a time lapse from that point until the guest virtual machine actually reboots.

#virsh reboot {domain-id, domain-name or domain-uuid} [--mode method]

You can control the behavior of the rebooting guest virtual machine by modifying the on_reboot element in the guest virtual machine's configuration file.

By default, the hypervisor will try to pick a suitable shutdown method. To specify an alternative method, the --mode argument can specify a comma separated list which includes initctl, acpi, agent, signal. The order in which drivers will try each mode is undefined, and not related to the order specified in virsh. For strict control over ordering, use a single mode at a time and repeat the command.

⁠23.10.5. Forcing a guest virtual machine to stop

Force a guest virtual machine to stop with the virsh destroy command:

# virsh destroy {domain-id, domain-name or domain-uuid} [--graceful]

This command does an immediate ungraceful shutdown and stops the specified guest virtual machine. Using virsh destroy can corrupt guest virtual machine file systems. Use the destroy option only when the guest virtual machine is unresponsive. If you want to initiate a graceful shutdown, use the virsh destroy --graceful command.

⁠23.10.6. Resetting a virtual machine

virsh reset domain resets the guest virtual machine immediately without any guest shutdown. A reset emulates the power reset button on a machine, where all guest hardware sees the RST line and re-initializes the internal state. Note that without any guest virtual machine OS shutdown, there are risks for data loss.

⁠23.10.7. Connecting the serial console for the guest virtual machine

The $ virsh console <domain> [--devname <string>] [--force] [--safe] command connects the virtual serial console for the guest virtual machine. The optional --devname <string> parameter refers to the device alias of an alternate console, serial, or parallel device configured for the guest virtual machine. If this parameter is omitted, the primary console will be opened. The --force argument will force the console connection or when used with disconnect, will disconnect connections. Using the --safe argument will only allow the guest to connect if safe console handling is supported.

$ virsh console guest1 --safe

⁠23.10.8. Defining a guest virtual machine with an XML file

The virsh define <FILE> command defines a guest virtual machine from an XML file. The guest virtual machine definition in this case is registered but not started. If the guest virtual machine is already running, the changes will take effect on the next boot.

$ virsh define --file guest1.xml

⁠23.10.9. Injecting NMI

The $ virsh inject-nmi [domain] injects NMI (non-maskable interrupt) message to the guest virtual machine. This is used when response time is critical, such as non-recoverable hardware errors. To run this command:

$ virsh inject-nmi guest1

⁠23.10.10. Displaying device block statistics

This command will display the block statistics for a running guest virtual machine. You need to have both the guest virtual machine name and the device name (use the virsh domblklist to list the devices.)In this case a block device is the unique target name (<target dev='name'/>) or a source file (< source file ='name'/>). Note that not every hypervisor can display every field. To make sure that the output is presented in its most legible form use the --human argument, as shown:

# virsh domblklist guest1
Target     Source
------------------------------------------------
vda        /VirtualMachines/guest1.img
hdc        -

# virsh domblkstat --human guest1 vda
Device: vda
 number of read operations:      174670
 number of bytes read:           3219440128
 number of write operations:     23897
 number of bytes written:        164849664
 number of flush operations:     11577
 total duration of reads (ns):   1005410244506
 total duration of writes (ns):  1085306686457
 total duration of flushes (ns): 340645193294

⁠23.10.11. Retrieving network statistics

The virsh domnetstat [domain][interface-device] command displays the network interface statistics for the specified device running on a given guest virtual machine (domain).

# virsh domifstat guest1 eth0

⁠23.10.12. Modifying the link state of a guest virtual machine's virtual interface

The virsh domif-setlink command can either configure a specified interface as up or down. The virsh domif-setlink <domain> <interface> <state> [--config] modifies the status of the specified interface for the specified guest virtual machine. Note that if you only want the persistent configuration of the guest virtual machine to be modified, you need to use the --configargument. It should also be noted that for compatibility reasons, --persistent is an alias of --config. The "interface device" can be the interface's target name or the MAC address.

# virsh domif-setlink guest1 eth0 up

⁠23.10.13. Listing the link state of a guest virtual machine's virtual interface

The virsh domif-getlink command can be used to query the state of a specified interface on a given guest virtual machine. Note that if you only want the persistent configuration of the guest to be modified, you need to use the --configargument. It should also be noted that for compatibility reasons, --persistent is an alias of --config. The "interface device" can be the interface's target name or the MAC address.

# virsh domif-getlink guest1 eth0 up

⁠23.10.14. Setting network interface bandwidth parameters

The virsh domiftune sets the guest virtual machine's network interface bandwidth parameters. The following format should be used:

#virsh domiftune [domain] interface-device [[--config] [--live] | [--current]] [--inbound average,peak,burst] [--outbound average,peak,burst]

The only required parameter is the guest's name and interface device of the guest virtual machine, the --config, --live, and --current functions the same as in Section 23.23, “Setting schedule parameters”. If no limit is specified, it will query current network interface setting. Otherwise, alter the limits with the following flags:

<interface-device> This is mandatory and it will set or query the guest virtual machine’s network interface’s bandwidth parameters. interface-device can be the interface’s target name (<target dev=’name’/>), or the MAC address.

If no --inbound or --outbound is specified, this command will query and show the bandwidth settings. Otherwise, it will set the inbound or outbound bandwidth. average,peak,burst is the same as in attach-interface command. Refer to Section 23.12.6.2, “Attaching interface devices”

⁠23.10.15. Retrieving memory statistics for a running guest virtual machine

This command may return varied results depending on the hypervisor you are using.

The dommemstat [domain] [--period (sec)][[--config][--live]|[--current]] displays the memory statistics for a running guest virtual machine. Using the --period argument requires a time period in seconds. Setting this argument to a value larger than 0 will allow the balloon driver to return additional statistics which will be displayed by subsequent domemstat commands. Setting the --period argument to 0, will stop the balloon driver collection but does not clear the statistics in the balloon driver. You cannot use the --live, --config, or --current arguments without also setting the --period option in order to also set the collection period for the balloon driver. If the --live argument is specified, only the running guest's collection period is affected. If the --configargument is used, it will affect the next boot of a persistent guest. If --current argument is used, it will affect the current guest state

Both the --live and --config arguments may be used but --current is exclusive. If no flag is specified, the behavior will be different depending on the guest's state.

#virsh domemstat guest1--current

⁠23.10.16. Displaying errors on block devices

This command is best used following a domstate that reports that a guest virtual machine is paused due to an I/O error. The domblkerror domain command shows all block devices that are in error state on a given guest virtual machine and it displays the error message that the device is reporting.

# virsh domblkerror guest1

⁠23.10.17. Displaying the block device size

In this case a block device is the unique target name (<target dev='name'/>) or a source file (< source file ='name'/>). To retrieve a list you can run domblklist. This domblkinfo requires a domain name.

# virsh domblkinfo guest1

⁠23.10.18. Displaying the block devices associated with a domain

The domblklist domain --inactive--details displays a table of all block devices that are associated with the specified guest virtual machine.

If --inactive is specified, the result will show the devices that are to be used at the next boot and will not show those that are currently running in use by the running guest virtual machine. If --details is specified, the disk type and device value will be included in the table. The information displayed in this table can be used with the domblkinfo and snapshot-create.

#domblklist guest1 --details

⁠23.10.19. Displaying virtual interfaces associated with a guest virtual machine

Running the domiflist command results in a table that displays information of all the virtual interfaces that are associated with a specified domain. The domiflist requires a domain name and optionally can take the --inactive argument.

If --inactive is specified, the result will show the devices that are to be used at the next boot and will not show those that are currently running in use by the running guest virtual machine.

Commands that require a MAC address of a virtual interface (such as detach-interface or domif-setlink) will accept the output displayed by this command.

⁠23.10.20. Using blockcommit to shorten a backing chain

This section demonstrates how to use blockcommit to shorten a backing chain. For more background on backing chains, see Section 23.10.23, “Disk image management with live block copy”.

blockcommit copies data from one part of the chain down into a backing file, allowing you to pivot the rest of the chain in order to bypass the committed portions. For example, suppose this is the current state:

      base ← snap1 ← snap2 ← active.

Using blockcommit moves the contents of snap2 into snap1, allowing you to delete snap2 from the chain, making backups much quicker.

⁠

Procedure 23.3. virsh blockcommit

Run the following command:

# virsh blockcommit $dom $disk -base snap1 -top snap2 -wait -verbose

The contents of snap2 are moved into snap1, resulting in:

base ← snap1 ← active. Snap2 is no longer valid and can be deleted

Warning

blockcommit will corrupt any file that depends on the -base argument (other than files that depended on the -top argument, as those files now point to the base). To prevent this, do not commit changes into files shared by more than one guest. The -verbose option will allow the progress to be printed on the screen.

⁠23.10.21. Using blockpull to shorten a backing chain

blockpull can be used in in the following applications:

Flattens an image by populating it with data from its backing image chain. This makes the image file self-contained so that it no longer depends on backing images and looks like this:

Before: base.img ← Active

After: base.img is no longer used by the guest and Active contains all of the data.

Flattens part of the backing image chain. This can be used to flatten snapshots into the top-level image and looks like this:

Before: base ← sn1 ←sn2 ← active

After: base.img ← active. Note that active now contains all data from sn1 and sn2 and neither sn1 nor sn2 are used by the guest.

Moves the disk image to a new file system on the host. This is allows image files to be moved while the guest is running and looks like this:

Before (The original image file): /fs1/base.vm.img

After: /fs2/active.vm.qcow2 is now the new file system and /fs1/base.vm.img is no longer used.

Useful in live migration with post-copy storage migration. The disk image is copied from the source host to the destination host after live migration completes.

In short this is what happens: Before:/source-host/base.vm.img After:/destination-host/active.vm.qcow2./source-host/base.vm.img is no longer used.

⁠

Procedure 23.4. Using blockpull to shorten a backing chain

It may be helpful to run this command prior to running blockpull:

# virsh snapshot-create-as $dom $name - disk-only

If the chain looks like this: base ← snap1 ← snap2 ← active run the following:

# virsh blockpull $dom $disk snap1

This command makes 'snap1' the backing file of active, by pulling data from snap2 into active resulting in: base ← snap1 ← active.

Once the blockpull is complete, the libvirt tracking of the snapshot that created the extra image in the chain is no longer useful. Delete the tracking on the outdated snapshot with this command:

# virsh snapshot-delete $dom $name - metadata

Additional applications of blockpull can be done as follows:

To flatten a single image and populate it with data from its backing image chain:# virsh blockpull example-domain vda - wait

To flatten part of the backing image chain:# virsh blockpull example-domain vda - base /path/to/base.img - wait

To move the disk image to a new file system on the host:# virsh snapshot-create example-domain - xmlfile /path/to/new.xml - disk-only followed by # virsh blockpull example-domain vda - wait

To use live migration with post-copy storage migration:

On the destination run:

 # qemu-img create -f qcow2 -o backing_file=/source-host/vm.img /destination-host/vm.qcow2

On the source run:

# virsh migrate example-domain

On the destination run:

# virsh blockpull example-domain vda - wait

⁠23.10.22. Using blockresize to change the size of a guest virtual machine path

blockresize can be used to resize a block device of a guest virtual machine while the guest virtual machine is running, using the absolute path of the block device which also corresponds to a unique target name (<target dev="name"/>) or source file (<source file="name"/>). This can be applied to one of the disk devices attached to guest virtual machine (you can use the command domblklist to print a table showing the brief information of all block devices associated with a given guest virtual machine).

Note

Live image resizing will always resize the image, but may not immediately be picked up by guests. With recent guest kernels, the size of virtio-blk devices is automatically updated (older kernels require a guest reboot). With SCSI devices, it is required to manually trigger a re-scan in the guest with the command, echo > /sys/class/scsi_device/0:0:0:0/device/rescan. In addition, with IDE it is required to reboot the guest before it picks up the new size.

Run the following command: blockresize [domain] [path size] where:

Domain is the unique target name or source file of the guest virtual machine whose size you want to change

Path size is a scaled integer which defaults to KiB (blocks of 1024 bytes) if there is no suffix. You must use a suffix of "B" to for bytes.

⁠23.10.23. Disk image management with live block copy

Note

Live block copy is a feature that is not supported with the version of KVM that is supplied with Red Hat Enterprise Linux. Live block copy is available with the version of KVM that is supplied with Red Hat Virtualization. This version of KVM must be running on your physical host machine in order for the feature to be supported. Contact your representative at Red Hat for more details.

Live block copy allows you to copy an in use guest disk image to a destination image and switches the guest disk image to the destination guest image while the guest is running. Whilst live migration moves the memory and registry state of the host, the guest is kept in shared storage. Live block copy allows you to move the entire guest contents to another host on the fly while the guest is running. Live block copy may also be used for live migration without requiring permanent share storage. In this method the disk image is copied to the destination host after migration, but while the guest is running.

Live block copy is especially useful for the following applications:

moving the guest image from local storage to a central location

when maintenance is required, guests can be transferred to another location, with no loss of performance

allows for management of guest images for speed and efficiency

image format conversions can be done without having to shut down the guest

⁠

Example 23.5. Example (live block copy)

This example shows what happens when live block copy is performed. The example has a backing file (base) that is shared between a source and destination. It also has two overlays (sn1 and sn2) that are only present on the source and must be copied.

The backing file chain at the beginning looks like this:

base ← sn1 ← sn2

The components are as follows:

base - the original disk image

sn1 - the first snapshot that was taken of the base disk image

sn2 - the most current snapshot

active - the copy of the disk

When a copy of the image is created as a new image on top of sn2 the result is this:

base ← sn1 ← sn2 ← active

At this point the read permissions are all in the correct order and are set automatically. To make sure write permissions are set properly, a mirror mechanism redirects all writes to both sn2 and active, so that sn2 and active read the same at any time (and this mirror mechanism is the essential difference between live block copy and image streaming).

A background task that loops over all disk clusters is executed. For each cluster, there are the following possible cases and actions:

The cluster is already allocated in active and there is nothing to do.

Use bdrv_is_allocated() to follow the backing file chain. If the cluster is read from base (which is shared) there is nothing to do.

If bdrv_is_allocated() variant is not feasible, rebase the image and compare the read data with write data in base in order to decide if a copy is needed.

In all other cases, copy the cluster into active

When the copy has completed, the backing file of active is switched to base (similar to rebase)

To reduce the length of a backing chain after a series of snapshots, the following commands are helpful: blockcommit and blockpull. See Section 23.10.20, “Using blockcommit to shorten a backing chain” for more information.

⁠23.10.24. Displaying a URI for connection to a graphical display

Running the virsh domdisplay command will output a URI which can then be used to connect to the graphical display of the guest virtual machine via VNC, SPICE, or RDP. If the argument --include-password is used, the SPICE channel password will be included in the URI.

⁠23.10.25. Discarding blocks not in use

The virsh domfstrim doman --minium --mountpoint command will issue a fstrim on all mounted files ystems within a running specified guest virtual machine. It will discard the blocks not in use by the file system. if the argument --minimum is used, an amount in bytes must be specified. This amount will be sent to the guest kernel as its length of contiguous free range. Values smaller than this amount may be ignored. Increasing this value will create competition with file systems with badly fragmented free space. Note that not all blocks in this case are discarded. The default minimum is zero which means that every free block is discarded. If a user only wants to trim one mount point, the --mountpoint argument should be used and a mount point should be specified.

⁠23.10.26. Guest virtual machine retrieval commands

The following commands will display different information about a given guest virtual machine

virsh domhostname <domain> displays the hostname of the specified guest virtual machine provided the hypervisor can publish it. This command may also be used with the option [--domain] <string> where the name of the guest is included as the <string>. For example:

virsh domhostname guest1

or

virsh domhostname --domainguest1

.

virsh dominfo <domain> displays basic information about a specified guest virtual machine. This command may also be used with the option [--domain] <string> where the name of the guest is included as the <string>. For example:

virsh dominfoguest1

or

virsh dominfo --domainguest1

.

virsh domid <domain>|<ID> converts a given guest virtual machine name or UUID into an ID. If the ID given is valid and the guest virtual machine is shut off, the machine name will be displayed as a series of dashes ('-----'). This command may also be used with the option [--domain] <string> where the name of the guest is included as the <string>. For example:

virsh domidguest1

or

virsh domid --domainguest1

.

virsh domjobabort <domain> aborts the currently running job on the specified guest virtual machine. This command may also be used with the option [--domain] <string> where the name of the guest is included as the <string>. For example:

virsh domjobabortguest1

or

virsh domjobabort --domainguest1

.

virsh domjobinfo <domain> displays information about jobs running on the specified guest virtual machine, including migration statistics. This command may also be used with the option [--domain] <string> where the name of the guest is included as the <string> as well as --completed to return information on the statistics of a recently completed job. For example:

virsh domjobabortguest1

or

virsh domjobabort --domainguest1 --completed

.

virsh domname <domain><domain ID>|<UUID> converts a given guest virtual machine ID or UUID into a guest virtual machine name. This command may also be used with the option [--domain] <string> where the name of the guest is included as the <string>. For example:

virsh domnameguest1

or

virsh domname --domainguest1

.

virsh domstate <domain> displays the state of the given guest virtual machine. Using the --reason argument will also display the reason for the displayed state.This command may also be used with the option [--domain] <string> where the name of the guest is included as the <string> as well as the option --reason, which displays the reason for the state. For example:

virsh domnameguest1

or

virsh domname --domainguest1 --reason

.

virsh domcontrol <domain> displays the state of an interface to VMM that were used to control a guest virtual machine. For states that are not OK or Error, it will also print the number of seconds that have elapsed since the control interface entered the displayed state. This command may also be used with the option [--domain] <string> where the name of the guest is included as the <string>. For example:

virsh domcontrolguest1

or

virsh domcontrol --domainguest1

.

⁠

Example 23.6. Example of statistical feedback

In order to get information about the guest virtual machine, run the following command:

# virsh domjobinfo guest1
Job type:         Unbounded
Time elapsed:     1603         ms
Data processed:   47.004 MiB
Data remaining:   658.633 MiB
Data total:       1.125 GiB
Memory processed: 47.004 MiB
Memory remaining: 658.633 MiB
Memory total:     1.125 GiB
Constant pages:   114382
Normal pages:     12005
Normal data:      46.895 MiB
Expected downtime: 0            ms
Compression cache: 64.000 MiB
Compressed data:  0.000 B
Compressed pages: 0
Compression cache misses: 12005
Compression overflows: 0

⁠23.10.27. Converting QEMU arguments to domain XML

The virsh domxml-from-native provides a way to convert an existing set of QEMU arguments into a guest description using libvirt Domain XML that can then be used by libvirt. Please note that this command is intended to be used only to convert existing qemu guests previously started from the command line in order to allow them to be managed through libvirt. The method described here should not be used to create new guests from scratch. New guests should be created using either virsh or virt-manager. Additional information can be found here.

Suppose you have a QEMU guest with the following args file:

 $ cat demo.args
LC_ALL=C
PATH=/bin
HOME=/home/test
USER=test
LOGNAME=test /usr/bin/qemu -S -M pc -m 214 -smp 1 -nographic -monitor pty -no-acpi -boot c -hda /dev/HostVG/QEMUGuest1 -net none -serial none -parallel none -usb

To convert this to a domain XML file so that the guest can be managed by libvirt, run:

$ virsh domxml-from-native qemu-argv demo.args

This command turns the args file above, into this domain XML file:

<domain type='qemu'>
  <uuid>00000000-0000-0000-0000-000000000000</uuid>
  <memory>219136</memory>
  <currentMemory>219136</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='i686' machine='pc'>hvm</type>
    <boot dev='hd'/>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu</emulator>
    <disk type='block' device='disk'>
      <source dev='/dev/HostVG/QEMUGuest1'/>
      <target dev='hda' bus='ide'/>
    </disk>
  </devices>
</domain>

⁠23.10.28. Creating a dump file of a guest virtual machine's core

Sometimes it is necessary (especially in the cases of troubleshooting), to create a dump file containing the core of the guest virtual machine so that it can be analyzed. In this case, running virsh dump domain corefilepath --bypass-cache --live |--crash |--reset --verbose --memory-only dumps the guest virtual machine core to a file specified by the corefilepath. Note that some hypervisors may gave restrictions on this action and may require the user to manually ensure proper permissions on the file and path specified in the corefilepath parameter. This command is supported with SR-IOV devices as well as other passthrough devices. The following arguments are supported and have the following effect:

--bypass-cache the file saved will not contain the file system cache. Note that selecting this option may slow down dump operation.

--live will save the file as the guest virtual machine continues to run and will not pause or stop the guest virtual machine.

--crash puts the guest virtual machine in a crashed status rather than leaving it in a paused state while the dump file is saved.

--reset once the dump file is successfully saved, the guest virtual machine will reset.

--verbose displays the progress of the dump process

--memory-only running a dump using this option will create a dump file where the contents of the dump file will only contain the guest virtual machine's memory and CPU common register file. This option should be used in cases where running a full dump will fail. This may happen when a guest virtual machine cannot be live migrated (due to a passthrough PCI device).

Note that the entire process can be monitored using the domjobinfo command and can be canceled using the domjobabort command.

⁠23.10.29. Creating a virtual machine XML dump (configuration file)

Output a guest virtual machine's XML configuration file with virsh:

# virsh dumpxml {guest-id, guestname or uuid}

This command outputs the guest virtual machine's XML configuration file to standard out (stdout). You can save the data by piping the output to a file. An example of piping the output to a file called guest.xml:

# virsh dumpxml GuestID > guest.xml

This file guest.xml can recreate the guest virtual machine (refer to Section 23.11, “Editing a guest virtual machine's configuration file”. You can edit this XML configuration file to configure additional devices or to deploy additional guest virtual machines.

An example of virsh dumpxml output:

# virsh dumpxml guest1
<domain type='kvm'>
  <name>guest1-rhel6-64</name>
  <uuid>b8d7388a-bbf2-db3a-e962-b97ca6e514bd</uuid>
  <memory>2097152</memory>
  <currentMemory>2097152</currentMemory>
  <vcpu>2</vcpu>
  <os>
    <type arch='x86_64' machine='rhel6.2.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='none' io='threads'/>
      <source file='/home/guest-images/guest1-rhel6-64.img'/>
      <target dev='vda' bus='virtio'/>
      <shareable/<
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
    <interface type='bridge'>
      <mac address='52:54:00:b9:35:a9'/>
      <source bridge='br0'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
    <input type='tablet' bus='usb'/>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes'/>
    <sound model='ich6'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </sound>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </memballoon>
  </devices>
</domain>

Note that the <shareable/> flag is set. This indicates the device is expected to be shared between guest virtual machines (assuming the hypervisor and OS support this), which means that caching should be deactivated for that device.

⁠23.10.30. Creating a guest virtual machine from a configuration file

Guest virtual machines can be created from XML configuration files. You can copy existing XML from previously created guest virtual machines or use the dumpxml option (refer to Section 23.10.29, “Creating a virtual machine XML dump (configuration file)”). To create a guest virtual machine with virsh from an XML file:

# virsh create configuration_file.xml

⁠23.11. Editing a guest virtual machine's configuration file

Instead of using the dumpxml option (refer to Section 23.10.29, “Creating a virtual machine XML dump (configuration file)”) guest virtual machines can be edited either while they run or while they are offline. The virsh edit command provides this functionality. For example, to edit the guest virtual machine named guest1:

# virsh edit guest1

This opens a text editor. The default text editor is the $EDITOR shell parameter (set to vi by default).

⁠23.11.1. Adding multifunction PCI devices to KVM guest virtual machines

This section will demonstrate how to add multi-function PCI devices to KVM guest virtual machines.

Run the virsh edit [guestname] command to edit the XML configuration file for the guest virtual machine.

In the address type tag, add a multifunction='on' entry for function='0x0'.

This enables the guest virtual machine to use the multifunction PCI devices.

<disk type='file' device='disk'>
<driver name='qemu' type='raw' cache='none'/>
<source file='/var/lib/libvirt/images/rhel62-1.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0' multifunction='on'/
</disk>

For a PCI device with two functions, amend the XML configuration file to include a second device with the same slot number as the first device and a different function number, such as function='0x1'.

For Example:

<disk type='file' device='disk'>
<driver name='qemu' type='raw' cache='none'/>
<source file='/var/lib/libvirt/images/rhel62-1.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0' multifunction='on'/>
</disk>
<disk type='file' device='disk'>
<driver name='qemu' type='raw' cache='none'/>
<source file='/var/lib/libvirt/images/rhel62-2.img'/>
<target dev='vdb' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x1'/>
</disk>

lspci output from the KVM guest virtual machine shows:

$ lspci

00:05.0 SCSI storage controller: Red Hat, Inc Virtio block device
00:05.1 SCSI storage controller: Red Hat, Inc Virtio block device

Note

The SeaBIOS application runs in real mode for compatibility with BIOS interfaces. This limits the amount of memory available. As a consequence, SeaBIOS is only able to handle a limited number of disks. Currently, the supported number of disks is:

virtio-scsi — 64

virtio-blk — 4

ahci/sata — 24 (4 controllers with all 6 ports connected)

usb-storage — 4

As a workaround for this problem, when attaching a large number of disks to your virtual machine, make sure that your system disk has a small pci slot number, so SeaBIOS sees it first when scanning the pci bus. It is also recommended to use the virtio-scsi device instead of virtio-blk as the per-disk memory overhead is smaller.

⁠23.11.2. Stopping a running guest virtual machine in order to restart it later

virsh managedsave domain --bypass-cache --running | --paused | --verbose saves and destroys (stops) a running guest virtual machine so that it can be restarted from the same state at a later time. When used with a virsh start command it is automatically started from this save point. If it is used with the --bypass-cache argument the save will avoid the filesystem cache. Note that this option may slow down the save process speed.

--verbose displays the progress of the dump process

Under normal conditions, the managed save will decide between using the running or paused state as determined by the state the guest virtual machine is in when the save is done. However, this can be overridden by using the --running argument to indicate that it must be left in a running state or by using --paused argument which indicates it is to be left in a paused state.

To remove the managed save state, use the virsh managedsave-remove command which will force the guest virtual machine to do a full boot the next time it is started.

Note that the entire managed save process can be monitored using the domjobinfo command and can also be canceled using the domjobabort command.

⁠23.11.3. Displaying CPU statistics for a specified guest virtual machine

The virsh cpu-stats domain --total start count command provides the CPU statistical information on the specified guest virtual machine. By default it shows the statistics for all CPUs, as well as a total. The --total argument will only display the total statistics.

⁠23.11.4. Saving a screenshot

The virsh screenshot command takes a screenshot of a current guest virtual machine console and stores it into a file. If however the hypervisor supports more displays for a guest virtual machine, using the --screen and giving a screen ID will specify which screen to capture. In the case where there are multiple graphics cards, where the heads are numerated before their devices, screen ID 5 addresses the second head on the second card.

⁠23.11.5. Sending a keystroke combination to a specified guest virtual machine

Using the virsh send-key domain --codeset --holdtime keycode command you can send a sequence as a keycode to a specific guest virtual machine.

Each keycode can either be a numeric value or a symbolic name from the corresponding codeset. If multiple keycodes are specified, thay are all sent simultaneously to the guest virtual machine and as such may be received in random order. If you need distinct keycodes, you must send the send-key command multiple times.

# virsh send-key guest1 --holdtime 1000 0xf

If a --holdtime is given, each keystroke will be held for the specified amount in milliseconds. The --codeset allows you to specify a code set, the default being Linux, but the following options are permitted:

linux - choosing this option causes the symbolic names to match the corresponding Linux key constant macro names and the numeric values are those offered by the Linux generic input event subsystems.

xt- this will send a value that is defined by the XT keyboard controller. No symbolic names are provided.

atset1 - the numeric values are those that are defined by the AT keyboard controller, set1 (XT compatible set). Extended keycodes from the atset1 may differ from extended keycodes in the XT codeset. No symbolic names are provided.

atset2 - The numeric values are those defined by the AT keyboard controller, set 2. No symbolic names are provided.

atset3 - The numeric values are those defined by the AT keyboard controller, set 3 (PS/2 compatible). No symbolic names are provided.

os_x - The numeric values are those defined by the OS-X keyboard input subsystem. The symbolic names match the corresponding OS-X key constant macro names.

xt_kbd - The numeric values are those defined by the Linux KBD device. These are a variant on the original XT codeset, but often with different encoding for extended keycodes. No symbolic names are provided.

win32 - The numeric values are those defined by the Win32 keyboard input subsystem. The symbolic names match the corresponding Win32 key constant macro names.

usb - The numeric values are those defined by the USB HID specification for keyboard input. No symbolic names are provided.

rfb - The numeric values are those defined by the RFB extension for sending raw keycodes. These are a variant on the XT codeset, but extended keycodes have the low bit of the second bite set, instead of the high bit of the first byte. No symbolic names are provided.

⁠23.11.6. Sending process signal names to virtual processes

Using the virsh send-process-signal domain-ID PID signame you can send a signal signame to a specified virtual process (as identified by its Process ID or PID) within a running guest virtual machine given it's domain-ID. In addition to an integer signal constant number, one or more of the following signames can be sent:

nop , stkflt

hup, cont

int, chld

quit, stop

ill, tstp

trap, ttin

abrt, ttou

bus, urg

fpe, xcpu

kill, xfsz

usr1, vtalrm

segv, prof

usr2, winch

pipe, poll

alrm, pwr

term, sys

More options are on the Virsh MAN page. Note that these sumbols may also be prefixed with sigor sig_ and it is not case sensitive.

# virsh send-process-signal guest1 187 kill

⁠23.11.7. Displaying the IP address and port number for the VNC display

The virsh vncdisplay will print the IP address and port number of the VNC display for the specified guest virtual machine. If the information is unavailable the exit code 1 will be displayed.

# virsh vncdisplay guest1
127.0.0.1:0

⁠23.12. NUMA node management

This section contains the commands needed for NUMA node management.

⁠23.12.1. Displaying node information

The nodeinfo command displays basic information about the node, including the model number, number of CPUs, type of CPU, and size of the physical memory. The output corresponds to virNodeInfo structure. Specifically, the "CPU socket(s)" field indicates the number of CPU sockets per NUMA cell.

$ virsh nodeinfo
CPU model:           x86_64
CPU(s):              4
CPU frequency:       1199 MHz
CPU socket(s):       1
Core(s) per socket:  2
Thread(s) per core:  2
NUMA cell(s):        1
Memory size:         3715908 KiB

⁠23.12.2. Setting NUMA parameters

The virsh numatune can either set or retrieve the NUMA parameters for a specified guest virtual machine. Within the Domain XML file these parameters are nested within the <numatune> element. Without using flags, only the current settings are displayed. The numatune domain command requires a specified guest virtual machine and can take the following arguments:

--mode - The mode can be set to either strict, interleave, or preferred. Running domains cannot have their mode changed while live unless the guest virtual machine was started within strict mode.

--nodeset contains a list of NUMA nodes that are used by the host physical machine for running the guest virtual machine. The list contains nodes, each separated by a comma, with a dash - used for node ranges and a caret ^ used for excluding a node.

Only one of the three following flags can be used per instance

--config will effect the next boot of a persistent guest virtual machine

--live will set the scheduler information of a running guest virtual machine.

--current will effect the current state of the guest virtual machine.

⁠23.12.3. Displaying the amount of free memory in a NUMA cell

The virsh freecell displays the available amount of memory on the machine within a specified NUMA cell. This command can provide one of three different displays of available memory on the machine depending on the options specified. If no options are used, the total free memory on the machine is displayed. Using the --all option, it displays the free memory in each cell and the total free memory on the machine. By using a numeric argument or with --cellno along with a cell number it will display the free memory for the specified cell.

⁠23.12.4. Displaying a CPU list

The nodecpumap command displays the number of CPUs that are available to the node, whether they are online or not and it also lists the number that are currently online.

$ virsh nodecpumap
   CPUs present: 4
   CPUs online: 1
   CPU map: y

⁠23.12.5. Displaying CPU statistics

The nodecpustats command displays statistical information about the specified CPU, if the CPU is given. If not, it will display the CPU status of the node. If a percent is specified, it will display the percentage of each type of CPU statistics that were recorded over an one (1) second interval.

This example shows no CPU specified:

$ virsh nodecpustats
user:               1056442260000000
system:              401675280000000
idle:               7549613380000000
iowait:               94593570000000

This example shows the statistical percentages for CPU number 2:

$ virsh nodecpustats 2 --percent
usage:            2.0%
user:             1.0%
system:           1.0%
idle:            98.0%
iowait:           0.0%

⁠23.12.6. Managing devices

⁠23.12.6.1. Attaching and updating a device with virsh

For information on attaching storage devices, refer to Section 15.5.1, “Adding file based storage to a guest”.

⁠

Procedure 23.5. Hotplugging USB devices for use by the guest virtual machine

USB devices can be either attached to the virtual machine is running by hotplugging, or while the guest is shut off. The device you want to emulate needs to be attached to the host machine.

Locate the USB device you want to attach with the following command:

# lsusb -v

idVendor           0x17ef Lenovo
idProduct          0x480f Integrated Webcam [R5U877]

Create an XML file and give it a logical name (usb_device.xml, for example). Copy the vendor and product IDs exactly as was displayed in your search.

⁠

​
​   <hostdev mode='subsystem' type='usb' managed='yes'>
​      <source>
​        <vendor id='0x17ef'/>
​        <product id='0x480f'/>
​      </source>
​    </hostdev>
​

Figure 23.2. USB devices XML snippet

Attach the device with the following command:

# virsh attach-device guest1 --file usb_device.xml --config

In this example, guest1 is the name of the virtual machine and usb_device.xml is the file you created in the previous step.

For the change take effect at the next reboot, use the --config. For the change to take effect on the current guest virtual machine, use the --current argument. See the virsh MAN page for additional arguments.

To detach the device (hot unplug), use the following command, where guest1 is the name of the virtual machine and usb_device.xml is the device file:

# virsh detach-device guest1 --file usb_device.xml

⁠23.12.6.2. Attaching interface devices

The virsh attach-interface guest type source command can take the following arguments:

--live - get value from running guest virtual machine

--config - take effect at next boot

--current - get value according to current guest virtual machine state

--target - indicates the target device in the guest virtual machine.

--mac - use this to specify the MAC address of the network interface

--script - use this to specify a path to a script file handling a bridge instead of the default one.

--model - use this to specify the model type.

--inbound - controls the inbound bandwidth of the interface. Acceptable values are average, peak, and burst.

--outbound - controls the outbound bandwidth of the interface. Acceptable values are average, peak, and burst.

The type can be either network to indicate a physical network device, or bridge to indicate a bridge to a device. source is the source of the device. To remove the attached device, use the virsh detach-device.

⁠23.12.6.3. Changing the media of a CDROM

Changing the media of a CDROM to another source or format

# virsh change-media domain path source --eject --insert --update --current --live --config --force

--path - A string containing a fully-qualified path or target of disk device

--source - A string containing the source of the media

--eject - Eject the media

--insert - Insert the media

--update - Update the media

--current - can be either or both of --live and --config, depends on implementation of hypervisor driver

--live - alter live configuration of running guest virtual machine

--config - alter persistent configuration, effect observed on next boot

--force - force media changing

⁠23.12.7. Suspending the host physical machine

The nodesuspend command puts the host physical machine into a system-wide sleep state similar to that of Suspend-to-RAM (s3), Suspend-to-Disk (s4), or Hybrid-Suspend and sets up a Real-Time-Clock to wake up the node after the duration that is set has past. The --target argument can be set to either mem,disk, or hybrid. These options indicate to set the memory, disk, or combination of the two to suspend. Setting the --duration instructs the host physical machine to wake up after the set duration time has run out. It is set in seconds. It is recommended that the duration time be longer than 60 seconds.

$ virsh nodesuspend disk 60

⁠23.12.8. Setting and displaying the node memory parameters

The node-memory-tune [shm-pages-to-scan] [shm-sleep-milisecs] [shm-merge-across-nodes] command displays and allows you to set the node memory parameters. There are three parameters that may be set with this command:

shm-pages-to-scan - sets the number of pages to scan before the shared memory service goes to sleep.

shm-sleep-milisecs - sets the number of miliseconds that the shared memory service will sleep before the next scan

shm-merge-across-nodes - specifies if pages from different NUMA nodes can be merged. Values allowed are 0 and 1. When set to 0, the only pages that can be merged are those that are physically residing in the memory area of the same NUMA node. When set to 1, pages from all of the NUMA nodes can be merged. The default setting is 1.

⁠23.12.9. Creating devices on host nodes

The virsh nodedev-create file command allows you to create a device on a host node and then assign it to a guest virtual machine. libvirt normally detects which host nodes are available for use automatically, but this command allows for the registration of host hardware that libvirt did not detect. The file should contain the XML for the top level <device> description of the node device.

To stop this device, use the nodedev-destroy device command.

⁠23.12.10. Detaching a node device

The virsh nodedev-detach detaches the nodedev from the host so it can be safely used by guests via <hostdev> passthrough. This action can be reversed with the nodedev-reattach command but it is done automatically for managed services. This command also accepts nodedev-dettach.

Note that different drivers expect the device to be bound to different dummy devices. Using the --driver argument allows you to specify the desired backend driver.

⁠23.12.11. Dump a Device

The virsh nodedev-dumpxml device dumps the <device> XML representation for the given node device, including information such as the device name, which BUS owns the device, the vendor, and product ID as well as any capabilities of the device as is usable by libvirt, where it may specify what is supported. The argument device can either be a device name or WWN pair in WWNN, WWPN format (HBA only).

⁠23.12.12. List devices on a node

The virsh nodedev-list cap --tree command lists all the devices available on the node that are known by libvirt. cap is used to filter the list by capability types, each separated by a comma and cannot be used with --tree. Using the argument --tree, puts the output into a tree structure as shown:

   #  virsh nodedev-list --tree
   computer
  |
  +- net_lo_00_00_00_00_00_00
  +- net_macvtap0_52_54_00_12_fe_50
  +- net_tun0
  +- net_virbr0_nic_52_54_00_03_7d_cb
  +- pci_0000_00_00_0
  +- pci_0000_00_02_0
  +- pci_0000_00_16_0
  +- pci_0000_00_19_0
  |   |
  |   +- net_eth0_f0_de_f1_3a_35_4f

(this is a partial screen)

⁠23.12.13. Triggering a reset for a node

The nodedev-reset nodedev command triggers a device reset for the specified nodedev. Running this command is useful prior to transferring a node device between guest virtual machine pass through or the host physical machine. libvirt will do this action implicitly when required, but this command allows an explicit reset when needed.

⁠23.13. Retrieving guest virtual machine information

⁠23.13.1. Getting the domain ID of a guest virtual machine

To get the domain ID of a guest virtual machine:

# virsh domid {domain-name or domain-uuid}

Note, domid returns '-' for domains that are in shut off state.

⁠23.13.2. Getting the domain name of a guest virtual machine

To get the domain name of a guest virtual machine:

# virsh domname {domain-id or domain-uuid}

⁠23.13.3. Getting the UUID of a guest virtual machine

To get the Universally Unique Identifier (UUID) for a guest virtual machine:

# virsh domuuid {domain-id or domain-name}

An example of virsh domuuid output:

# virsh domuuid r5b2-mySQL01
4a4c59a7-ee3f-c781-96e4-288f2862f011

⁠23.13.4. Displaying guest virtual machine information

Use virsh dominfo with a guest virtual machine's domain ID, domain name or UUID to display information on that guest virtual machine:

# virsh dominfo {domain-id, domain-name or domain-uuid}

This is an example of virsh dominfo output:

# virsh dominfo vr-rhel6u1-x86_64-kvm
Id:             9
Name:           vr-rhel6u1-x86_64-kvm
UUID:           a03093a1-5da6-a2a2-3baf-a845db2f10b9
OS Type:        hvm
State:          running
CPU(s):         1
CPU time:       21.6s
Max memory:     2097152 kB
Used memory:    1025000 kB
Persistent:     yes
Autostart:      disable
Security model: selinux
Security DOI:   0
Security label: system_u:system_r:svirt_t:s0:c612,c921 (permissive)

⁠23.14. Storage pool commands

Using libvirt, you can manage various storage solutions, including files, raw partitions, and domain-specific formats, used to provide the storage volumes visible as devices within virtual machines. For more detailed information, see libvirt.org. Many of the commands for administering storage pools are similar to the ones used for domains.

⁠23.14.1. Searching for a storage pool XML

The find-storage-pool-sources type srcSpec command displays the XML describing all storage pools of a given type that could be found. If srcSpec is provided, it is a file that contains XML to further restrict the query for pools.

The find-storage-pool-sources-as type host port initiator displays the XML describing all storage pools of a given type that could be found. If host, port, or initiator are provided, they control where the query is performed.

The pool-info pool-or-uuid command will list the basic information about the specified storage pool object. This command requires the name or UUID of the storage pool. To retrieve this information, use the pool-list

The pool-list --inactive --all --persistent --transient --autostart --no-autostart --details<type> command lists all storage pool objects known to libvirt. By default, only active pools are listed; but using the --inactive argument lists just the inactive pools, and using the --all argument lists all of the storage pools.

In addition to those arguments there are several sets of filtering flags that can be used to filter the content of the list. --persistent restricts the list to persistent pools, --transient restricts the list to transient pools, --autostart restricts the list to autostarting pools and finally --no-autostart restricts the list to the storage pools that have autostarting disabled.

For all storage pool commands which require a type, the pool types must be separated by comma. The valid pool types include: dir, fs, netfs, logical, disk, iscsi, scsi, mpath, rbd, sheepdog and gluster.

The --details option instructs virsh to additionally display pool persistence and capacity related information where available.

Note

When this command is used with older servers, it is forced to use a series of API calls with an inherent race, where a pool might not be listed or might appear more than once if it changed its state between calls while the list was being collected. Newer servers however, do not have this problem.

The pool-refresh pool-or-uuid refreshes the list of volumes contained in pool.

⁠23.14.2. Creating, defining, and starting storage pools

⁠23.14.2.1. Building a storage pool

The pool-build pool-or-uuid --overwrite --no-overwrite command builds a pool with a specified pool name or UUID. The arguments --overwrite and --no-overwrite can only be used for a pool whose type is file system.

If --no-overwrite is specified, it probes to determine if a file system already exists on the target device, returning an error if it exists, or using mkfs to format the target device if it does not. If --overwrite is specified, then the mkfs command is executed and any existing data on the target device is overwritten.

⁠23.14.2.2. Creating and defining a storage pool from an XML file

The pool-create file command creates and starts a storage pool from its associated XML file.

The pool-define file command creates, but does not start, a storage pool object from the XML file.

⁠23.14.2.3. Creating and starting a storage pool from raw parameters

The pool-create-as name --print-xml type source-host source-path source-dev source-name <target> --source-format <format> command creates and starts a pool object name from the raw parameters given.

The pool-define-as name --print-xml type source-host source-path source-dev source-name <target> --source-format <format> command creates, but does not start, a pool object name from the raw parameters given.

If --print-xml is specified, then it prints the XML of the pool object without creating or defining the pool. Otherwise, the pool requires a specified type to be built. For all storage pool commands which require a type, the pool types must be separated by comma. The valid pool types include: dir, fs, netfs, logical, disk, iscsi, scsi, mpath, rbd, sheepdog, and gluster.

The pool-start pool-or-uuid starts the specified storage pool, which was previously defined but inactive.

⁠23.14.2.4. Auto-starting a storage pool

The pool-autostart pool-or-uuid --enable command enables a storage pool to automatically start at boot. This command requires the pool name or UUID. To disable the pool-autostart command use the --disable argument instead.

⁠23.14.3. Stopping and deleting storage pools

The pool-destroy pool-or-uuid command stops a storage pool. Once stopped, libvirt will no longer manage the pool but the raw data contained in the pool is not changed, and can be later recovered with the pool-create command.

The pool-delete pool-or-uuid command destroys the resources used by the specified storage pool. It is important to note that this operation is non-recoverable and non-reversible. However, the pool structure will still exist after this command, ready to accept the creation of new storage volumes.

The pool-undefine pool-or-uuid command undefines the configuration for an inactive pool.

⁠23.14.4. Creating an XML dump file for a pool

The pool-dumpxml --inactive pool-or-uuid command returns the XML information about the specified storage pool object. Using --inactive dumps the configuration that will be used on next start of the pool instead of the current pool configuration.

⁠23.14.5. Editing the storage pool's configuration file

The pool-edit pool-or-uuid opens the specified storage pool's XML configuration file for editing.

This method is the only method that should be used to edit an XML configuration file as it does error checking before applying.

⁠23.15. Storage Volume Commands

This section covers commands for creating, deleting, and managing storage volumes. It is best to do this once you have created a storage pool as the storage pool name or UUID will be required. For information on storage pools refer to Chapter 14, Storage pools. For information on storage volumes refer to, Chapter 15, Storage Volumes .

⁠23.15.1. Creating storage volumes

The virsh vol-create-from pool-or-uuid FILE [--inputpool pool-or-uuid] vol-name-or-key-or-path [--prealloc-metadata] [--reflink] creates a volume, using another volume as input. This command requires a pool-or-uuid which is the name or UUID of the storage pool to create the volume in.

The file argument is the path to the file containing the volume definition. The --inputpool pool-or-uuid argument specifies the name or UUID of the storage pool the source volume is in. The vol-name-or-key-or-path argument specifies the name or key or path of the source volume. For examples, refer to Section 15.2, “Creating volumes”.

The vol-create-as command creates a volume from a set of arguments. The pool-or-uuid argument contains the name or UUID of the storage pool to create the volume in.

vol-create-as pool-or-uuid name capacity --allocation <size> --format <string> --backing-vol <vol-name-or-key-or-path> --backing-vol-format <string> [--prealloc-metadata]

name is the name of the new volume. capacity is the size of the volume to be created, as a scaled integer, defaulting to bytes if there is no suffix. --allocation <size> is the initial size to be allocated in the volume, also as a scaled integer defaulting to bytes. --format <string>is used in file based storage pools to specify the volume file format. Acceptable formats include raw, bochs, qcow, qcow2, vmdk, and qed. --backing-vol vol-name-or-key-or-path is the source backing volume to be used if taking a snapshot of an existing volume. --backing-vol-format string is the format of the snapshot backing volume. Accepted values include: raw, bochs, qcow, qcow2, qed, vmdk, and host_device. These are, however, only meant for file based storage pools. By default the qcow version that is used is version 3. If you want to change the version, refer to Section 26.20.2, “Setting target elements”.

⁠23.15.1.1. Creating a storage volume from an XML file

The vol-create pool-or-uuid file [--prealloc-metadata] creates a storage volume from an XML file. This command also requires the pool-or-uuid, which is the name or UUID of the storage pool to create the volume in. The file argument contains the path with the volume definition's XML file. An easy way to create the XML file is to use the vol-dumpxml command to obtain the definition of a pre-existing volume.

virsh vol-dumpxml --pool storagepool1 appvolume1 > newvolume.xml
virsh edit newvolume.xml
virsh vol-create differentstoragepool newvolume.xml

⁠23.15.1.2. Cloning a storage volume

The vol-clone --pool pool-or-uuid vol-name-or-key-or-path name [--prealloc-metadata] [--reflink] command clones an existing storage volume. Although the vol-create-from may also be used, it is not the recommended way to clone a storage volume. The --pool pool-or-uuid argument is the name or UUID of the storage pool to create the volume in. The vol-name-or-key-or-path argument is the name or key or path of the source volume. Using a name argument refers to the name of the new volume. For additional examples, refer to Section 15.3, “Cloning volumes”.

⁠23.15.2. Deleting storage volumes

The vol-delete --pool pool-or-uuid vol-name-or-key-or-path command deletes a given volume. The command requires a specific --pool pool-or-uuid which is the name or UUID of the storage pool the volume is in. The option vol-name-or-key-or-path is the name or key or path of the volume to delete.

The vol-wipe --pool pool-or-uuid --algorithm algorithm vol-name-or-key-or-pathcommand wipes a volume, to ensure data previously on the volume is not accessible to future reads. The command requires a --pool pool-or-uuid which is the name or UUID of the storage pool the volume is in. The vol-name-or-key-or-path contains the name or key or path of the volume to wipe. Note that it is possible to choose different wiping algorithms instead of re-writing volume with zeroes, via the argument --algorithm and using one of the following supported algorithm types:

Note

The availability of algorithms may be limited by the version of the "scrub" binary installed on the host.

zero - 1-pass all zeroes

nnsa - 4-pass NNSA Policy Letter NAP-14.1-C (XVI-8) for sanitizing removable and non-removable hard disks: random x2, 0x00, verify.

dod - 4-pass DoD 5220.22-M section 8-306 procedure for sanitizing removeable and non-removeable rigid disks: random, 0x00, 0xff, verify.

bsi - 9-pass method recommended by the German Center of Security in Information Technologies (http://www.bsi.bund.de): 0xff, 0xfe, 0xfd, 0xfb, 0xf7, 0xef, 0xdf, 0xbf, 0x7f.

gutmann - The canonical 35-pass sequence described in Gutmann’s paper.

schneier - 7-pass method described by Bruce Schneier in "Applied Cryptography" (1996): 0x00, 0xff, random x5.

pfitzner7 - Roy Pfitzner’s 7-random-pass method: random x7

pfitzner33 - Roy Pfitzner’s 33-random-pass method: random x33.

random - 1-pass pattern: random.

⁠23.15.3. Dumping storage volume information to an XML file

vol-dumpxml --pool pool-or-uuid vol-name-or-key-or-path command takes the volume information as an XML dump to a specified file.

This command requires a --pool pool-or-uuid, which is the name or UUID of the storage pool the volume is in. vol-name-or-key-or-path is the name or key or path of the volume to place the resulting XML file.

⁠23.15.4. Listing volume information

The vol-info --pool pool-or-uuid vol-name-or-key-or-path command lists basic information about the given storage volume --pool, where pool-or-uuid is the name or UUID of the storage pool the volume is in. vol-name-or-key-or-path is the name or key or path of the volume to return information for.

The vol-list--pool pool-or-uuid --details lists all of volumes in the specified storage pool. This command requires --pool pool-or-uuid which is the name or UUID of the storage pool. The --details option instructs virsh to additionally display volume type and capacity related information where available.

⁠23.15.5. Retrieving storage volume information

The vol-pool --uuid vol-key-or-path command returns the pool name or UUID for a given volume. By default, the pool name is returned. If the --uuid option is given, the pool UUID is returned instead. The command requires the vol-key-or-path which is the key or path of the volume for which to return the requested information.

The vol-path --pool pool-or-uuid vol-name-or-key command returns the path for a given volume. The command requires --pool pool-or-uuid, which is the name or UUID of the storage pool the volume is in. It also requires vol-name-or-key which is the name or key of the volume for which the path has been requested.

The vol-name vol-key-or-path command returns the name for a given volume, where vol-key-or-path is the key or path of the volume to return the name for.

The vol-key --pool pool-or-uuid vol-name-or-path command returns the volume key for a given volume where --pool pool-or-uuid is the name or UUID of the storage pool the volume is in and vol-name-or-path is the name or path of the volume to return the volume key for.

⁠23.15.6. Uploading and downloading storage volumes

The vol-upload --pool pool-or-uuid --offset bytes --length bytes vol-name-or-key-or-path local-file command uploads the contents of specified local-file to a storage volume. The command requires --pool pool-or-uuid, which is the name or UUID of the storage pool the volume is in. It also requires vol-name-or-key-or-path which is the name or key or path of the volume to upload. The --offset argument is the position in the storage volume at which to start writing the data. --length length dictates an upper limit for the amount of data to be uploaded. An error will occur if the local-file is greater than the specified --length.

The vol-download --pool pool-or-uuid --offset bytes -length bytes vol-name-or-key-or-path local-file command downloads the contents of local-file from a storage volume.

The command requires a --pool pool-or-uuid which is the name or UUID of the storage pool that the volume is in. It also requires vol-name-or-key-or-path which is the name or key or path of the volume to download. Using the argument --offset dictates the position in the storage volume at which to start reading the data. --length length dictates an upper limit for the amount of data to be downloaded.

⁠23.15.7. Resizing storage volumes

The vol-resize --pool pool-or-uuid vol-name-or-path pool-or-uuid capacity --allocate --delta --shrink command resizes the capacity of the given volume, in bytes.The command requires --pool pool-or-uuid which is the name or UUID of the storage pool the volume is in. This command also requires vol-name-or-key-or-path is the name or key or path of the volume to resize.

The new capacity might be sparse unless --allocate is specified. Normally, capacity is the new size, but if --delta is present, then it is added to the existing size. Attempts to shrink the volume will fail unless --shrink is present.

Note that capacity cannot be negative unless --shrink is provided and a negative sign is not necessary. capacity is a scaled integer which defaults to bytes if there is no suffix. Note too that this command is only safe for storage volumes not in use by an active guest. Refer to Section 23.10.22, “Using blockresize to change the size of a guest virtual machine path” for live resizing.

⁠23.16. Displaying per-guest virtual machine information

⁠23.16.1. Displaying the guest virtual machines

To display the guest virtual machine list and their current states with virsh:

# virsh list

Other options available include:

--inactive option lists the inactive guest virtual machines (that is, guest virtual machines that have been defined but are not currently active)

--all option lists all guest virtual machines. For example:

# virsh list --all
 Id Name                 State
----------------------------------
  0 Domain-0             running
  1 Domain202            paused
  2 Domain010            inactive
  3 Domain9600           crashed

There are seven states that can be visible using this command:

Running - The running state refers to guest virtual machines which are currently active on a CPU.

Idle - The idle state indicates that the guest virtual machine is idle, and may not be running or able to run. This can be caused because the guest virtual machine is waiting on IO (a traditional wait state) or has gone to sleep because there was nothing else for it to do.

Paused - The paused state lists domains that are paused. This occurs if an administrator uses the paused button in virt-manager or virsh suspend. When a guest virtual machine is paused it consumes memory and other resources but it is ineligible for scheduling and CPU resources from the hypervisor.

Shutdown - The shutdown state is for guest virtual machines in the process of shutting down. The guest virtual machine is sent a shutdown signal and should be in the process of stopping its operations gracefully. This may not work with all guest virtual machine operating systems; some operating systems do not respond to these signals.

Shut off - The shut off state indicates that the guest virtual machine is not running. This can be caused when a guest virtual machine completely shuts down or has not been started.

Crashed - The crashed state indicates that the guest virtual machine has crashed and can only occur if the guest virtual machine has been configured not to restart on crash.

Dying - Domains in the dying state are in is in process of dying, which is a state where the guest virtual machine has not completely shut-down or crashed.

--managed-save Although this flag alone does not filter the domains, it will list the domains that have managed save state enabled. In order to actually list the domains seperately you will need to use the --inactive flag as well.

--name is specified guest virtual machine names are printed in a list. If --uuid is specified the donain's UUID is printed instead. Using the flag --table specifies that a table style output should be used. All three commands are mutually exclusive

--title This command must be used with --table output. --titlewill cause an extra column to be created in the table with the short guest virtual machine description (title).

--persistentincludes persistent domains in a list. Use the --transient argument.

--with-managed-save lists the domains that have been configured with managed save. To list the commands without it, use the command --without-managed-save

--state-running filters out for the domains that are running, --state-paused for paused domains, --state-shutoff for domains that are turned off, and --state-other lists all states as a fallback.

--autostart this argument will cause the auto-starting domains to be listed. To list domains with this feature disabled, use the argument --no-autostart.

--with-snapshot will list the domains whose snapshot images can be listed. To filter for the domains without a snapshot, use the argument --without-snapshot

$ virsh list --title --name

    Id       Name                                          State     Title
    0        Domain-0                                      running   Mailserver1
    2        rhelvm                                        paused

⁠23.16.2. Displaying virtual CPU information

To display virtual CPU information from a guest virtual machine with virsh:

# virsh vcpuinfo {domain-id, domain-name or domain-uuid}

An example of virsh vcpuinfo output:

# virsh vcpuinfo guest1
VCPU:           0
CPU:            2
State:          running
CPU time:       7152.4s
CPU Affinity:   yyyy

VCPU:           1
CPU:            2
State:          running
CPU time:       10889.1s
CPU Affinity:   yyyy

⁠23.16.3. Pinning vCPU to a host physical machine's CPU

The virsh vcpupin assigns a virtual CPU to a physical one.

# virsh vcpupin guest1
VCPU: CPU Affinity
----------------------------------
   0: 0-3
   1: 0-3

The vcpupin can take the following arguments:

--vcpu requires the vcpu number

[--cpulist] >string< lists the host physical machine's CPU number(s) to set, or omit option to query

--config affects next boot

--live affects the running guest virtual machine

--current affects the current guest virtual machine

⁠23.16.4. Displaying information about the virtual CPU counts of a given domian

virsh vcpucount requires a domain name or a domain ID

# virsh vcpucount guest1
maximum      config         2
maximum      live           2
current      config         2
current      live           2

The vcpucount can take the following arguments:

--maximum get maximum cap on vcpus

--active get number of currently active vcpus

--live get value from running guest virtual machine

--config get value to be used on next boot

--current get value according to current guest virtual machine state

--guest count that is returned is from the perspective of the guest

⁠23.16.5. Configuring virtual CPU affinity

To configure the affinity of virtual CPUs with physical CPUs:

# virsh vcpupin domain-id vcpu cpulist

The domain-id parameter is the guest virtual machine's ID number or name.

The vcpu parameter denotes the number of virtualized CPUs allocated to the guest virtual machine.The vcpu parameter must be provided.

The cpulist parameter is a list of physical CPU identifier numbers separated by commas. The cpulist parameter determines which physical CPUs the VCPUs can run on.

Additional parameters such as --config effect the next boot, whereas --live effects the running guest virtual machine and --currenteffects the current guest virtual machine

⁠23.16.6. Configuring virtual CPU count

Use this command to change the number of virtual CPUs active in a guest guest virtual machine. By default, this command works on active guest virtual machines. To change the settings for an inactive guest virtual machine, use the --config flag.To modify the number of CPUs assigned to a guest virtual machine with virsh:

# virsh setvcpus {domain-name, domain-id or domain-uuid} count [[--config] [--live] | [--current]]

For example:

virsh setvcpus guestVM1 2 --live

will increase the number of vCPUs to guestVM1 by two and this action will be performed while the guestVM1 is running.

Likewise, to hot unplug the same CPU run the following:

virsh setvcpus guestVM1 1 --live

The count value may be limited by host, hypervisor, or a limit coming from the original description of the guest virtual machine. For Xen, you can only adjust the virtual CPUs of a running guest virtual machine if the guest virtual machine is paravirtualized.

If the --config flag is specified, the change is made to the stored XML configuration for the guest virtual machine, and will only take effect when the guest is started.

If --live is specified, the guest virtual machine must be active, and the change takes place immediately. This option will allow hotplugging of a vCPU. Both the --config and --live flags may be specified together if supported by the hypervisor.

If --current is specified, the flag affects the current guest virtual machine state. When no flags are given, the --live flag is assumed which will fail if the guest virtual machine is not active. In this situation it is up to the hypervisor whether the --config flag is also assumed, and therefore whether the XML configuration is adjusted to make the change persistent.

The --maximum flag controls the maximum number of virtual cpus that can be hot-plugged the next time the guest virtual machine is booted. As such, it must only be used with the --config flag, and not with the --live flag.

It is important to note that the count value cannot exceed the number of CPUs that were assigned to the guest virtual machine when it was created.

⁠23.16.7. Configuring memory allocation

To modify a guest virtual machine's memory allocation with virsh:

# virsh setmem {domain-id or domain-name} count

# virsh setmem vr-rhel6u1-x86_64-kvm --kilobytes 1025000

You must specify the count in kilobytes. The new count value cannot exceed the amount you specified when you created the guest virtual machine. Values lower than 64 MB are unlikely to work with most guest virtual machine operating systems. A higher maximum memory value does not affect active guest virtual machines. If the new value is lower than the available memory, it will shrink possibly causing the guest virtual machine to crash.

This command has the following options

[--domain] <string> domain name, id or uuid

[--size] <number> new memory size, as scaled integer (default KiB)

Valid memory units include:

b or bytes for bytes

KB for kilobytes (10³ or blocks of 1,000 bytes)

k or KiB for kibibytes (2¹⁰ or blocks of 1024 bytes)

MB for megabytes (10⁶ or blocks of 1,000,000 bytes)

M or MiB for mebibytes (2²⁰ or blocks of 1,048,576 bytes)

GB for gigabytes (10⁹ or blocks of 1,000,000,000 bytes)

G or GiB for gibibytes (2³⁰ or blocks of 1,073,741,824 bytes)

TB for terabytes (10¹² or blocks of 1,000,000,000,000 bytes)

T or TiB for tebibytes (2⁴⁰ or blocks of 1,099,511,627,776 bytes)

Note that all values will be rounded up to the nearest kibibyte by libvirt, and may be further rounded to the granularity supported by the hypervisor. Some hypervisors also enforce a minimum, such as 4000KiB (or 4000 x 2¹⁰ or 4,096,000 bytes). The units for this value are determined by the optional attribute memory unit, which defaults to the kibibytes (KiB) as a unit of measure where the value given is multiplied by 2¹⁰ or blocks of 1024 bytes.

--config takes affect next boot

--live controls the memory of the running guest virtual machine

--current controls the memory on the current guest virtual machine

⁠23.16.8. Changing the memory allocation for the domain

The virsh setmaxmem domain size --config --live --current allows the setting of the maximum memory allocation for a guest virtual machine as shown:

virsh setmaxmem guest1 1024 --current

The size that can be given for the maximum memory is a scaled integer that by default is expressed in kibibytes, unless a supported suffix is provided. The following arguments can be used with this command:

--config - takes affect next boot

--live - controls the memory of the running guest virtual machine, providing the hypervisor supports this action as not all hypervisors allow live changes of the maximum memory limit.

--current - controls the memory on the current guest virtual machine

⁠23.16.9. Displaying guest virtual machine block device information

Use virsh domblkstat to display block device statistics for a running guest virtual machine.

# virsh domblkstat GuestName block-device

⁠23.16.10. Displaying guest virtual machine network device information

Use virsh domifstat to display network interface statistics for a running guest virtual machine.

# virsh domifstat GuestName interface-device

⁠23.17. Managing virtual networks

This section covers managing virtual networks with the virsh command. To list virtual networks:

# virsh net-list

This command generates output similar to:

# virsh net-list
Name                 State      Autostart
-----------------------------------------
default              active     yes
vnet1	             active     yes
vnet2	             active     yes

To view network information for a specific virtual network:

# virsh net-dumpxml NetworkName

This displays information about a specified virtual network in XML format:

# virsh net-dumpxml vnet1
<network>
  <name>vnet1</name>
  <uuid>98361b46-1581-acb7-1643-85a412626e70</uuid>
  <forward dev='eth0'/>
  <bridge name='vnet0' stp='on' forwardDelay='0' />
  <ip address='192.168.100.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.100.128' end='192.168.100.254' />
    </dhcp>
  </ip>
</network>

Other virsh commands used in managing virtual networks are:

virsh net-autostart network-name — Autostart a network specified as network-name.

virsh net-create XMLfile — generates and starts a new network using an existing XML file.

virsh net-define XMLfile — generates a new network device from an existing XML file without starting it.

virsh net-destroy network-name — destroy a network specified as network-name.

virsh net-name networkUUID — convert a specified networkUUID to a network name.

virsh net-uuid network-name — convert a specified network-name to a network UUID.

virsh net-start nameOfInactiveNetwork — starts an inactive network.

virsh net-undefine nameOfInactiveNetwork — removes the definition of an inactive network.

libvirt has the capability to define virtual networks which can then be used by domains and linked to actual network devices. For more detailed information about this feature see the documentation at libvirt's website . Many of the commands for virtual networks are similar to the ones used for domains, but the way to name a virtual network is either by its name or UUID.

⁠23.17.1. Autostarting a virtual network

This command will configure a virtual network to be started automatically when the guest virtual machine boots. To run this command:

# virsh net-autostart network [--disable]

This command accepts the --disable option which disables the autostart command.

⁠23.17.2. Creating a virtual network from an XML file

This command creates a virtual network from an XML file. Refer to libvirt's website to get a description of the XML network format used by libvirt. In this command file is the path to the XML file. To create the virtual network from an XML file, run:

# virsh net-create file

⁠23.17.3. Defining a virtual network from an XML file

This command defines a virtual network from an XML file, the network is just defined but not instantiated. To define the virtual network, run:

# net-define file

⁠23.17.4. Stopping a virtual network

This command destroys (stops) a given virtual network specified by its name or UUID. This takes effect immediately. To stop the specified network network is required.

# net-destroy network

⁠23.17.5. Creating a dump file

This command outputs the virtual network information as an XML dump to stdout for the specified virtual network. If --inactive is specified, then physical functions are not expanded into their associated virtual functions. To create the dump file, run:

# virsh net-dumpxml network [--inactive]

⁠23.17.6. Eding a virtual network's XML configuration file

This command edits the XML configuration file for a network. This is equivalent to:

#virsh net-dumpxml --inactive network > network.xml
vi network.xml (or make changes with your other text editor)
virsh net-define network.xml

except that it does some error checking. The editor used can be supplied by the $VISUAL or $EDITOR environment variables, and defaults to "vi". To edit the network, run:

#virsh net-edit network

⁠23.17.7. Getting information about a virtual network

This command returns basic information about the network object. To get the network information, run:

# virsh net-info network

⁠23.17.8. Listing information about a virtual network

Returns the list of active networks, if --all is specified this will also include defined but inactive networks, if --inactive is specified only the inactive ones will be listed. You may also want to filter the returned networks by --persistent to list the persitent ones, --transient to list the transient ones, --autostart to list the ones with autostart enabled, and --no-autostart to list the ones with autostart disabled.

Note: When talking to older servers, this command is forced to use a series of API calls with an inherent race, where a pool might not be listed or might appear more than once if it changed state between calls while the list was being collected. Newer servers do not have this problem.

To list the virtual networks, run:

# net-list [--inactive | --all] [--persistent] [<--transient>] [--autostart] [<--no-autostart>]

⁠23.17.9. Converting a network UUID to network name

This command converts a network UUID to network name. To do this run:

# virsh net-name network-UUID

⁠23.17.10. Starting a (previously defined) inactive network

This command starts a (previously defined) inactive network. To do this, run:

# virsh net-start network

⁠23.17.11. Undefining the configuration for an inactive network

This command undefines the configuration for an inactive network. To do this, run:

# net-undefine network

⁠23.17.12. Converting a network name to network UUID

This command converts a network name to network UUID. To do this, run:

# virsh net-uuid network-name

⁠23.17.13. Updating an existing network definition file

This command updates the given section of an existing network definition, taking effect immediately, without needing to destroy and re-start the network. This command is one of "add-first", "add-last", "add" (a synonym for add-last), "delete", or "modify". section is one of ""bridge", "domain", "ip", "ip-dhcp-host", "ip-dhcp-range", "forward", "forward-interface", "forward-pf", "portgroup", "dns-host", "dns-txt", or "dns-srv", each section being named by a concatenation of the xml element hierarchy leading to the element being changed. For example, "ip-dhcp-host" will change a <host> element that is contained inside a <dhcp> element inside an <ip> element of the network. xml is either the text of a complete xml element of the type being changed (e.g. "<host mac="00:11:22:33:44:55’ ip=’1.2.3.4’/>", or the name of a file that contains a complete xml element. Disambiguation is done by looking at the first character of the provided text - if the first character is "<", it is xml text, if the first character is not ">", it is the name of a file that contains the xml text to be used. The **parent-index option is used to specify which of several parent elements the requested element is in (0-based). For example, a dhcp <host> element could be in any one of multiple <ip> elements in the network; if a parent-index isn’t provided, the "most appropriate" <ip> element will be selected (usually the only one that already has a <dhcp> element), but if --parent-index is given, that particular instance of <ip> will get the modification. If --live is specified, affect a running network. If --config is specified, affect the next startup of a persistent network. If -- current is specified, affect the current network state. Both --live and --config flags may be given, but --current is exclusive. Not specifying any flag is the same as specifying --current.

To update the configuration file, run:

# virsh net-update network command section xml [--parent-index index] [[--live] [--config] | [--current]]

⁠23.17.14. Migrating guest virtual machines with virsh

Information on migration using virsh is located in the section entitled Live KVM Migration with virsh Refer to Section 17.4, “Live KVM migration with virsh”

⁠23.17.15. Setting a static IP address for the guest virtual machine

In cases where a guest virtual machine is configured to acquire its IP address from DHCP, but you still need it to have a predictable static IP address, you can use the following procedure to modify the DHCP server configuration used by libvirt. This procedure requires that you know the MAC address of the guest interface in order to make this change. Therefore you will need to perform the operation after the guest has been created, or decide on a MAC address for the guest prior to creating it, and then set this same address manually when creating the guest virtual machine.

In addition, you should note that this procedure only works for guest interfaces that are connected to a libvirt virtual network with a forwarding mode of "nat", "route", or no forwarding mode at all. This procedure will not work if the network has been configured with forward mode="bridge" or "hostdev" . In those cases, the DCHP server is located elsewhere on the network, and is therefore not under control of libvirt. In this case the static IP entry would need to be made on the remote DHCP server. To do that refer to the documentation that is supplied with the server.

⁠

Procedure 23.6. Setting a static IP address

This procedure is performed on the host physical machine.

Check the guest XML configuration file

Display the guest's network configuration settings by running the virsh domiflist guest1 command. Substitute the name of your virtual machine in place of guest1. A table is displayed. Look in the Source column. That is the name of your network. In this example the network is called default. This name will be used for the rest of the procedure as well as the MAC address.

# virsh domiflist guest1
Interface  Type       Source     Model       MAC
-------------------------------------------------------
vnet4      network    default    virtio      52:54:00:48:27:1D

Verify the DHCP range

The IP address that you set must be within the dhcp range that is specified for the network. In addition it must also not conflict with any other existing static IP addresses on the network. To check the range of addresses available as well as addresses used, run the following command as root in a terminal on the host physical machine:

# virsh net-dumpxml default | egrep 'range|host\ mac'

<range start='198.51.100.2' end='198.51.100.254'/>
<host mac='52:54:00:48:27:1C:1D' ip='198.51.100.2'/>

The output you see will differ from the example and you may see more lines and multiple host mac lines. Each guest static IP address will have one line.

Set a static IP address

Run the following command as root in a terminal on the host physical machine. default is the name of the network. Substitute the name of your network in its place. The --live option allows this change to immediately take place and the --config option makes the change persistent. This command will also work for guest virtual machines that you have not yet created as long as you use a valid IP and MAC address. The MAC address should be a valid unicast MAC address (6 hexadecimal digit pairs separated by ":", with the first digit pair being an even number); when libvirt creates a new random MAC address, it uses "52:54:00" for the first three digit pairs, and it's recommended (but not required) to follow this convention.

# virsh net-update default add ip-dhcp-host '<host mac='52:54:00:48:27:1D' ip='198.51.100.3"/>'--live --config

Restart the interface (optional)

If the guest virtual machine is currently running you will need to force the guest virtual machine to re-request a DHCP address. If the guest is not running, the new IP address will be implemented the next time you start it. To restart the interface, run the following commands as root in a terminal on the host physical machine:

# virsh domif-setlink guest1 52:54:00:48:27:1D down
# sleep 10
# virsh domif-setlink guest1 52:54:00:48:27:1D up

This command makes the guest virtual machine's operating system think that the Ethernet cable has been unplugged, and then re-plugged after ten seconds. The sleep command is important because many DHCP clients allow for a short disconnect of the cable without re-requesting the IP address. Ten seconds is long enough so that the DHCP client forgets the old IP address and will request a new one once the up command is executed. If for some reason this command fails, you will have to reset the guest's interface from the guest operating system's management interface.

⁠23.18. Interface Commands

The following commands manipulate host interfaces and as such should not be run from the guest virtual machine. These commands should be run from a terminal on the host physical machine.

Warning

The commands in this section are only supported if the machine has the NetworkManager service disabled, and is using the network service instead.

Often, these host interfaces can then be used by name within guest virtual machine <interface> elements (such as a system-created bridge interface), but there is no requirement that host interfaces be tied to any particular guest configuration XML at all. Many of the commands for host interfaces are similar to the ones used for guest virtual machines, and the way to name an interface is either by its name or its MAC address. However, using a MAC address for an iface argument only works when that address is unique (if an interface and a bridge share the same MAC address, which is often the case, then using that MAC address results in an error due to ambiguity, and you must resort to a name instead).

⁠23.18.1. Defining and starting a host physical machine interface via an XML file

The virsh iface-define file command define a host interface from an XML file. This command will only define the interface and will not start it.

virsh iface-define iface.xml

To start an interface which has already been defined, run iface-start interface, where interface is the interface name.

⁠23.18.2. Editing the XML configuration file for the host interface

The command iface-edit interface edits the XML configuration file for a host interface. This is the only recommended way to edit the XML configuration file. (Refer to Chapter 26, Manipulating the domain XML for more information about these files.)

⁠23.18.3. Listing active host interfaces

The iface-list --inactive --all displays a list of active host interfaces. If --all is specified, this list will also include interfaces that are defined but are inactive. If --inactive is specified only the inactive interfaces will be listed.

⁠23.18.4. Converting a MAC address into an interface name

The iface-name interface command converts a host interface MAC to an interface name, provided the MAC address is unique among the host’s interfaces. This command requires interface which is the interface's MAC address.

The iface-mac interface command will convert a host's interface name to MAC address where in this case interface, is the interface name.

⁠23.18.5. Stopping a specific host physical machine interface

The virsh iface-destroy interface command destroys (stops) a given host interface, which is the same as running if-down on the host. This command will disable that interface from active use and takes effect immediately.

To undefine the interface, use the iface-undefine interface command along with the interface name.

⁠23.18.6. Displaying the host configuration file

virsh iface-dumpxml interface --inactive displays the host interface information as an XML dump to stdout. If the --inactive argument is specified, then the output reflects the persistent state of the interface that will be used the next time it is started.

⁠23.18.7. Creating bridge devices

The iface-bridge creates a bridge device named bridge, and attaches the existing network device interface to the new bridge, which starts working immediately, with STP enabled and a delay of 0.

# virsh iface-bridge interface bridge --no-stp delay --no-start

Note that these settings can be altered with --no-stp, --no-start, and an integer number of seconds for delay. All IP address configuration of interface will be moved to the new bridge device. Refer to Section 23.18.8, “Tearing down a bridge device” for information on tearing down the bridge.

⁠23.18.8. Tearing down a bridge device

The iface-unbridge bridge --no-start command tears down a specified bridge device named bridge, releases its underlying interface back to normal usage, and moves all IP address configuration from the bridge device to the underlying device. The underlying interface is restarted unless --no-start argument is used, but keep in mind not restarting is generally not recommended. Refer to Section 23.18.7, “Creating bridge devices” for the command to use to create a bridge.

⁠23.18.9. Manipulating interface snapshots

The iface-begin command creates a snapshot of current host interface settings, which can later be committed (with iface-commit) or restored (iface-rollback). If a snapshot already exists, then this command will fail until the previous snapshot has been committed or restored. Undefined behavior will result if any external changes are made to host interfaces outside of the libvirt API between the time of the creation of a snapshot and its eventual commit or rollback.

Use the iface-commit command to declare all changes made since the last iface-begin as working, and then delete the rollback point. If no interface snapshot has already been started via iface-begin, then this command will fail.

Use the iface-rollback to revert all host interface settings back to the state that recorded the last time the iface-begin command was executed. If iface-begin command had not been previously executed, then iface-rollback will fail. Note that if the host physical machine is rebooted before virsh iface-commit is run, an automatic rollback will be performed which will restore the host's configuration to the state it was at the time that the virsh iface-begin was executed. This is useful in cases where an improper change to the network configuration renders the host unreachable for purposes of undoing the change, but the host is either power-cycled or otherwise forced to reboot.

⁠23.19. Managing snapshots

The sections that follow describe actions that can be done in order to manipulate guest virtual machine snapshots. Snapshots take the disk, memory, and device state of a guest virtual machine at a specified point-in-time, and save it for future use. Snapshots have many uses, from saving a "clean" copy of an OS image to saving a guest virtual machine’s state before what may be a potentially destructive operation. Snapshots are identified with a unique name. See the libvirt website for documentation of the XML format used to represent properties of snapshots.

live snapshots

In all cases where --live is listed take note that live snapshots are not supported with Red Hat Enterprise Linux 7 but you can create a snapshot while the guest virtual machine is powered down. Live snapshot creation is available on Red Hat Enterprise Virtualization . Call your service representative for details.

⁠23.19.1. Creating Snapshots

The virsh snapshot create command creates a snapshot for guest virtual machine with the properties specified in the guest virtual machine's XML file (such as <name> and <description> elements, as well as <disks>). To create a snapshot run:

#snapshot-create <domain> <xmlfile> [--redefine [--current] [--no-metadata] [--halt] [--disk-only] [--reuse-external] [--quiesce] [--atomic] [--live]

The guest virtual machine name, id, or uid may be used as the guest virtual machine requirement. The XML requirement is a string that must in the very least contain the <name>, <description> and <disks> elements.

The remaining optional arguments are as follows:

--disk-only - causes the rest of the fields to be ignored, and automatically filled in by libvirt.

If the XML file string is completely omitted, then libvirt will choose a value for all fields. The new snapshot will become current, as listed by snapshot-current. In addition the snapshot will only include the disk state rather than the usual system checkpoint with guest virtual machine state. Disk snapshots are faster than full system checkpoints, but reverting to a disk snapshot may require fsck or journal replays, since it is like the disk state at the point when the power cord is abruptly pulled. Note that mixing --halt and --disk-only loses any data that was not flushed to disk at the time.

--halt - causes the guest virtual machine to be left in an inactive state after the snapshot is created. Mixing --halt and --disk-only loses any data that was not flushed to disk at the time

--redefine specifies that if all XML elements produced by snapshot-dumpxml are valid; it can be used to migrate snapshot hierarchy from one machine to another, to recreate hierarchy for the case of a transient guest virtual machine that goes away and is later recreated with the same name and UUID, or to make slight alterations in the snapshot metadata (such as host-specific aspects of the guest virtual machine XML embedded in the snapshot). When this flag is supplied, the xmlfile argument is mandatory, and the guest virtual machine’s current snapshot will not be altered unless the --current flag is also given.

--no-metadata creates the snapshot, but any metadata is immediately discarded (that is, libvirt does not treat the snapshot as current, and cannot revert to the snapshot unless --redefine is later used to teach libvirt about the metadata again).

--reuse-external, if used and snapshot XML requests an external snapshot with a destination of an existing file, then the destination must exist, and is reused; otherwise, a snapshot is refused to avoid losing contents of the existing files.

--quiesce libvirt will try to to freeze and unfreeze the guest virtual machine’s mounted file system(s), using the guest agent. However, if the guest virtual machine doesn't have a guest agent, snapshot creation will fail. Currently, this requires --disk-only to be passed as well.

--atomic causes libvirt to guarantee that the snapshot either succeeds, or fails with no changes. Note that not all hypervisors support this. If this flag is not specified, then some hypervisors may fail after partially performing the action, and dumpxml must be used to see whether any partial changes occurred.

--live Refer to live snapshots. This option causes libvirt to take the snapshot while the guest is running. This increases the size of the memory image of the external checkpoint. This is currently supported only for external checkpoints. Existence of snapshot metadata will prevent attempts to undefine a persistent guest virtual machine. However, for transient guest virtual machines, snapshot metadata is silently lost when the guest virtual machine quits running (whether by command such as destroy or by internal guest action).

⁠23.19.2. Creating a snapshot for the current guest virtual machine

The virsh snapshot-create-as command creates a snapshot for guest virtual machine with the properties specified in the domain XML file (such as <name> and <description> elements). If these values are not included in the XML string, libvirt will choose a value. To create a snapshot run:

# snapshot-create-as domain {[--print-xml] | [--no-metadata] [--halt] [--reuse-external]} [name] [description] [--disk-only [--quiesce]] [--atomic] [[--live] [--memspec memspec]] [--diskspec] diskspec]

The remaining optional arguments are as follows:

--print-xmlcreates appropriate XML for snapshot-create as output, rather than actually creating a snapshot.

--halt keeps the guest virtual machine in an inactive state after the snapshot is created.

--disk-only creates a snapshot that does not include the guest virtual machine state.

--memspec can be used to control whether a checkpoint is internal or external. The flag is mandatory, followed by a memspec of the form [file=]name[,snapshot=type], where type can be none, internal, or external. To include a literal comma in file=name, escape it with a second comma.

--diskspec option can be used to control how --disk-only and external checkpoints create external files. This option can occur multiple times, according to the number of <disk> elements in the domain XML. Each <diskspec> is in the form disk[,snapshot=type][,driver=type][,file=name]. To include a literal comma in disk or in file=name, escape it with a second comma. A literal --diskspec must precede each diskspec unless all three of <domain>, <name>, and <description> are also present. For example, a diskspec of vda,snapshot=external,file=/path/to,,new results in the following XML:

<disk name=’vda’ snapshot=’external’>
   <source file=’/path/to,new’/>
</disk>

--reuse-external is specified, and the domain XML or diskspec option requests an external snapshot with a destination of an existing file, then the destination must exist, and is reused; otherwise, a snapshot is refused to avoid losing contents of the existing files.

--quiesce is specified, libvirt will try to use guest agent to freeze and unfreeze guest virtual machine’s mounted file systems. However, if domain has no guest agent, snapshot creation will fail. Currently, this requires --disk-only to be passed as well.

--no-metadata creates snapshot data but any metadata is immediately discarded (that is,libirt does not treat the snapshot as current, and cannot revert to the snapshot unless snapshot-create is later used to teach libvirt about the metadata again). This flag is incompatible with --print-xml

--atomicwill cause libvirt to guarantee that the snapshot either succeeds, or fails with no changes. It should be noted that not all hypervisors support this. If this flag is not specified, then some hypervisors may fail after partially performing the action, and dumpxml must be used to see whether any partial changes occurred.

--live Refer to live snapshots. This option causes libvirt to take the snapshot while the guest virtual machine is running. This increases the size of the memory image of the external checkpoint. This is currently supported only for external checkpoints.

⁠23.19.3. Taking a snapshot of the current guest virtual machine

This command is used to query which snapshot is currently in use. To use, run:

# virsh snapshot-current domain {[--name] | [--security-info] | [snapshotname]}

If snapshotname is not used, snapshot XML for the guest virtual machine’s current snapshot (if there is one) will be displayed as output. If --name is specified, just the current snapshot name instead of the full XML will be sent as output. If --security-info is supplied security sensitive information will be included in the XML. Using snapshotname, generates a request to make the existing named snapshot become the current snapshot, without reverting it to the guest virtual machine.

⁠23.19.4. snapshot-edit-domain

This command is used to edit the snapshot that is currently in use. To use, run:

#virsh snapshot-edit domain [snapshotname] [--current] {[--rename] [--clone]}

If both snapshotname and --current are specified, it forces the edited snapshot to become the current snapshot. If snapshotname is omitted, then --current must be supplied, in order to edit the current snapshot.

This is equivalent to the following command sequence below, but it also includes some error checking:

# virsh snapshot-dumpxml dom name > snapshot.xml
# vi snapshot.xml [note - this can be any editor]
# virsh snapshot-create dom snapshot.xml --redefine [--current]

If --rename is specified, then the resulting edited file gets saved in a different file name. If --clone is specified, then changing the snapshot name will create a clone of the snapshot metadata. If neither is specified, then the edits will not change the snapshot name. Note that changing a snapshot name must be done with care, since the contents of some snapshots, such as internal snapshots within a single qcow2 file, are accessible only from the original snapshot filename.

⁠23.19.5. snapshot-info-domain

snapshot-info-domain displays information about the snapshots. To use, run:

# snapshot-info domain {snapshot | --current}

Outputs basic information about a specified snapshot , or the current snapshot with --current.

⁠23.19.6. snapshot-list-domain

List all of the available snapshots for the given guest virtual machine, defaulting to show columns for the snapshot name, creation time, and guest virtual machine state. To use, run:

#virsh snapshot-list domain [{--parent | --roots | --tree}] [{[--from] snapshot | --current} [--descendants]] [--metadata] [--no-metadata] [--leaves] [--no-leaves] [--inactive] [--active] [--disk-only] [--internal] [--external]

The remaining optional arguments are as follows:

--parent adds a column to the output table giving the name of the parent of each snapshot. This option may not be used with --roots or --tree.

--roots filters the list to show only the snapshots that have no parents. This option may not be used with --parent or --tree.

--tree displays output in a tree format, listing just snapshot names. These three options are mutually exclusive. This option may not be used with --roots or --parent.

--from filters the list to snapshots which are children of the given snapshot; or if --current is provided, will cause the list to start at the current snapshot. When used in isolation or with --parent, the list is limited to direct children unless --descendants is also present. When used with --tree, the use of --descendants is implied. This option is not compatible with --roots. Note that the starting point of --from or --current is not included in the list unless the --tree option is also present.

--leaves is specified, the list will be filtered to just snapshots that have no children. Likewise, if --no-leaves is specified, the list will be filtered to just snapshots with children. (Note that omitting both options does no filtering, while providing both options will either produce the same list or error out depending on whether the server recognizes the flags) Filtering options are not compatible with --tree..

--metadata is specified, the list will be filtered to just snapshots that involve libvirt metadata, and thus would prevent the undefining of a persistent guest virtual machine, or be lost on destroy of a transient guest virtual machine. Likewise, if --no-metadata is specified, the list will be filtered to just snapshots that exist without the need for libvirt metadata.

--inactive is specified, the list will be filtered to snapshots that were taken when the guest virtual machine was shut off. If --active is specified, the list will be filtered to snapshots that were taken when the guest virtual machine was running, and where the snapshot includes the memory state to revert to that running state. If --disk-only is specified, the list will be filtered to snapshots that were taken when the guest virtual machine was running, but where the snapshot includes only disk state.

--internal is specified, the list will be filtered to snapshots that use internal storage of existing disk images. If --external is specified, the list will be filtered to snapshots that use external files for disk images or memory state.

⁠23.19.7. snapshot-dumpxml domain snapshot

virsh snapshot-dumpxml domain snapshot outputs the snapshot XML for the guest virtual machine’s snapshot named snapshot. To use, run:

# virsh snapshot-dumpxml domain snapshot [--security-info]

The --security-info option will also include security sensitive information. Use snapshot-current to easily access the XML of the current snapshot.

⁠23.19.8. snapshot-parent guest virtual machine

Outputs the name of the parent snapshot, if any, for the given snapshot, or for the current snapshot with --current. To use, run:

#virsh snapshot-parent domain {snapshot | --current}

⁠23.19.9. snapshot-revert guest virtual machine

Reverts the given domain to the snapshot specified by snapshot, or to the current snapshot with --current.

Warning

Be aware that this is a destructive action; any changes in the domain since the last snapshot was taken will be lost. Also note that the state of the domain after snapshot-revert is complete will be the state of the domain at the time the original snapshot was taken.

To revert the snapshot, run

# snapshot-revert domain {snapshot | --current} [{--running | --paused}] [--force]

Normally, reverting to a snapshot leaves the domain in the state it was at the time the snapshot was created, except that a disk snapshot with no guest virtual machine state leaves the domain in an inactive state. Passing either the --running or --paused flag will perform additional state changes (such as booting an inactive domain, or pausing a running domain). Since transient domains cannot be inactive, it is required to use one of these flags when reverting to a disk snapshot of a transient domain.

There are two cases where a snapshot revert involves extra risk, which requires the use of --force to proceed. One is the case of a snapshot that lacks full domain information for reverting configuration; since libvirt cannot prove that the current configuration matches what was in use at the time of the snapshot, supplying --force assures libvirt that the snapshot is compatible with the current configuration (and if it is not, the domain will likely fail to run). The other is the case of reverting from a running domain to an active state where a new hypervisor has to be created rather than reusing the existing hypervisor, because it implies drawbacks such as breaking any existing VNC or Spice connections; this condition happens with an active snapshot that uses a provably incompatible configuration, as well as with an inactive snapshot that is combined with the --start or --pause flag.

⁠23.19.10. snapshot-delete domain

snapshot-delete domain deletes the snapshot for the specified domain. To do this, run:

# virsh snapshot-delete domain {snapshot | --current} [--metadata] [{--children | --children-only}]

This command Deletes the snapshot for the domain named snapshot, or the current snapshot with --current. If this snapshot has child snapshots, changes from this snapshot will be merged into the children. If the option --children is used, then it will delete this snapshot and any children of this snapshot. If --children-only is used, then it will delete any children of this snapshot, but leave this snapshot intact. These two flags are mutually exclusive.

The --metadata is used it will delete the snapshot's metadata maintained by libvirt, while leaving the snapshot contents intact for access by external tools; otherwise deleting a snapshot also removes its data contents from that point in time.

⁠23.20. Guest virtual machine CPU model configuration

⁠23.20.1. Introduction

Every hypervisor has its own policy for what a guest virtual machine will see for its CPUs by default. Whereas some hypervisors decide which CPU host physical machine features will be available for the guest virtual machine, QEMU/KVM presents the guest virtual machine with a generic model named qemu32 or qemu64. These hypervisors perform more advanced filtering, classifying all physical CPUs into a handful of groups and have one baseline CPU model for each group that is presented to the guest virtual machine. Such behavior enables the safe migration of guest virtual machines between host physical machines, provided they all have physical CPUs that classify into the same group. libvirt does not typically enforce policy itself, rather it provides the mechanism on which the higher layers define their own desired policy. Understanding how to obtain CPU model information and define a suitable guest virtual machine CPU model is critical to ensure guest virtual machine migration is successful between host physical machines. Note that a hypervisor can only emulate features that it is aware of and features that were created after the hypervisor was released may not be emulated.

⁠23.20.2. Learning about the host physical machine CPU model

The virsh capabilities command displays an XML document describing the capabilities of the hypervisor connection and host physical machine. The XML schema displayed has been extended to provide information about the host physical machine CPU model. One of the big challenges in describing a CPU model is that every architecture has a different approach to exposing their capabilities. On x86, the capabilities of a modern CPU are exposed via the CPUID instruction. Essentially this comes down to a set of 32-bit integers with each bit given a specific meaning. Fortunately AMD and Intel agree on common semantics for these bits. Other hypervisors expose the notion of CPUID masks directly in their guest virtual machine configuration format. However, QEMU/KVM supports far more than just the x86 architecture, so CPUID is clearly not suitable as the canonical configuration format. QEMU ended up using a scheme which combines a CPU model name string, with a set of named flags. On x86, the CPU model maps to a baseline CPUID mask, and the flags can be used to then toggle bits in the mask on or off. libvirt decided to follow this lead and uses a combination of a model name and flags.

It is not practical to have a database listing all known CPU models, so libvirt has a small list of baseline CPU model names. It chooses the one that shares the greatest number of CPUID bits with the actual host physical machine CPU and then lists the remaining bits as named features. Notice that libvirt does not display which features the baseline CPU contains. This might seem like a flaw at first, but as will be explained in this section, it is not actually necessary to know this information.

⁠23.20.3. Determining support for VFIO IOMMU devices

Use the virsh domcapabilities command to determine support for VFIO. See the following example output:

⁠

​
​
​# virsh domcapabilities
​
​[...output truncated...]
​
​<enum name='pciBackend'>
​        <value>default</value>
​        <value>vfio</value>
​
​[...output truncated...]

Figure 23.3. Determining support for VFIO

⁠23.20.4. Determining a compatible CPU model to suit a pool of host physical machines

Now that it is possible to find out what CPU capabilities a single host physical machine has, the next step is to determine what CPU capabilities are best to expose to the guest virtual machine. If it is known that the guest virtual machine will never need to be migrated to another host physical machine, the host physical machine CPU model can be passed straight through unmodified. A virtualized data center may have a set of configurations that can guarantee all servers will have 100% identical CPUs. Again the host physical machine CPU model can be passed straight through unmodified. The more common case, though, is where there is variation in CPUs between host physical machines. In this mixed CPU environment, the lowest common denominator CPU must be determined. This is not entirely straightforward, so libvirt provides an API for exactly this task. If libvirt is provided a list of XML documents, each describing a CPU model for a host physical machine, libvirt will internally convert these to CPUID masks, calculate their intersection, and convert the CPUID mask result back into an XML CPU description.

Here is an example of what libvirt reports as the capabilities on a basic workstation, when the virsh capabilitiesis executed:

⁠

​
​
​<capabilities>
​  <host>
​    <cpu>
​      <arch>i686</arch>
​      <model>pentium3</model>
​      <topology sockets='1' cores='2' threads='1'/>
​      <feature name='lahf_lm'/>
​      <feature name='lm'/>
​      <feature name='xtpr'/>
​      <feature name='cx16'/>
​      <feature name='ssse3'/>
​      <feature name='tm2'/>
​      <feature name='est'/>
​      <feature name='vmx'/>
​      <feature name='ds_cpl'/>
​      <feature name='monitor'/>
​      <feature name='pni'/>
​      <feature name='pbe'/>
​      <feature name='tm'/>
​      <feature name='ht'/>
​      <feature name='ss'/>
​      <feature name='sse2'/>
​      <feature name='acpi'/>
​      <feature name='ds'/>
​      <feature name='clflush'/>
​      <feature name='apic'/>
​    </cpu>
​ </host>
​</capabilities>

Figure 23.4. Pulling host physical machine's CPU model information

Now compare that to any random server, with the same virsh capabilities command:

⁠

​
​
​<capabilities>
​  <host>
​    <cpu>
​      <arch>x86_64</arch>
​      <model>phenom</model>
​      <topology sockets='2' cores='4' threads='1'/>
​      <feature name='osvw'/>
​      <feature name='3dnowprefetch'/>
​      <feature name='misalignsse'/>
​      <feature name='sse4a'/>
​      <feature name='abm'/>
​      <feature name='cr8legacy'/>
​      <feature name='extapic'/>
​      <feature name='cmp_legacy'/>
​      <feature name='lahf_lm'/>
​      <feature name='rdtscp'/>
​      <feature name='pdpe1gb'/>
​      <feature name='popcnt'/>
​      <feature name='cx16'/>
​      <feature name='ht'/>
​      <feature name='vme'/>
​    </cpu>
​    ...snip...

Figure 23.5. Generate CPU description from a random server

To see if this CPU description is compatible with the previous workstation CPU description, use the virsh cpu-compare command.

The reduced content was stored in a file named virsh-caps-workstation-cpu-only.xml and the virsh cpu-compare command can be executed on this file:

# virsh cpu-compare virsh-caps-workstation-cpu-only.xml
Host physical machine CPU is a superset of CPU described in virsh-caps-workstation-cpu-only.xml

As seen in this output, libvirt is correctly reporting that the CPUs are not strictly compatible. This is because there are several features in the server CPU that are missing in the client CPU. To be able to migrate between the client and the server, it will be necessary to open the XML file and comment out some features. To determine which features need to be removed, run the virsh cpu-baseline command, on the both-cpus.xml which contains the CPU information for both machines. Running # virsh cpu-baseline both-cpus.xml, results in:

⁠

​
​
​<cpu match='exact'>
​  <model>pentium3</model>
​  <feature policy='require' name='lahf_lm'/>
​  <feature policy='require' name='lm'/>
​  <feature policy='require' name='cx16'/>
​  <feature policy='require' name='monitor'/>
​  <feature policy='require' name='pni'/>
​  <feature policy='require' name='ht'/>
​  <feature policy='require' name='sse2'/>
​  <feature policy='require' name='clflush'/>
​  <feature policy='require' name='apic'/>
​</cpu>

Figure 23.6. Composite CPU baseline

This composite file shows which elements are in common. Everything that is not in common should be commented out.

⁠23.21. Configuring the guest virtual machine CPU model

For simple defaults, the guest virtual machine CPU configuration accepts the same basic XML representation as the host physical machine capabilities XML exposes. In other words, the XML from the cpu-baseline virsh command can now be copied directly into the guest virtual machine XML at the top level under the <domain> element. In the previous XML snippet, there are a few extra attributes available when describing a CPU in the guest virtual machine XML. These can mostly be ignored, but for the curious here is a quick description of what they do. The top level <cpu> element has an attribute called match with possible values of:

match='minimum' - the host physical machine CPU must have at least the CPU features described in the guest virtual machine XML. If the host physical machine has additional features beyond the guest virtual machine configuration, these will also be exposed to the guest virtual machine.

match='exact' - the host physical machine CPU must have at least the CPU features described in the guest virtual machine XML. If the host physical machine has additional features beyond the guest virtual machine configuration, these will be masked out from the guest virtual machine.

match='strict' - the host physical machine CPU must have exactly the same CPU features described in the guest virtual machine XML.

The next enhancement is that the <feature> elements can each have an extra 'policy' attribute with possible values of:

policy='force' - expose the feature to the guest virtual machine even if the host physical machine does not have it. This is usually only useful in the case of software emulation.

policy='require' - expose the feature to the guest virtual machine and fail if the host physical machine does not have it. This is the sensible default.

policy='optional' - expose the feature to the guest virtual machine if it happens to support it.

policy='disable' - if the host physical machine has this feature, then hide it from the guest virtual machine.

policy='forbid' - if the host physical machine has this feature, then fail and refuse to start the guest virtual machine.

The 'forbid' policy is for a niche scenario where an incorrectly functioning application will try to use a feature even if it is not in the CPUID mask, and you wish to prevent accidentally running the guest virtual machine on a host physical machine with that feature. The 'optional' policy has special behavior with respect to migration. When the guest virtual machine is initially started the flag is optional, but when the guest virtual machine is live migrated, this policy turns into 'require', since you cannot have features disappearing across migration.

⁠23.22. Managing resources for guest virtual machines

virsh allows the grouping and allocation of resources on a per guest virtual machine basis. This is managed by the libvirt daemon which creates cgroups and manages them on behalf of the guest virtual machine. The only thing that is left for the system administrator to do is to either query or set tunables against specified guest virtual machines. The following tunables may used:

memory - The memory controller allows for setting limits on RAM and swap usage and querying cumulative usage of all processes in the group

cpuset - The CPU set controller binds processes within a group to a set of CPUs and controls migration between CPUs.

cpuacct - The CPU accounting controller provides information about CPU usage for a group of processes.

cpu -The CPU scheduler controller controls the prioritization of processes in the group. This is similar to granting nice level privileges.

devices - The devices controller grants access control lists on character and block devices.

freezer - The freezer controller pauses and resumes execution of processes in the group. This is similar to SIGSTOP for the whole group.

net_cls - The network class controller manages network utilization by associating processes with a tc network class.

In creating a group hierarchy cgroup will leave mount point and directory setup entirely to the administrators’ discretion and is more complex than just adding some mount points to /etc/fstab. It is necessary to setup the directory hierarchy and decide how processes get placed within it. This can be done with the following virsh commands:

schedinfo - described in Section 23.23, “Setting schedule parameters”

blkdeviotune - described in Section 23.24, “Disk I/O throttling”

blkiotune- described in Section 23.25, “Display or set block I/O parameters”

domiftune- described in Section 23.10.14, “Setting network interface bandwidth parameters”

memtune - described in Section 23.26, “Configuring memory Tuning”

⁠23.23. Setting schedule parameters

schedinfo allows scheduler parameters to be passed to guest virtual machines. The following command format should be used:

#virsh schedinfo domain --set --weight --cap --current --config --live

Each parameter is explained below:

domain - this is the guest virtual machine domain

--set - the string placed here is the controller or action that is to be called. Additional parameters or values if required should be added as well.

--current - when used with --set, will use the specified set string as the current scheduler information. When used without will display the current scheduler information.

--config - - when used with --set, will use the specified set string on the next reboot. When used without will display the scheduler information that is saved in the configuration file.

--live - when used with --set, will use the specified set string on a guest virtual machine that is currently running. When used without will display the configuration setting currently used by the running virtual machine

The scheduler can be set with any of the following parameters: cpu_shares, vcpu_period and vcpu_quota.

⁠

Example 23.7. schedinfo show

This example shows the shell guest virtual machine's schedule information

# virsh schedinfo shell
Scheduler      : posix
cpu_shares     : 1024
vcpu_period    : 100000
vcpu_quota     : -1

⁠

Example 23.8. schedinfo set

In this example, the cpu_shares is changed to 2046. This effects the current state and not the configuration file.

# virsh schedinfo --set cpu_shares=2046 shell
Scheduler      : posix
cpu_shares     : 2046
vcpu_period    : 100000
vcpu_quota     : -1

⁠23.24. Disk I/O throttling

virsh blkdeviotune sets disk I/O throttling for a specified guest virtual machine. This can prevent a guest virtual machine from over utilizing shared resources and thus impacting the performance of other guest virtual machines. The following format should be used:

# virsh blkdeviotune <domain> <device> [[--config] [--live] | [--current]] [[total-bytes-sec] | [read-bytes-sec] [write-bytes-sec]] [[total-iops-sec] [read-iops-sec] [write-iops-sec]]

The only required parameter is the domain name of the guest virtual machine. To list the domain name, run the domblklist command. The --config, --live, and --current arguments function the same as in Section 23.23, “Setting schedule parameters”. If no limit is specified, it will query current I/O limits setting. Otherwise, alter the limits with the following flags:

--total-bytes-sec - specifies total throughput limit in bytes per second.

--read-bytes-sec - specifies read throughput limit in bytes per second.

--write-bytes-sec - specifies write throughput limit in bytes per second.

--total-iops-sec - specifies total I/O operations limit per second.

--read-iops-sec - specifies read I/O operations limit per second.

--write-iops-sec - specifies write I/O operations limit per second.

For more information refer to the blkdeviotune section of the virsh MAN page. For an example domain XML refer to Figure 26.27, “Devices - Hard drives, floppy disks, CD-ROMs”.

⁠23.25. Display or set block I/O parameters

blkiotune sets and or displays the I/O parameters for a specified guest virtual machine. The following format should be used:

# virsh blkiotune domain [--weight weight] [--device-weights device-weights] [[--config] [--live] | [--current]]

More information on this command can be found in the Virtualization Tuning and Optimization Guide

⁠23.26. Configuring memory Tuning

The virsh memtune virtual_machine --parameter size is covered in the Virtualization Tuning and Opitimization Guide.

⁠Chapter 24. Guest virtual machine disk access with offline tools

⁠24.1. Introduction

Red Hat Enterprise Linux 7 comes with tools to access, edit and create guest virtual machine disks or other disk images. There are several uses for these tools, including:

Viewing or downloading files located on a guest virtual machine disk.

Editing or uploading files onto a guest virtual machine disk.

Reading or writing guest virtual machine configuration.

Preparing new disk images containing files, directories, file systems, partitions, logical volumes and other options.

Rescuing and repairing guest virtual machines that fail to boot or those that need boot configuration changes.

Monitoring disk usage of guest virtual machines.

Auditing compliance of guest virtual machines, for example to organizational security standards.

Deploying guest virtual machines by cloning and modifying templates.

Reading CD and DVD ISO and floppy disk images.

Warning

You must never use these tools to write to a guest virtual machine or disk image which is attached to a running virtual machine, not even to open such a disk image in write mode. Doing so will result in disk corruption of the guest virtual machine. The tools try to prevent you from doing this, however do not catch all cases. If there is any suspicion that a guest virtual machine might be running, it is strongly recommended that the tools not be used, or at least always use the tools in read-only mode.

Where to find further documentation

The primary source for documentation for libguestfs and the tools are the Unix man pages. The API is documented in guestfs(3). guestfish is documented in guestfish(1). The virt tools are documented in their own man pages (eg. virt-df(1)). For troubleshooting information, refer to Section A.17, “libguestfs troubleshooting”

⁠24.1.1. Caution about using remote connections

Some virt commands in Red Hat Enterprise Linux 7 allow you to specify a remote libvirt connection. For example:

# virt-df -c  qemu://remote/system -d Guest

libguestfs in Red Hat Enterprise Linux 7 cannot access the disks of remote libvirt guests, and commands using remote URLs like this do not work as expected.

However beginning with Red Hat Enterprise Linux 7, libguestfs can access remote disk sources over NBD. You can export a disk image from a remote machine using the qemu-nbd command, and access it using a nbd:// URL,. You may need to open a port on your firewall (port 10809) as shown here:

(on remote system) qemu-nbd -t disk.img

(on local system) virt-df -a nbd://remote

The following libguestfs commands are affected:

- guestfish

- guestmount

- virt-alignment-scan

- virt-cat

- virt-copy-in

- virt-copy-out

- virt-df

- virt-edit

- virt-filesystems

- virt-inspector

- virt-ls

- virt-rescue

- virt-sysprep

- virt-tar-in

- virt-tar-out

- virt-win-reg

⁠24.2. Terminology

This section explains the terms used throughout this chapter.

libguestfs (GUEST FileSystem LIBrary) - the underlying C library that provides the basic functionality for opening disk images, reading and writing files and so on. You can write C programs directly to this API, but it is quite low level.

guestfish (GUEST Filesystem Interactive SHell) is an interactive shell that you can use from the command line or from shell scripts. It exposes all of the functionality of the libguestfs API.

Various virt tools are built on top of libguestfs, and these provide a way to perform specific single tasks from the command line. Tools include virt-df, virt-rescue, virt-resize and virt-edit.

hivex and augeas are libraries for editing the Windows Registry and Linux configuration files respectively. Although these are separate from libguestfs, much of the value of libguestfs comes from the combination of these tools.

guestmount is an interface between libguestfs and FUSE. It is primarily used to mount file systems from disk images on your host physical machine. This functionality is not necessary, but can be useful.

⁠24.3. Installation

To install libguestfs, guestfish, the libguestfs tools, guestmount and support for Windows guest virtual machines, subscribe to the RHEL V2WIN channel, go to the Red Hat Website and run the following command:

# yum install libguestfs guestfish libguestfs-tools libguestfs-mount libguestfs-winsupport

Important

Red Hat Enterprise Linux 7 Windows guests are only supported under specific subscription programs such as Advanced Mission Critical (AMC). If you are unsure whether your subscription model includes support for Windows guests, please contact customer support.

To install every libguestfs-related package including the language bindings, run the following command:

# yum install '*guestf*'

⁠24.4. The guestfish shell

guestfish is an interactive shell that you can use from the command line or from shell scripts to access guest virtual machine file systems. All of the functionality of the libguestfs API is available from the shell.

To begin viewing or editing a virtual machine disk image, run the following command, substituting the path to your desired disk image:

$ guestfish --ro -a /path/to/disk/image

--ro means that the disk image is opened read-only. This mode is always safe but does not allow write access. Only omit this option when you are certain that the guest virtual machine is not running, or the disk image is not attached to a live guest virtual machine. It is not possible to use libguestfs to edit a live guest virtual machine, and attempting to will result in irreversible disk corruption.

/path/to/disk/image is the path to the disk. This can be a file, a host physical machine logical volume (such as /dev/VG/LV), a host physical machine device (/dev/cdrom) or a SAN LUN (/dev/sdf3).

Note

libguestfs and guestfish do not require root privileges. You only need to run them as root if the disk image being accessed needs root to read and/or write.

When you start guestfish interactively, it will display this prompt:

$ guestfish --ro -a /path/to/disk/image

Welcome to guestfish, the libguestfs filesystem interactive shell for editing virtual machine filesystems.

 Type: 'help' for help on commands
       'man' to read the manual
       'quit' to quit the shell

><fs>

At the prompt, type run to initiate the library and attach the disk image. This can take up to 30 seconds the first time it is done. Subsequent starts will complete much faster.

Note

libguestfs will use hardware virtualization acceleration such as KVM (if available) to speed up this process.

Once the run command has been entered, other commands can be used, as the following section demonstrates.

⁠24.4.1. Viewing file systems with guestfish

⁠24.4.1.1. Manual listing and viewing

The list-filesystems command will list file systems found by libguestfs. This output shows a Red Hat Enterprise Linux 4 disk image:

><fs> run
><fs> list-filesystems
/dev/vda1: ext3
/dev/VolGroup00/LogVol00: ext3
/dev/VolGroup00/LogVol01: swap

This output shows a Windows disk image:

><fs> run
><fs> list-filesystems
/dev/vda1: ntfs
/dev/vda2: ntfs

Other useful commands are list-devices, list-partitions, lvs, pvs, vfs-type and file. You can get more information and help on any command by typing help command, as shown in the following output:

><fs> help vfs-type
 NAME
    vfs-type - get the Linux VFS type corresponding to a mounted device

 SYNOPSIS
     vfs-type device

 DESCRIPTION
    This command gets the filesystem type corresponding to the filesystem on
    "device".

    For most filesystems, the result is the name of the Linux VFS module
    which would be used to mount this filesystem if you mounted it without
    specifying the filesystem type. For example a string such as "ext3" or
    "ntfs".

To view the actual contents of a file system, it must first be mounted. This example uses one of the Windows partitions shown in the previous output (/dev/vda2), which in this case is known to correspond to the C:\ drive:

><fs> mount-ro /dev/vda2 /
><fs> ll /
total 1834753
 drwxrwxrwx  1 root root       4096 Nov  1 11:40 .
 drwxr-xr-x 21 root root       4096 Nov 16 21:45 ..
 lrwxrwxrwx  2 root root         60 Jul 14  2009 Documents and Settings
 drwxrwxrwx  1 root root       4096 Nov 15 18:00 Program Files
 drwxrwxrwx  1 root root       4096 Sep 19 10:34 Users
 drwxrwxrwx  1 root root      16384 Sep 19 10:34 Windows

You can use guestfish commands such as ls, ll, cat, more, download and tar-out to view and download files and directories.

Note

There is no concept of a current working directory in this shell. Unlike ordinary shells, you cannot for example use the cd command to change directories. All paths must be fully qualified starting at the top with a forward slash (/) character. Use the Tab key to complete paths.

To exit from the guestfish shell, type exit or enter Ctrl+d.

⁠24.4.1.2. Via guestfish inspection

Instead of listing and mounting file systems by hand, it is possible to let guestfish itself inspect the image and mount the file systems as they would be in the guest virtual machine. To do this, add the -i option on the command line:

$ guestfish --ro -a /path/to/disk/image -i

Welcome to guestfish, the libguestfs filesystem interactive shell for
 editing virtual machine filesystems.

 Type: 'help' for help on commands
       'man' to read the manual
       'quit' to quit the shell

 Operating system: Red Hat Enterprise Linux AS release 4 (Nahant Update 8)
 /dev/VolGroup00/LogVol00 mounted on /
 /dev/vda1 mounted on /boot

 ><fs> ll /
 total 210
 drwxr-xr-x. 24 root root  4096 Oct 28 09:09 .
 drwxr-xr-x  21 root root  4096 Nov 17 15:10 ..
 drwxr-xr-x.  2 root root  4096 Oct 27 22:37 bin
 drwxr-xr-x.  4 root root  1024 Oct 27 21:52 boot
 drwxr-xr-x.  4 root root  4096 Oct 27 21:21 dev
 drwxr-xr-x. 86 root root 12288 Oct 28 09:09 etc
 [etc]

Because guestfish needs to start up the libguestfs back end in order to perform the inspection and mounting, the run command is not necessary when using the -i option. The -i option works for many common Linux and Windows guest virtual machines.

⁠24.4.1.3. Accessing a guest virtual machine by name

A guest virtual machine can be accessed from the command line when you specify its name as known to libvirt (in other words, as it appears in virsh list --all). Use the -d option to access a guest virtual machine by its name, with or without the -i option:

$ guestfish --ro -d GuestName -i

⁠24.4.2. Adding files with guestfish

To add a file with guestfish you need to have the complete URI. Once you have the URI, use the following command:

# guestfish -a  ssh://root@example.com/disk.img

The format used for the URI should be like any of these examples where the file is named disk.img, use /// when the file is local:

guestfish -a disk.img

guestfish -a file:///path/to/disk.img

guestfish -a ftp://[user@]example.com[:port]/disk.img

guestfish -a ftps://[user@]example.com[:port]/disk.img

guestfish -a http://[user@]example.com[:port]/disk.img

guestfish -a https://[user@]example.com[:port]/disk.img

guestfish -a tftp://[user@]example.com[:port]/disk.img

⁠24.4.3. Modifying files with guestfish

To modify files, create directories or make other changes to a guest virtual machine, first heed the warning at the beginning of this section: your guest virtual machine must be shut down. Editing or changing a running disk with guestfish will result in disk corruption. This section gives an example of editing the /boot/grub/grub.conf file. When you are sure the guest virtual machine is shut down you can omit the --ro flag in order to get write access via a command such as:

$ guestfish -d RHEL3 -i

Welcome to guestfish, the libguestfs filesystem interactive shell for
 editing virtual machine filesystems.

 Type: 'help' for help on commands
       'man' to read the manual
       'quit' to quit the shell

 Operating system: Red Hat Enterprise Linux AS release 3 (Taroon Update 9)
 /dev/vda2 mounted on /
 /dev/vda1 mounted on /boot

><fs> edit /boot/grub/grub.conf

Commands to edit files include edit, vi and emacs. Many commands also exist for creating files and directories, such as write, mkdir, upload and tar-in.

⁠24.4.4. Other actions with guestfish

You can also format file systems, create partitions, create and resize LVM logical volumes and much more, with commands such as mkfs, part-add, lvresize, lvcreate, vgcreate and pvcreate.

⁠24.4.5. Shell scripting with guestfish

Once you are familiar with using guestfish interactively, according to your needs, writing shell scripts with it may be useful. The following is a simple shell script to add a new MOTD (message of the day) to a guest:

#!/bin/bash -
 set -e
 guestname="$1"

 guestfish -d "$guestname" -i <<'EOF'
   write /etc/motd "Welcome to Acme Incorporated."
   chmod 0644 /etc/motd
 EOF

⁠24.4.6. Augeas and libguestfs scripting

Combining libguestfs with Augeas can help when writing scripts to manipulate Linux guest virtual machine configuration. For example, the following script uses Augeas to parse the keyboard configuration of a guest virtual machine, and to print out the layout. Note that this example only works with guest virtual machines running Red Hat Enterprise Linux:

#!/bin/bash -
 set -e
 guestname="$1"

 guestfish -d "$1" -i --ro <<'EOF'
   aug-init / 0
   aug-get /files/etc/sysconfig/keyboard/LAYOUT
 EOF

Augeas can also be used to modify configuration files. You can modify the above script to change the keyboard layout:

#!/bin/bash -
 set -e
 guestname="$1"

 guestfish -d "$1" -i <<'EOF'
   aug-init / 0
   aug-set /files/etc/sysconfig/keyboard/LAYOUT '"gb"'
   aug-save
 EOF

Note the three changes between the two scripts:

The --ro option has been removed in the second example, giving the ability to write to the guest virtual machine.

The aug-get command has been changed to aug-set to modify the value instead of fetching it. The new value will be "gb" (including the quotes).

The aug-save command is used here so Augeas will write the changes out to disk.

Note

More information about Augeas can be found on the website http://augeas.net.

guestfish can do much more than we can cover in this introductory document. For example, creating disk images from scratch:

guestfish -N fs

Or copying out whole directories from a disk image:

><fs> copy-out /home /tmp/home

For more information see the man page guestfish(1).

⁠24.5. Other commands

This section describes tools that are simpler equivalents to using guestfish to view and edit guest virtual machine disk images.

virt-cat is similar to the guestfish download command. It downloads and displays a single file to the guest virtual machine. For example:

# virt-cat RHEL3 /etc/ntp.conf | grep ^server
 server	    127.127.1.0	      # local clock

virt-edit is similar to the guestfish edit command. It can be used to interactively edit a single file within a guest virtual machine. For example, you may need to edit the grub.conf file in a Linux-based guest virtual machine that will not boot:

# virt-edit LinuxGuest /boot/grub/grub.conf

virt-edit has another mode where it can be used to make simple non-interactive changes to a single file. For this, the -e option is used. This command, for example, changes the root password in a Linux guest virtual machine to having no password:

# virt-edit LinuxGuest /etc/passwd -e 's/^root:.*?:/root::/'

virt-ls is similar to the guestfish ls, ll and find commands. It is used to list a directory or directories (recursively). For example, the following command would recursively list files and directories under /home in a Linux guest virtual machine:

# virt-ls -R LinuxGuest /home/ | less

⁠24.6. virt-rescue: The rescue shell

⁠24.6.1. Introduction

This section describes virt-rescue, which can be considered analogous to a rescue CD for virtual machines. It boots a guest virtual machine into a rescue shell so that maintenance can be performed to correct errors and the guest virtual machine can be repaired.

There is some overlap between virt-rescue and guestfish. It is important to distinguish their differing uses. virt-rescue is for making interactive, ad-hoc changes using ordinary Linux file system tools. It is particularly suited to rescuing a guest virtual machine that has failed . virt-rescue cannot be scripted.

In contrast, guestfish is particularly useful for making scripted, structured changes through a formal set of commands (the libguestfs API), although it can also be used interactively.

⁠24.6.2. Running virt-rescue

Before you use virt-rescue on a guest virtual machine, make sure the guest virtual machine is not running, otherwise disk corruption will occur. When you are sure the guest virtual machine is not live, enter:

$ virt-rescue -d GuestName

(where GuestName is the guest name as known to libvirt), or:

$ virt-rescue -a /path/to/disk/image

(where the path can be any file, any logical volume, LUN, or so on) containing a guest virtual machine disk.

You will first see output scroll past, as virt-rescue boots the rescue VM. In the end you will see:

Welcome to virt-rescue, the libguestfs rescue shell.

 Note: The contents of / are the rescue appliance.
 You have to mount the guest virtual machine's partitions under /sysroot
 before you can examine them.

 bash: cannot set terminal process group (-1): Inappropriate ioctl for device
 bash: no job control in this shell
 ><rescue>

The shell prompt here is an ordinary bash shell, and a reduced set of ordinary Red Hat Enterprise Linux commands is available. For example, you can enter:

><rescue> fdisk -l /dev/vda

The previous command will list disk partitions. To mount a file system, it is suggested that you mount it under /sysroot, which is an empty directory in the rescue machine for the user to mount anything you like. Note that the files under / are files from the rescue VM itself:

><rescue> mount /dev/vda1 /sysroot/
EXT4-fs (vda1): mounted filesystem with ordered data mode. Opts: (null)
><rescue> ls -l /sysroot/grub/
 total 324
 -rw-r--r--. 1 root root     63 Sep 16 18:14 device.map
 -rw-r--r--. 1 root root  13200 Sep 16 18:14 e2fs_stage1_5
 -rw-r--r--. 1 root root  12512 Sep 16 18:14 fat_stage1_5
 -rw-r--r--. 1 root root  11744 Sep 16 18:14 ffs_stage1_5
 -rw-------. 1 root root   1503 Oct 15 11:19 grub.conf
 [...]

When you are finished rescuing the guest virtual machine, exit the shell by entering exit or Ctrl+d.

virt-rescue has many command line options. The options most often used are:

--ro: Operate in read-only mode on the guest virtual machine. No changes will be saved. You can use this to experiment with the guest virtual machine. As soon as you exit from the shell, all of your changes are discarded.

--network: Enable network access from the rescue shell. Use this if you need to, for example, download RPM or other files into the guest virtual machine.

⁠24.7. virt-df: Monitoring disk usage

⁠24.7.1. Introduction

This section describes virt-df, which displays file system usage from a disk image or a guest virtual machine. It is similar to the Linux df command, but for virtual machines.

⁠24.7.2. Running virt-df

To display file system usage for all file systems found in a disk image, enter the following:

# virt-df -a /dev/vg_guests/RHEL7
 Filesystem                   1K-blocks       Used  Available  Use%
 RHEL6:/dev/sda1                 101086      10233      85634   11%
 RHEL6:/dev/VolGroup00/LogVol00 7127864    2272744    4493036   32%

(Where /dev/vg_guests/RHEL7 is a Red Hat Enterprise Linux 7 guest virtual machine disk image. The path in this case is the host physical machine logical volume where this disk image is located.)

You can also use virt-df on its own to list information about all of your guest virtual machines (ie. those known to libvirt). The virt-df command recognizes some of the same options as the standard df such as -h (human-readable) and -i (show inodes instead of blocks).

virt-df also works on Windows guest virtual machines:

# virt-df -h -d domname
 Filesystem                       Size       Used  Available  Use%
 F14x64:/dev/sda1               484.2M      66.3M     392.9M   14%
 F14x64:/dev/vg_f14x64/lv_root    7.4G       3.0G       4.4G   41%
 RHEL6brewx64:/dev/sda1         484.2M      52.6M     406.6M   11%
 RHEL6brewx64:/dev/vg_rhel6brewx64/lv_root
                                 13.3G       3.4G       9.2G   26%
 Win7x32:/dev/sda1              100.0M      24.1M      75.9M   25%
 Win7x32:/dev/sda2   		 19.9G	     7.4G      12.5G   38%

Note

You can use virt-df safely on live guest virtual machines, since it only needs read-only access. However, you should not expect the numbers to be precisely the same as those from a df command running inside the guest virtual machine. This is because what is on disk will be slightly out of synch with the state of the live guest virtual machine. Nevertheless it should be a good enough approximation for analysis and monitoring purposes.

virt-df is designed to allow you to integrate the statistics into monitoring tools, databases and so on. This allows system administrators to generate reports on trends in disk usage, and alerts if a guest virtual machine is about to run out of disk space. To do this you should use the --csv option to generate machine-readable Comma-Separated-Values (CSV) output. CSV output is readable by most databases, spreadsheet software and a variety of other tools and programming languages. The raw CSV looks like the following:

# virt-df --csv -d WindowsGuest
 Virtual Machine,Filesystem,1K-blocks,Used,Available,Use%
 Win7x32,/dev/sda1,102396,24712,77684,24.1%
 Win7x32,/dev/sda2,20866940,7786652,13080288,37.3%

For resources and ideas on how to process this output to produce trends and alerts, refer to the following URL: http://virt-tools.org/learning/advanced-virt-df/.

⁠24.8. virt-resize: resizing guest virtual machines offline

⁠24.8.1. Introduction

This section describes virt-resize, a tool for expanding or shrinking guest virtual machines. It only works for guest virtual machines which are offline (shut down). It works by copying the guest virtual machine image and leaving the original disk image untouched. This is ideal because you can use the original image as a backup, however there is a trade-off as you need twice the amount of disk space.

⁠24.8.2. Expanding a disk image

This section demonstrates a simple case of expanding a disk image:

Locate the disk image to be resized. You can use the command virsh dumpxml GuestName for a libvirt guest virtual machine.

Decide on how you wish to expand the guest virtual machine. Run virt-df -h and virt-filesystems on the guest virtual machine disk, as shown in the following output:

# virt-df -h -a /dev/vg_guests/RHEL6
Filesystem                      Size       Used  Available  Use%
RHEL6:/dev/sda1                98.7M      10.0M      83.6M   11%
RHEL6:/dev/VolGroup00/LogVol00  6.8G       2.2G       4.3G   32%

# virt-filesystems -a disk.img --all --long -h
/dev/sda1 ext3 101.9M
/dev/sda2 pv 7.9G

This example will demonstrate how to:

Increase the size of the first (boot) partition, from approximately 100MB to 500MB.

Increase the total disk size from 8GB to 16GB.

Expand the second partition to fill the remaining space.

Expand /dev/VolGroup00/LogVol00 to fill the new space in the second partition.

Make sure the guest virtual machine is shut down.

Rename the original disk as the backup. How you do this depends on the host physical machine storage environment for the original disk. If it is stored as a file, use the mv command. For logical volumes (as demonstrated in this example), use lvrename:

# lvrename /dev/vg_guests/RHEL6 /dev/vg_guests/RHEL6.backup

Create the new disk. The requirements in this example are to expand the total disk size up to 16GB. Since logical volumes are used here, the following command is used:

# lvcreate -L 16G -n RHEL6 /dev/vg_guests
Logical volume "RHEL6" created

The requirements from step 2 are expressed by this command:

# virt-resize \
       /dev/vg_guests/RHEL6.backup /dev/vg_guests/RHEL6 \
       --resize /dev/sda1=500M \
       --expand /dev/sda2 \
       --LV-expand /dev/VolGroup00/LogVol00

The first two arguments are the input disk and output disk. --resize /dev/sda1=500M resizes the first partition up to 500MB. --expand /dev/sda2 expands the second partition to fill all remaining space. --LV-expand /dev/VolGroup00/LogVol00 expands the guest virtual machine logical volume to fill the extra space in the second partition.

virt-resize describes what it is doing in the output:

Summary of changes:
   /dev/sda1: partition will be resized from 101.9M to 500.0M
   /dev/sda1: content will be expanded using the 'resize2fs' method
   /dev/sda2: partition will be resized from 7.9G to 15.5G
   /dev/sda2: content will be expanded using the 'pvresize' method
   /dev/VolGroup00/LogVol00: LV will be expanded to maximum size
   /dev/VolGroup00/LogVol00: content will be expanded using the 'resize2fs' method
   Copying /dev/sda1 ...
   [#####################################################]
   Copying /dev/sda2 ...
   [#####################################################]
   Expanding /dev/sda1 using the 'resize2fs' method
   Expanding /dev/sda2 using the 'pvresize' method
   Expanding /dev/VolGroup00/LogVol00 using the 'resize2fs' method

Try to boot the virtual machine. If it works (and after testing it thoroughly) you can delete the backup disk. If it fails, shut down the virtual machine, delete the new disk, and rename the backup disk back to its original name.

Use virt-df and/or virt-filesystems to show the new size:

# virt-df -h -a /dev/vg_pin/RHEL6
   Filesystem                      Size       Used  Available  Use%
   RHEL6:/dev/sda1               484.4M      10.8M     448.6M    3%
   RHEL6:/dev/VolGroup00/LogVol00 14.3G       2.2G      11.4G   16%

Resizing guest virtual machines is not an exact science. If virt-resize fails, there are a number of tips that you can review and attempt in the virt-resize(1) man page. For some older Red Hat Enterprise Linux guest virtual machines, you may need to pay particular attention to the tip regarding GRUB.

⁠24.9. virt-inspector: inspecting guest virtual machines

⁠24.9.1. Introduction

virt-inspector is a tool for inspecting a disk image to find out what operating system it contains.

⁠24.9.2. Installation

To install virt-inspector and the documentation, enter the following command:

# yum install libguestfs-tools libguestfs-devel

To process Windows guest virtual machines you must also install libguestfs-winsupport. Refer to Section 24.10.2, “Installation” for details. The documentation, including example XML output and a Relax-NG schema for the output, will be installed in /usr/share/doc/libguestfs-devel-*/ where "*" is replaced by the version number of libguestfs.

⁠24.9.3. Running virt-inspector

You can run virt-inspector against any disk image or libvirt guest virtual machine as shown in the following example:

$ virt-inspector --xml -a disk.img > report.xml

Or as shown here:

$ virt-inspector --xml -d GuestName > report.xml

The result will be an XML report (report.xml). The main components of the XML file are a top-level <operatingsytems> element containing usually a single <operatingsystem> element, similar to the following:

 <operatingsystems>
   <operatingsystem>

     <!-- the type of operating system and Linux distribution -->
     <name>linux</name>
     <distro>rhel</distro>
     <!-- the name, version and architecture -->
     <product_name>Red Hat Enterprise Linux Server release 6.4 </product_name>
     <major_version>6</major_version>
     <minor_version>4</minor_version>
     <package_format>rpm</package_format>
     <package_management>yum</package_management>
     <root>/dev/VolGroup/lv_root</root>
     <!-- how the filesystems would be mounted when live -->
     <mountpoints>
       <mountpoint dev="/dev/VolGroup/lv_root">/</mountpoint>
       <mountpoint dev="/dev/sda1">/boot</mountpoint>
       <mountpoint dev="/dev/VolGroup/lv_swap">swap</mountpoint>
     </mountpoints>

    < !-- filesystems-->
      <filesystem dev="/dev/VolGroup/lv_root">
        <label></label>
        <uuid>b24d9161-5613-4ab8-8649-f27a8a8068d3</uuid>
        <type>ext4</type>
        <content>linux-root</content>
        <spec>/dev/mapper/VolGroup-lv_root</spec>
      </filesystem>
      <filesystem dev="/dev/VolGroup/lv_swap">
        <type>swap</type>
        <spec>/dev/mapper/VolGroup-lv_swap</spec>
      </filesystem>
     <!-- packages installed -->
     <applications>
       <application>
         <name>firefox</name>
         <version>3.5.5</version>
         <release>1.fc12</release>
       </application>
     </applications>

   </operatingsystem>
 </operatingsystems>

Processing these reports is best done using W3C standard XPath queries. Red Hat Enterprise Linux 7 comes with a command line program (xpath) which can be used for simple instances; however, for long-term and advanced usage, you should consider using an XPath library along with your favorite programming language.

As an example, you can list out all file system devices using the following XPath query:

$ virt-inspector --xml GuestName | xpath //filesystem/@dev
 Found 3 nodes:
 -- NODE --
 dev="/dev/sda1"
 -- NODE --
 dev="/dev/vg_f12x64/lv_root"
 -- NODE --
 dev="/dev/vg_f12x64/lv_swap"

Or list the names of all applications installed by entering:

$ virt-inspector --xml GuestName | xpath //application/name
 [...long list...]

⁠24.10. virt-win-reg: Reading and editing the Windows Registry

⁠24.10.1. Introduction

virt-win-reg is a tool that manipulates the Registry in Windows guest virtual machines. It can be used to read and change registry keys in Windows guest virtual machines.

Important

Red Hat Enterprise Linux 7 Windows guests are only supported under specific subscription programs such as Advanced Mission Critical (AMC). If you are unsure whether your subscription model includes support for Windows guests, please contact customer support.

Warning

Never merge or write changes into live/running guest virtual machines with virt-win-reg, as it will result in disk corruption. Always stop the guest first.

⁠24.10.2. Installation

To install virt-win-reg, run the following yum command as the root user:

# yum install libguestfs-tools libguestfs-winsupport

⁠24.10.3. Using virt-win-reg

To read Registry keys, you must specify the name of the guest to be changed. This guest can be local or remote, or you can specify its disk image file. Also specify the full path to the desired Registry key. You must use single quotes to surround the name of the desired Registry key. See the following example commands for guests (local and remote) and a guest image file, used against the example HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall key, and with output (optionally) piped to the less pager.

# virt-win-reg WindowsGuest \
  'HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall' | less

# virt-win-reg -c qemu+ssh://host_ip/system WindowsGuest \
  'HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall' | less

# virt-win-reg WindowsGuest.qcow2 \
  'HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall' | less

The output is in the same text-based format used by .reg files on Windows.

Note

Hex-quoting is used for strings because the format does not properly define a portable encoding method for strings. This is the only way to ensure fidelity when transporting .reg files from one machine to another.

You can make hex-quoted strings printable by piping the output of virt-win-reg through this simple Perl script:

perl -MEncode -pe's?hex\((\d+)\):(\S+)?$t=$1;$_=$2;s,\,,,g;"str($t):\"".decode(utf16le=>pack("H*",$_))."\""?eg'

Warning

Modifying the Windows Registry is inherently risky. Ensure that the target guest is offline, and that you have reliable backups.

To merge changes into the Windows Registry of an offline guest virtual machine, you must first prepare a suitable .reg file that contains your changes. See this Microsoft KB article for detailed information on this file format. When you have prepared a .reg file, use a command similar to the following examples, which show local, remote, and local image file options:

# virt-win-reg --merge WindowsGuest changes.reg

# virt-win-reg -c qemu+ssh://host_ip/system WindowsGuest --merge changes.reg

# virt-win-reg --merge WindowsGuest.qcow2 changes.reg

⁠24.11. Using the API from Programming Languages

The libguestfs API can be used directly from the following languages in Red Hat Enterprise Linux 7: C, C++, Perl, Python, Java, Ruby and OCaml.

To install C and C++ bindings, enter the following command:

# yum install libguestfs-devel

To install Perl bindings:

# yum install 'perl(Sys::Guestfs)'

To install Python bindings:

# yum install python-libguestfs

To install Java bindings:

# yum install libguestfs-java libguestfs-java-devel libguestfs-javadoc

To install Ruby bindings:

# yum install ruby-libguestfs

To install OCaml bindings:

# yum install ocaml-libguestfs ocaml-libguestfs-devel

The binding for each language is essentially the same, but with minor syntactic changes. A C statement:

guestfs_launch (g);

Would appear like the following in Perl:

$g->launch ()

Or like the following in OCaml:

g#launch ()

Only the API from C is detailed in this section.

In the C and C++ bindings, you must manually check for errors. In the other bindings, errors are converted into exceptions; the additional error checks shown in the examples below are not necessary for other languages, but conversely you may wish to add code to catch exceptions. Refer to the following list for some points of interest regarding the architecture of the libguestfs API:

The libguestfs API is synchronous. Each call blocks until it has completed. If you want to make calls asynchronously, you have to create a thread.

The libguestfs API is not thread safe: each handle should be used only from a single thread, or if you want to share a handle between threads you should implement your own mutex to ensure that two threads cannot execute commands on one handle at the same time.

You should not open multiple handles on the same disk image. It is permissible if all the handles are read-only, but still not recommended.

You should not add a disk image for writing if anything else could be using that disk image (eg. a live VM). Doing this will cause disk corruption.

Opening a read-only handle on a disk image which is currently in use (eg. by a live VM) is possible; however, the results may be unpredictable or inconsistent particularly if the disk image is being heavily written to at the time you are reading it.

⁠24.11.1. Interaction with the API via a C program

Your C program should start by including the <guestfs.h> header file, and creating a handle:

#include <stdio.h>
#include <stdlib.h>
#include <guestfs.h>

int
main (int argc, char *argv[])
{
  guestfs_h *g;

  g = guestfs_create ();
  if (g == NULL) {
    perror ("failed to create libguestfs handle");
    exit (EXIT_FAILURE);
   }

   /* ... */

   guestfs_close (g);

   exit (EXIT_SUCCESS);
 }

Save this program to a file (test.c). Compile this program and run it with the following two commands:

gcc -Wall test.c -o test -lguestfs
./test

At this stage it should print no output. The rest of this section demonstrates an example showing how to extend this program to create a new disk image, partition it, format it with an ext4 file system, and create some files in the file system. The disk image will be called disk.img and be created in the current directory.

The outline of the program is:

Create the handle.

Add disk(s) to the handle.

Launch the libguestfs back end.

Create the partition, file system and files.

Close the handle and exit.

Here is the modified program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <guestfs.h>

 int
 main (int argc, char *argv[])
 {
   guestfs_h *g;
   size_t i;

   g = guestfs_create ();
   if (g == NULL) {
     perror ("failed to create libguestfs handle");
     exit (EXIT_FAILURE);
  }

   /* Create a raw-format sparse disk image, 512 MB in size. */
   int fd = open ("disk.img", O_CREAT|O_WRONLY|O_TRUNC|O_NOCTTY, 0666);
   if (fd == -1) {
     perror ("disk.img");
     exit (EXIT_FAILURE);
   }
   if (ftruncate (fd, 512 * 1024 * 1024) == -1) {
     perror ("disk.img: truncate");
     exit (EXIT_FAILURE);
   }
   if (close (fd) == -1) {
     perror ("disk.img: close");
     exit (EXIT_FAILURE);
   }

   /* Set the trace flag so that we can see each libguestfs call. */
   guestfs_set_trace (g, 1);

   /* Set the autosync flag so that the disk will be synchronized
    * automatically when the libguestfs handle is closed.
    */
   guestfs_set_autosync (g, 1);

   /* Add the disk image to libguestfs. */
   if (guestfs_add_drive_opts (g, "disk.img",
         GUESTFS_ADD_DRIVE_OPTS_FORMAT, "raw", /* raw format */
         GUESTFS_ADD_DRIVE_OPTS_READONLY, 0,   /* for write */
         -1 /* this marks end of optional arguments */ )
       == -1)
     exit (EXIT_FAILURE);

   /* Run the libguestfs back-end. */
   if (guestfs_launch (g) == -1)
     exit (EXIT_FAILURE);

   /* Get the list of devices.  Because we only added one drive
    * above, we expect that this list should contain a single
    * element.
    */
   char **devices = guestfs_list_devices (g);
   if (devices == NULL)
     exit (EXIT_FAILURE);
   if (devices[0] == NULL || devices[1] != NULL) {
     fprintf (stderr,
              "error: expected a single device from list-devices\n");
     exit (EXIT_FAILURE);
   }

   /* Partition the disk as one single MBR partition. */
   if (guestfs_part_disk (g, devices[0], "mbr") == -1)
     exit (EXIT_FAILURE);

   /* Get the list of partitions.  We expect a single element, which
    * is the partition we have just created.
    */
   char **partitions = guestfs_list_partitions (g);
   if (partitions == NULL)
     exit (EXIT_FAILURE);
   if (partitions[0] == NULL || partitions[1] != NULL) {
     fprintf (stderr,
              "error: expected a single partition from list-partitions\n");
     exit (EXIT_FAILURE);
   }

   /* Create an ext4 filesystem on the partition. */
   if (guestfs_mkfs (g, "ext4", partitions[0]) == -1)
     exit (EXIT_FAILURE);

   /* Now mount the filesystem so that we can add files. */
   if (guestfs_mount_options (g, "", partitions[0], "/") == -1)
     exit (EXIT_FAILURE);

   /* Create some files and directories. */
   if (guestfs_touch (g, "/empty") == -1)
     exit (EXIT_FAILURE);

   const char *message = "Hello, world\n";
   if (guestfs_write (g, "/hello", message, strlen (message)) == -1)
     exit (EXIT_FAILURE);

   if (guestfs_mkdir (g, "/foo") == -1)
     exit (EXIT_FAILURE);

   /* This uploads the local file /etc/resolv.conf into the disk image. */
   if (guestfs_upload (g, "/etc/resolv.conf", "/foo/resolv.conf") == -1)
     exit (EXIT_FAILURE);

   /* Because 'autosync' was set (above) we can just close the handle
    * and the disk contents will be synchronized.  You can also do
    * this manually by calling guestfs_umount_all and guestfs_sync.
    */
   guestfs_close (g);

   /* Free up the lists. */
   for (i = 0; devices[i] != NULL; ++i)
     free (devices[i]);
   free (devices);
   for (i = 0; partitions[i] != NULL; ++i)
     free (partitions[i]);
   free (partitions);

   exit (EXIT_SUCCESS);
 }

Compile and run this program with the following two commands:

gcc -Wall test.c -o test -lguestfs
./test

If the program runs to completion successfully then you should be left with a disk image called disk.img, which you can examine with guestfish:

guestfish --ro -a disk.img -m /dev/sda1
><fs> ll /
><fs> cat /foo/resolv.conf

By default (for C and C++ bindings only), libguestfs prints errors to stderr. You can change this behavior by setting an error handler. The guestfs(3) man page discusses this in detail.

⁠24.12. virt-sysprep: resetting virtual machine settings

The virt-sysprep command line tool can be used to reset or unconfigure a guest virtual machine so that clones can be made from it. This process involves removing SSH host keys, removing persistent network MAC configuration, and removing user accounts. Virt-sysprep can also customize a virtual machine, for instance by adding SSH keys, users or logos. Each step can be enabled or disabled as required.

virt-sysprep modifies the guest or disk image in place. To use virt-sysprep, the guest virtual machine must be offline, so you must shut it down before running the commands. If you want to preserve the existing contents of the guest virtual machine, you must snapshot, copy or clone the disk first. Refer to libguestfs.org for more information on copying and cloning disks.

You do not need to run virt-sysprep as root, and it is recommended that you do not. The only time where you might want to run it as root is when you need root in order to access the disk image, but even in this case, it is better to change the permissions on the disk image to be writable by the non-root user running virt-sysprep.

To install virt-sysprep, run the following command:

$ sudo yum install virt-sysprep

The following commands are available to use with virt-sysprep:

⁠

Table 24.1. virt-sysprep commands

Command
Description
Example

--help
Displays a brief help entry about a particular command or about the whole package. For additional help, see the virt-sysprep man page.
$ virt-sysprep --help

-a [file] or --add [file]
Adds the specified file which should be a disk image from a guest virtual machine. The format of the disk image is auto-detected. To override this and force a particular format, use the --format option.
$ virt-sysprep --add /dev/vms/disk.img

-a [URI] or --add [URI]
Adds a remote disk. The URI format is compatible with guestfish. Refer to Section 24.4.2, “Adding files with guestfish” for more information.
$ virt-sysprep -a http://[user@]example.com[:port]/disk.img

-c [URI] or --connect [URI]
Connects to the given URI, if using libvirt. If omitted, then it connects via the KVM hypervisor. If you specify guest block devices directly (virt-sysprep -a), then libvirt is not used at all.
$ virt-sysprep -c qemu:///system

-d [guest] or --domain [guest]
Adds all the disks from the specified guest virtual machine. Domain UUIDs can be used instead of domain names.
$ virt-sysprep --domain 90df2f3f-8857-5ba9-2714-7d95907b1c9e

-n or --dry-run
Performs a read-only "dry run" sysprep operation on the guest virtual machine. This runs the sysprep operation, but throws away any changes to the disk at the end.
$ virt-sysprep -n

--enable [operations]
Enables the specified operations. To list the possible operations, use the --list command.
$ virt-sysprep --enable ssh-hotkeys,udev-persistent-net

--operation or --operations
Chooses which sysprep operations to perform. To disable an operation, use the - before the operation name.
$ virt-sysprep --operations ssh-hotkeys,udev-persistent-net would enable both operations, while $ virt-sysprep --operations firewall-rules,-tmp-files would enable the firewall-rules operation and disable the tmp-files operation. Refer to libguestfs.org for a list of valid operations.

--format [raw|qcow2|auto]
The default for the -a option is to auto-detect the format of the disk image. Using this forces the disk format for -a options which follow on the command line. Using --format auto switches back to auto-detection for subsequent -a options (see the -a command above).
$ virt-sysprep --format raw -a disk.img forces raw format (no auto-detection) for disk.img, but virt-sysprep --format raw -a disk.img --format auto -a another.img forces raw format (no auto-detection) for disk.img and reverts to auto-detection for another.img. If you have untrusted raw-format guest disk images, you should use this option to specify the disk format. This avoids a possible security problem with malicious guests.

--list-operations
List the operations supported by the virt-sysprep program. These are listed one per line, with one or more single-space-separated fields. The first field in the output is the operation name, which can be supplied to the --enable flag. The second field is a * character if the operation is enabled by default, or is blank if not. Additional fields on the same line include a description of the operation.

   $ virt-sysprep --list-operations
 bash-history * Remove the bash history in the guest
 cron-spool * Remove user at-jobs and cron-jobs
 dhcp-client-state * Remove DHCP client leases
 dhcp-server-state * Remove DHCP server leases
...

--mount-options
Sets the mount options for each mount point in the guest virtual machine. Use a semicolon-separated list of mountpoint:options pairs. You may need to place quotes around this list to protect it from the shell.
$ virt-sysprep --mount-options "/:notime" will mount the root directory with the notime operation.

-q or --quiet
Prevents the printing of log messages.
$ virt-sysprep -q

-v or --verbose
Enables verbose messages for debugging purposes.
$ virt-sysprep -v

-V or --version
Displays the virt-sysprep version number and exits.
$ virt-sysprep -V

For more information, refer to the libguestfs documentation.

⁠Chapter 25. Graphic User Interface tools for guest virtual machine management

In addition to virt-manager, there are other tools that can allow you to have access to your guest virtual machine's console. The sections that follow describe and explain these tools.

⁠25.1. Using virt-viewer command line

virt-viewer is a minimal, limited in feature tool for displaying the graphical console of a guest virtual machine. The console is accessed using the VNC or SPICE protocol. The guest can be referred to based on its name, ID, or UUID. If the guest is not already running, then the viewer can be told to wait until is starts before attempting to connect to the console. The viewer can connect to remote hosts to get the console information and then also connect to the remote console using the same network transport.

To install the virt-viewer tool, run:

# sudo yum install virt-viewer

The basic virt viewer commands are as follows:

# virt-viewer [OPTIONS] {domain-name|id|uuid}

The following options may be used with virt-viewer

-h, or --help - Displays the command line help summary.

-V, or --version - Displays the virt-viewer version number.

-v, or --verbose - Displays information about the connection to the guest virtual machine.

-c URI, or --connect=URI - Specifies the hypervisor connection URI.

-w, or --wait - Causes the domain to start up before attempting to connect to the console.

-r, or --reconnect - Automatically reconnects to the domain if it shuts down and restarts.

-z PCT, or --zoom=PCT - Adjusts the zoom level of the display window in the specified percentage. Accepted range 10-200%.

-a, or --attach - Uses libvirt to directly attach to a local display, instead of making a TCP/UNIX socket connection. This avoids the need to authenticate with the remote display, if authentication with libvirt is already allowed. This option does not work with remote displays.

-f, or --full-screen - Starts with the windows maximized to fullscreen.

-h hotkeys, or --hotkeys hotkeys - Overrides the default hotkey settings with the new specified hotkey. Refer to Example 25.5, “Setting hot keys”.

--debug - Prints debugging information.

⁠

Example 25.1. Connecting to a guest virtual machine

If using a XEN hypervisor:

# virt-viewer guest-name

If using a KVM-QEMU hypervisor:

# virt-viewer --connect qemu:///system 7

⁠

Example 25.2. To wait for a spefic guest to start before connecting

Enter the following command:

# virt-viewer --reconnect --wait 66ab33c0-6919-a3f7-e659-16c82d248521

⁠

Example 25.3. To connect to a remote console using TLS

Enter the following command:

# virt-viewer --connect xen://example.org/ demo

⁠

Example 25.4. To connect to a remote host using SSH

Look up the guest configuration and then make a direct non-tunneled connection to the console:

# virt-viewer --direct --connect xen+ssh://root@example.org/ demo

⁠

Example 25.5. Setting hot keys

To create a customized hotkey, run the following command:

# virt-viewer --hotkeys=action1=key-combination1, action2=key-combination2

The following actions can be assigned to a hotkey:

toggle-fullscreen

release-cursor

smartcard-insert

smartcard-remove

Key name combination hotkeys are case insensitive. Each hotkey setting should have a unique key combination.

For example, to create a hotkey to change to full screen mode:

# virt-viewer --hotkeys=toggle-fullscreen=shift+f11 qemu:///system 7

⁠25.2. remote-viewer

The remote-viewer is a simple remote desktop display client that supports SPICE and VNC.

To install the remote-viewer tool, run:

# sudo yum install remote-viewer

The basic remote viewer commands are as follows:

remote-viewer [OPTIONS] -- URI

The following options may be used with remote-viewer:

-h, or --help - Displays the command line help summary.

-V, or --version - Displays the remote-viewer version number.

-v, or --verbose - Displays information about the connection to the guest virtual machine.

-z PCT, or --zoom=PCT - Adjusts the zoom level of the display window in the specified percentage. Accepted range 10-200%.

-f, or --full-screen=auto-conf - Starts with the windows maximized to its full screen size. If the optional argument 'auto-conf' is given, the remote display will be reconfigured to match at best the client physical monitor configuration on initialization, by enabling or disabling extra monitors as necessary. This is currently implemented by the Spice backend only.

-t title, or --title title - Sets the window title to the string given.

--spice-controller - Uses the SPICE controller to initialize the connection with the SPICE server. This option is used by the SPICE browser plug-ins to allow web page to start a client.

--debug - Prints debugging information.

For more information see the MAN page for the remote-viewer.

⁠25.3. GNOME Boxes

Boxes is a lightweight graphical desktop virtualization tool used to view and access virtual machines and remote systems. Boxes provides a way to test different operating systems and applications from the desktop with minimal configuration.

To install Boxes, run:

# sudo yum install gnome-boxes

Open Boxes through Applications > System Tools.

The main screen shows the available guest virtual machines. The right-hand side of the screen has two buttons:

The search button, a magnifying glass. the search button, to search for guest virtual machines by name, and

The selection button, a tick. the selection button.

Clicking on the selection button allows you to select one or more guest virtual machines in order to perform operations individually or as a group. The available operations are shown at the bottom of the screen on the operations bar:

⁠

Figure 25.1. The Operations Bar

There are four operations that can be performed:

Favorite: Adds a heart to selected guest virtual machines and moves them to the top of the list of guests. This becomes increasingly helpful as the number of guests grows.

Pause: The selected guest virtual machines will stop running.

Delete: Removes selected guest virtual machines.

Properties: Shows the properties of the selected guest virtual machine.

Create new guest virtual machines using the New button on the left-hand side of the main screen.

⁠

Procedure 25.1. Creating a new guest virtual machine with Boxes

Click New

This opens the Introduction screen. Click Continue.

⁠

Figure 25.2. Introduction screen

Select source

The Source Selection screen has three options:

Available media: Any immediately available installation media will be shown here. Clicking on any of these will take you directly to the Review screen.

Enter a URL: Type in a URL to specify a local URI or path to an ISO file. This can also be used to access a remote machine. The address should follow the pattern of protocol://IPaddress?port;, for example:

spice://192.168.122.1?port=5906;

Protocols can be spice://, qemu://, or vnc://.

Select a file: Open a file directory to search for installation media manually.

⁠

Figure 25.3. Source Selection screen

Review the details

The Review screen shows the details of the guest virtual machine.

⁠

Figure 25.4. Review screen

These details can be left as is, in which case proceed to the final step, or:

Optional: customize the details

Clicking on Customize allows you to adjust the configuration of the guest virtual machine, such as the memory and disk size.

⁠

Figure 25.5. Customization screen

Create

Click Create. The new guest virtual machine will open.

⁠Chapter 26. Manipulating the domain XML

This chapter explains in detail the components of the domain.xml configuration file. In this chapter, the term domain refers to the root <domain> element required for all guest virtual machines. The domain XML has two attributes: type and id. type specifies the hypervisor used for running the domain. The allowed values are driver specific, but include KVM and others. id is a unique integer identifier for the running guest virtual machine. Inactive machines have no id value. The sections in this chapter will describe the components of the domain XML. Additional chapters in this manual may refer to this chapter when manipulation of the domain XML is required.

Important

Use only supported management interfaces (such as virsh) and commands (such as virsh edit) to edit the components of the domain XML file. Do not open and edit the domain XML file directly with a text editor such as vim or gedit.

⁠26.1. General information and meta-data

This information is in this part of the domain XML:

⁠

​
​
​<domain type='kvm' id='3'>
​  <name>fv0</name>
​  <uuid>4dea22b31d52d8f32516782e98ab3fa0</uuid>
​  <title>A short description - title - of the domain</title>
​  <description>A human readable description</description>
​  <metadata>
​    <app1:foo xmlns:app1="http://app1.org/app1/">..</app1:foo>
​    <app2:bar xmlns:app2="http://app1.org/app2/">..</app2:bar>
​  </metadata>
​  ...
​</domain>

Figure 26.1. Domain XML meta-data

The components of this section of the domain XML are as follows:

⁠

Table 26.1. General meta-data elements

Element | Description
<name> | Assigns a name for the virtual machine. This name should consist only of alpha-numeric characters and is required to be unique within the scope of a single host physical machine. It is often used to form the file name for storing the persistent configuration files.
<uuid> | Assigns a globally unique identifier for the virtual machine. The format must be RFC 4122 compliant, for example, 3e3fce45-4f53-4fa7-bb32-11f34168b82b. If omitted when defining or creating a new machine, a random UUID is generated. It is also possible to provide the UUID via a sysinfo specification.
<title> | Creates space for a short description of the domain. The title should not contain any new lines.
<description> | Different from the title, this data is not used by libvirt. It can contain any information the user chooses to display.
<metadata> | Can be used by applications to store custom meta-data in the form of XML nodes/trees. Applications must use custom name spaces on XML nodes/trees, with only one top-level element per name space (if the application needs structure, they should have sub-elements to their name space element).

⁠26.2. Operating system booting

There are a number of different ways to boot virtual machines, including BIOS boot loader, host physical machine boot loader, direct kernel boot, and container boot.

⁠26.2.1. BIOS boot loader

Booting via the BIOS is available for hypervisors supporting full virtualization. In this case, the BIOS has a boot order priority (floppy, hard disk, CD-ROM, network) determining where to locate the boot image. The <os> section of the domain XML contains the following information:

⁠

​
​
​  ...
​  <os>
​    <type>hvm</type>
​    <loader>/usr/lib/kvm/boot/hvmloader</loader>
​    <boot dev='hd'/>
​    <boot dev='cdrom'/>
​    <bootmenu enable='yes'/>
​    <smbios mode='sysinfo'/>
​    <bios useserial='yes' rebootTimeout='0'/>
​  </os>
​  ...

Figure 26.2. BIOS boot loader domain XML

The components of this section of the domain XML are as follows:

⁠

Table 26.2. BIOS bootloader elements

Element
Description

<type>
Specifies the type of operating system to be booted on the guest virtual machine. hvm indicates that the operating system is designed to run on bare metal and requires full virtualization. linux refers to an operating system that supports the KVM hypervisor guest ABI. There are also two optional attributes: arch specifies the CPU architecture to virtualization, and machine refers to the machine type. Refer to Driver Capabilities for more information.

<loader>
Refers to a piece of firmware that is used to assist the domain creation process. It is only needed for using KVM fully virtualized domains.

<boot>
Specifies the next boot device to consider with one of the following values:fd, hd, cdrom or network. The boot element can be repeated multiple times to set up a priority list of boot devices to try in turn. Multiple devices of the same type are sorted according to their targets while preserving the order of buses. After defining the domain, its XML configuration returned by libvirt (through virDomainGetXMLDesc) lists devices in the sorted order. Once sorted, the first device is marked as bootable. For more information, see BIOS boot loader.

<bootmenu>
Determines whether or not to enable an interactive boot menu prompt on guest virtual machine start up. The enable attribute can be either yes or no. If not specified, the hypervisor default is used.

<smbios>
determines how SMBIOS information is made visible in the guest virtual machine. The mode attribute must be specified, as either emulate (allows the hypervisor generate all values), host (copies all of Block 0 and Block 1, except for the UUID, from the host physical machine's SMBIOS values; the virConnectGetSysinfo call can be used to see what values are copied), or sysinfo (uses the values in the sysinfo element). If not specified, the hypervisor's default setting is used.

<bios>
This element has attribute useserial with possible values yes or no. The attribute enables or disables the Serial Graphics Adapter which allows users to see BIOS messages on a serial port. Therefore, one needs to have serial port defined. The rebootTimeout attribute controls whether and after how long the guest virtual machine should start booting again in case the boot fails (according to the BIOS). The value is set in milliseconds with a maximum of 65535; setting -1 disables the reboot.

⁠26.2.2. Host physical machine boot loader

Hypervisors using para-virtualization do not usually emulate a BIOS, but instead the host physical machine is responsible for the operating system boot. This may use a pseudo-boot loader in the host physical machine to provide an interface to choose a kernel for the guest virtual machine. An example is PyGrub with KVM:

⁠

​
​ ...
​  <bootloader>/usr/bin/pygrub</bootloader>
​  <bootloader_args>--append single</bootloader_args>
​  ...

Figure 26.3. Host physical machine boot loader domain XML

The components of this section of the domain XML are as follows:

⁠

Table 26.3. BIOS boot loader elements

Element | Description
<bootloader> | Provides a fully qualified path to the boot loader executable in the host physical machine OS. This boot loader will choose which kernel to boot. The required output of the boot loader is dependent on the hypervisor in use.
<bootloader_args> | Allows command line arguments to be passed to the boot loader (optional command).

⁠26.2.3. Direct kernel boot

When installing a new guest virtual machine operating system, it is often useful to boot directly from a kernel and initrd stored in the host physical machine operating system, allowing command line arguments to be passed directly to the installer. This capability is usually available for both fully virtualized and para-virtualized guest virtual machines.

⁠

​
​
​  ...
​  <os>
​    <type>hvm</type>
​    <loader>/usr/lib/kvm/boot/hvmloader</loader>
​    <kernel>/root/f8-i386-vmlinuz</kernel>
​    <initrd>/root/f8-i386-initrd</initrd>
​    <cmdline>console=ttyS0 ks=http://example.com/f8-i386/os/</cmdline>
​    <dtb>/root/ppc.dtb</dtb>
​  </os>
​  ...
​

Figure 26.4. Direct kernel boot

The components of this section of the domain XML are as follows:

⁠

Table 26.4. Direct kernel boot elements

Element | Description
<type> | Same as described in the BIOS boot section.
<loader> | Same as described in the BIOS boot section.
<kernel> | Specifies the fully-qualified path to the kernel image in the host physical machine operating system.
<initrd> | Specifies the fully-qualified path to the (optional) ramdisk image in the host physical machine operating system.
<cmdline> | Specifies arguments to be passed to the kernel (or installer) at boot time. This is often used to specify an alternate primary console (such as a serial port), or the installation media source or kickstart file.

⁠26.2.4. Container boot

When booting a domain using container-based virtualization, instead of a kernel or boot image, a path to the init binary is required, using the init element. By default this will be launched with no arguments. To specify the initial argv, use the initarg element, repeated as many times as required. The cmdline element provides an equivalent to /proc/cmdline but will not affect <initarg>.

⁠

​
​
​  ...
​  <os>
​    <type>hvm</type>
​    <loader>/usr/lib/kvm/boot/hvmloader</loader>
​    <kernel>/root/f8-i386-vmlinuz</kernel>
​    <initrd>/root/f8-i386-initrd</initrd>
​    <cmdline>console=ttyS0 ks=http://example.com/f8-i386/os/</cmdline>
​    <dtb>/root/ppc.dtb</dtb>
​  </os>
​  ...

Figure 26.5. Container boot

⁠26.3. SMBIOS system information

Some hypervisors allow control over what system information is presented to the guest virtual machine (for example, SMBIOS fields can be populated by a hypervisor and inspected via the dmidecode command in the guest virtual machine). The optional sysinfo element covers all such categories of information.

⁠

​
​
​  ...
​  <os>
​    <smbios mode='sysinfo'/>
​    ...
​  </os>
​  <sysinfo type='smbios'>
​    <bios>
​      <entry name='vendor'>LENOVO</entry>
​    </bios>
​    <system>
​      <entry name='manufacturer'>Fedora</entry>
​      <entry name='vendor'>Virt-Manager</entry>
​    </system>
​  </sysinfo>
​  ...

Figure 26.6. SMBIOS system information

The <sysinfo> element has a mandatory attribute type that determines the layout of sub-elements, and may be defined as follows:

<smbios> - Sub-elements call out specific SMBIOS values, which will affect the guest virtual machine if used in conjunction with the smbios sub-element of the <os> element. Each sub-element of <sysinfo> names a SMBIOS block, and within those elements can be a list of entry elements that describe a field within the block. The following blocks and entries are recognized:

<bios> - This is block 0 of SMBIOS, with entry names drawn from vendor, version, date, and release.

<system> - This is block 1 of SMBIOS, with entry names drawn from manufacturer, product, version, serial, uuid, sku, and family. If a uuid entry is provided alongside a top-level uuid element, the two values must match.

⁠26.4. CPU allocation

⁠

​
​
​<domain>
​  ...
​  <vcpu placement='static' cpuset="1-4,^3,6" current="1">2</vcpu>
​  ...
​</domain>

Figure 26.7. CPU allocation

The <cpu> element defines the maximum number of virtual CPUs allocated for the guest virtual machine operating system, which must be between 1 and the maximum number supported by the hypervisor. This element can contain an optional cpuset attribute, which is a comma-separated list of physical CPU numbers that the domain process and virtual CPUs can be pinned to by default.

Note that the pinning policy of the domain process and virtual CPUs can be specified separately by using the cputune attribute. If the emulatorpin attribute is specified in <cputune>, cpuset specified by <vcpu> will be ignored.

Similarly, virtual CPUs that have set a value for vcpupin cause cpuset settings to be ignored. For virtual CPUs where vcpupin is not specified, it will be pinned to the physical CPUs specified by cpuset. Each element in the cpuset list is either a single CPU number, a range of CPU numbers, or a caret (^) followed by a CPU number to be excluded from a previous range. The attribute current can be used to specify whether fewer than the maximum number of virtual CPUs should be enabled.

The optional attribute placement can be used to indicate the CPU placement mode for domain process. Its value can be either static or auto, which defaults to placement, or numatune, or static if cpuset is specified. auto indicates the domain process will be pinned to the advisory nodeset from querying numad, and the value of attribute cpuset will be ignored if it is specified. If both cpuset and placement are not specified, or if placement is static, but no cpuset is specified, the domain process will be pinned to all the available physical CPUs.

⁠26.5. CPU tuning

⁠

​
​
​<domain>
​  ...
​  <cputune>
​    <vcpupin vcpu="0" cpuset="1-4,^2"/>
​    <vcpupin vcpu="1" cpuset="0,1"/>
​    <vcpupin vcpu="2" cpuset="2,3"/>
​    <vcpupin vcpu="3" cpuset="0,4"/>
​    <emulatorpin cpuset="1-3"/>
​    <shares>2048</shares>
​    <period>1000000</period>
​    <quota>-1</quota>
​    <emulator_period>1000000</emulator_period>
​    <emulator_quota>-1</emulator_quota>
​  </cputune>
​  ...
​</domain>

Figure 26.8. CPU tuning

Although all are optional, the components of this section of the domain XML are as follows:

⁠

Table 26.5. CPU tuning elements

Element
Description

<cputune>
Provides details regarding the CPU tunable parameters for the domain. This is optional.

<vcpupin>
Specifies which of host physical machine's physical CPUs the domain vCPU will be pinned to. If this is omitted, and the cpuset attribute of the <vcpu> element is not specified, the vCPU is pinned to all the physical CPUs by default. It contains two required attributes: the <vcpu> attribute specifies id, and the cpuset attribute is same as the cpuset attribute in the <vcpu> element.

<emulatorpin>
Specifies which of the host physical machine CPUs the "emulator" (a subset of a domains not including <vcpu>) will be pinned to. If this is omitted, and the cpuset attribute in the <vcpu> element is not specified, the "emulator" is pinned to all the physical CPUs by default. It contains one required cpuset attribute specifying which physical CPUs to pin to. emulatorpin is not allowed if the placement attribute in the <vcpu> element is set as auto.

<shares>
Specifies the proportional weighted share for the domain. If this is omitted, it defaults to the operating system provided defaults. If there is no unit for the value, it is calculated relative to the setting of the other guest virtual machine. For example, a guest virtual machine configured with a <shares> value of 2048 will get twice as much CPU time as a guest virtual machine configured with a <shares> value of 1024.

<period>
Specifies the enforcement interval in microseconds. By using <period>, each of the domain's vCPUs will not be allowed to consume more than its allotted quota worth of run time. This value should be within the following range: 1000-1000000. A <period> with a value of 0 means no value.

<quota>
Specifies the maximum allowed bandwidth in microseconds. A domain with <quota> as any negative value indicates that the domain has infinite bandwidth, which means that it is not bandwidth controlled. The value should be within the following range: 1000 - 18446744073709551 or less than 0. A quota with value of 0 means no value. You can use this feature to ensure that all vCPUs run at the same speed.

<emulator_period>
Specifies the enforcement interval in microseconds. Within an <emulator_period>, emulator threads (those excluding vCPUs) of the domain will not be allowed to consume more than the <emulator_quota> worth of run time. The <emulator_period> value should be in the following range: 1000 - 1000000. An <emulator_period> with value of 0 means no value.

<emulator_quota>
Specifies the maximum allowed bandwidth in microseconds for the domain's emulator threads (those excluding vCPUs). A domain with an <emulator_quota> as a negative value indicates that the domain has infinite bandwidth for emulator threads (those excluding vCPUs), which means that it is not bandwidth controlled. The value should be in the following range: 1000 - 18446744073709551, or less than 0. An <emulator_quota> with value 0 means no value.

⁠26.6. Memory backing

Memory backing allows the hypervisor to properly manage large pages within the guest virtual machine.

⁠

​
​
​<domain>
​  ...
​  <memoryBacking>
​    <hugepages/>
​  </memoryBacking>
​  ...
​</domain>

Figure 26.9. Memory backing

The optional <memoryBacking> element may have an <hugepages> element set within it. This tells the hypervisor that the guest virtual machine should have its memory allocated using hugepages instead of using the default native page size.

⁠26.7. Memory tuning

⁠

​
​
​<domain>
​  ...
​  <memtune>
​    <hard_limit unit='G'>1</hard_limit>
​    <soft_limit unit='M'>128</soft_limit>
​    <swap_hard_limit unit='G'>2</swap_hard_limit>
​    <min_guarantee unit='bytes'>67108864</min_guarantee>
​  </memtune>
​  ...
​</domain>

Figure 26.10. Memory tuning

Although <memtune> is optional, the components of this section of the domain XML are as follows:

⁠

Table 26.6. Memory tuning elements

Element
Description

<memtune>
Provides details regarding the memory tunable parameters for the domain. If this is omitted, it defaults to the operating system provided defaults. As parameters are applied to the process as a whole, when setting limits, determine values by adding the guest virtual machine RAM to the guest virtual machine video RAM, allowing for some memory overhead. For each tunable, it is possible to designate which unit the number is in on input, using the same values as for <memory>. For backwards compatibility, output is always in kibibytes (KiB).

<hard_limit>
The maximum memory the guest virtual machine can use. This value is expressed in kibibytes (blocks of 1024 bytes).

<soft_limit>
The memory limit to enforce during memory contention. This value is expressed in kibibytes (blocks of 1024 bytes).

<swap_hard_limit>
The maximum memory plus swap the guest virtual machine can use. This value is expressed in kibibytes (blocks of 1024 bytes). This must be more than <hard_limit> value.

<min_guarantee>
The guaranteed minimum memory allocation for the guest virtual machine. This value is expressed in kibibytes (blocks of 1024 bytes).

⁠26.8. Memory allocation

In cases where the guest virtual machine crashes, the optional attribute dumpCore can be used to control whether the guest virtual machine's memory should be included in the generated coredump (dumpCore='on') or not included (dumpCore='off'). Note that the default setting is on, so unless the parameter is set to off, the guest virtual machine memory will be included in the coredump file.

The currentMemory attribute determines the actual memory allocation for a guest virtual machine. This value can be less than the maximum allocation, to allow for the guest virtual machine memory to balloon as needed. If omitted, this defaults to the same value as the memory element. The unit attribute behaves the same as for memory.

⁠

​
​
​<domain>
​
​  <memory unit='KiB' dumpCore='off'>524288</memory>
​  <!-- changes the memory unit to KiB and does not allow the guest virtual machine's memory to be included in the generated coredump file -->
​  <currentMemory unit='KiB>524288</currentMemory>
​  <!-- makes the current memory unit 524288 KiB -->
​  ...
​</domain>

Figure 26.11. Memory unit

⁠26.9. NUMA node tuning

After NUMA node tuning is done using virsh edit, the following domain XML parameters are affected:

⁠

​
​
​<domain>
​  ...
​  <numatune>
​    <memory mode="strict" nodeset="1-4,^3"/>
​  </numatune>
​  ...
​</domain>

Figure 26.12. NUMA node tuning

Although all are optional, the components of this section of the domain XML are as follows:

⁠

Table 26.7. NUMA node tuning elements

Element
Description

<numatune>
Provides details of how to tune the performance of a NUMA host physical machine by controlling NUMA policy for domain processes.

<memory>
Specifies how to allocate memory for the domain processes on a NUMA host physical machine. It contains several optional attributes. The mode attribute can be set to interleave, strict, or preferred. If no value is given it defaults to strict. The nodeset attribute specifies the NUMA nodes, using the same syntax as the cpuset attribute of the <vcpu> element. Attribute placement can be used to indicate the memory placement mode for the domain process. Its value can be either static or auto. If the <nodeset> attribute is specified it defaults to the <placement> of <vcpu>, or static. auto indicates the domain process will only allocate memory from the advisory nodeset returned from querying numad and the value of the nodeset attribute will be ignored if it is specified. If the <placement> attribute in vcpu is set to auto, and the <numatune> attribute is not specified, a default <numatune> with <placement> auto and strict mode will be added implicitly.

⁠26.10. Block I/O tuning

⁠

​
​
​<domain>
​  ...
​  <blkiotune>
​    <weight>800</weight>
​    <device>
​      <path>/dev/sda</path>
​      <weight>1000</weight>
​    </device>
​    <device>
​      <path>/dev/sdb</path>
​      <weight>500</weight>
​    </device>
​  </blkiotune>
​  ...
​</domain>

Figure 26.13. Block I/O tuning

Although all are optional, the components of this section of the domain XML are as follows:

⁠

Table 26.8. Block I/O tuning elements

Element
Description

<blkiotune>
This optional element provides the ability to tune blkio cgroup tunable parameters for the domain. If this is omitted, it defaults to the operating system provided defaults.

<weight>
This optional weight element is the overall I/O weight of the guest virtual machine. The value should be within the range 100 - 1000.

<device>
The domain may have multiple <device> elements that further tune the weights for each host physical machine block device in use by the domain. Note that multiple guest virtual machine disks can share a single host physical machine block device. In addition, as they are backed by files within the same host physical machine file system, this tuning parameter is at the global domain level, rather than being associated with each guest virtual machine disk device (contrast this to the <iotune> element which can be applied to a single <disk>). Each device element has two mandatory sub-elements, <path> describing the absolute path of the device, and <weight> giving the relative weight of that device, which has an acceptable range of 100 - 1000.

⁠26.11. Resource partitioning

Hypervisors may allow for virtual machines to be placed into resource partitions, potentially with nesting of said partitions. The <resource> element groups together configurations related to resource partitioning. It currently supports a child element partition whose content defines the path of the resource partition in which to place the domain. If no partition is listed, then the domain will be placed in a default partition. The partition must be created prior to starting the guest virtual machine. Only the (hypervisor specific) default partition can be assumed to exist by default.

⁠

​
​<resource>
​    <partition>/virtualmachines/production</partition>
​  </resource>

Figure 26.14. Resource partitioning

Resource partitions are currently supported by the KVM and LXC drivers, which map partition paths to cgroups directories in all mounted controllers.

⁠26.12. CPU models and topology

This section covers the requirements for CPU models. Note that every hypervisor has its own policy for which CPU features guest will see by default. The set of CPU features presented to the guest by KVM depends on the CPU model chosen in the guest virtual machine configuration. qemu32 and qemu64 are basic CPU models, but there are other models (with additional features) available. Each model and its topology is specified using the following elements from the domain XML:

⁠

​
​<cpu match='exact'>
​    <model fallback='allow'>core2duo</model>
​    <vendor>Intel</vendor>
​    <topology sockets='1' cores='2' threads='1'/>
​    <feature policy='disable' name='lahf_lm'/>
​  </cpu>

Figure 26.15. CPU model and topology example 1

⁠

​
​<cpu mode='host-model'>
​   <model fallback='forbid'/>
​   <topology sockets='1' cores='2' threads='1'/>
​</cpu>

Figure 26.16. CPU model and topology example 2

⁠

​
​<cpu mode='host-passthrough'/>

Figure 26.17. CPU model and topology example 3

In cases where no restrictions are to be put on the CPU model or its features, a simpler <cpu> element such as the following may be used:

⁠

​
​<cpu>
​   <topology sockets='1' cores='2' threads='1'/>
​</cpu>

Figure 26.18. CPU model and topology example 4

The components of this section of the domain XML are as follows:

⁠

Table 26.9. CPU model and topology elements

Element
Description

<cpu>
This is the main container for describing guest virtual machine CPU requirements.

<match>
Specifies how the virtual CPU is provided to the guest virtual machine must match these requirements. The match attribute can be omitted if topology is the only element within <cpu>. Possible values for the match attribute are:

minimum - the specified CPU model and features describes the minimum requested CPU.

exact - the virtual CPU provided to the guest virtual machine will exactly match the specification.

strict - the guest virtual machine will not be created unless the host physical machine CPU exactly matches the specification.

Note that the match attribute can be omitted and will default to exact.

<mode>
This optional attribute may be used to make it easier to configure a guest virtual machine CPU to be as close to the host physical machine CPU as possible. Possible values for the mode attribute are:

custom - Describes how the CPU is presented to the guest virtual machine. This is the default setting when the mode attribute is not specified. This mode makes it so that a persistent guest virtual machine will see the same hardware no matter what host physical machine the guest virtual machine is booted on.

host-model - This is essentially a shortcut to copying host physical machine CPU definition from the capabilities XML into the domain XML. As the CPU definition is copied just before starting a domain, the same XML can be used on different host physical machines while still providing the best guest virtual machine CPU each host physical machine supports. Neither the match attribute nor any feature elements can be used in this mode. For more information see libvirt domain XML CPU models.

host-passthrough With this mode, the CPU visible to the guest virtual machine is exactly the same as the host physical machine CPU, including elements that cause errors within libvirt. The obvious the downside of this mode is that the guest virtual machine environment cannot be reproduced on different hardware and therefore this mode is recommended with great caution. Neither model nor feature elements are allowed in this mode.

Note that in both host-model and host-passthrough mode, the real (approximate in host-passthrough mode) CPU definition which would be used on current host physical machine can be determined by specifying VIR_DOMAIN_XML_UPDATE_CPU flag when calling the virDomainGetXMLDesc API. When running a guest virtual machine that might be prone to operating system reactivation when presented with different hardware, and which will be migrated between host physical machines with different capabilities, you can use this output to rewrite XML to the custom mode for more robust migration.

<model>
Specifies the CPU model requested by the guest virtual machine. The list of available CPU models and their definition can be found in the cpu_map.xml file installed in libvirt's data directory. If a hypervisor is unable to use the exact CPU model, libvirt automatically falls back to a closest model supported by the hypervisor while maintaining the list of CPU features. An optional fallback attribute can be used to forbid this behavior, in which case an attempt to start a domain requesting an unsupported CPU model will fail. Supported values for fallback attribute are: allow (the default), and forbid. The optional vendor_id attribute can be used to set the vendor ID seen by the guest virtual machine. It must be exactly 12 characters long. If not set, the vendor iID of the host physical machine is used. Typical possible values are AuthenticAMD and GenuineIntel.

<vendor>
Specifies the CPU vendor requested by the guest virtual machine. If this element is missing, the guest virtual machine runs on a CPU matching given features regardless of its vendor. The list of supported vendors can be found in cpu_map.xml.

<topology>
Specifies the requested topology of the virtual CPU provided to the guest virtual machine. Three non-zero values must be given for sockets, cores, and threads: the total number of CPU sockets, number of cores per socket, and number of threads per core, respectively.

<feature>
Can contain zero or more elements used to fine-tune features provided by the selected CPU model. The list of known feature names can be found in the cpu_map.xml file. The meaning of each feature element depends on its policy attribute, which has to be set to one of the following values:

force - forces the virtual to be supported, regardless of whether it is actually supported by host physical machine CPU.

require - dictates that guest virtual machine creation will fail unless the feature is supported by host physical machine CPU. This is the default setting,

optional - this feature is supported by virtual CPU but only if it is supported by host physical machine CPU.

disable - this is not supported by virtual CPU.

forbid - guest virtual machine creation will fail if the feature is supported by host physical machine CPU.

⁠26.12.1. Changing the feature set for a specified CPU

Although CPU models have an inherent feature set, the individual feature components can either be allowed or forbidden on a feature by feature basis, allowing for a more individualized configuration for the CPU.

⁠

Procedure 26.1. Enabling and disabling CPU features

To begin, shut down the guest virtual machine.

Open the guest virtual machine's configuration file by running the virsh edit [domain] command.

Change the parameters within the <feature> or <model> to include the attribute value 'allow' to force the feature to be allowed, or 'forbid' to deny support for the feature.

⁠

​
​
​<!-- original feature set -->
​<cpu mode='host-model'>
​   <model fallback='allow'/>
​   <topology sockets='1' cores='2' threads='1'/>
​</cpu>
​
​<!--changed feature set-->
​<cpu mode='host-model'>
​   <model fallback='forbid'/>
​   <topology sockets='1' cores='2' threads='1'/>
​</cpu>
​

Figure 26.19. Example for enabling or disabling CPU features

⁠

​
​
​<!--original feature set-->
​<cpu match='exact'>
​    <model fallback='allow'>core2duo</model>
​    <vendor>Intel</vendor>
​    <topology sockets='1' cores='2' threads='1'/>
​    <feature policy='disable' name='lahf_lm'/>
​</cpu>
​
​<!--changed feature set-->
​<cpu match='exact'>
​    <model fallback='allow'>core2duo</model>
​    <vendor>Intel</vendor>
​    <topology sockets='1' cores='2' threads='1'/>
​    <feature policy='enable' name='lahf_lm'/>
​  </cpu>
​

Figure 26.20. Example 2 for enabling or disabling CPU features

When you have completed the changes, save the configuration file and start the guest virtual machine.

⁠26.12.2. Guest virtual machine NUMA topology

Guest virtual machine NUMA topology can be specified using the <numa> element in the domain XML:

⁠

​
​
​  <cpu>
​    <numa>
​      <cell cpus='0-3' memory='512000'/>
​      <cell cpus='4-7' memory='512000'/>
​    </numa>
​  </cpu>
​  ...
​

Figure 26.21. Guest virtual machine NUMA topology

Each cell element specifies a NUMA cell or a NUMA node. cpus specifies the CPU or range of CPUs that are part of the node. memory specifies the node memory in kibibytes (blocks of 1024 bytes). Each cell or node is assigned a cellid or nodeid in increasing order starting from 0.

⁠26.13. Events configuration

Using the following sections of domain XML it is possible to override the default actions for various events:

⁠

​
​
​  <on_poweroff>destroy</on_poweroff>
​  <on_reboot>restart</on_reboot>
​  <on_crash>restart</on_crash>
​  <on_lockfailure>poweroff</on_lockfailure>

Figure 26.22. Events Configuration

The following collections of elements allow the actions to be specified when a guest virtual machine operating system triggers a life cycle operation. A common use case is to force a reboot to be treated as a power off when doing the initial operating system installation. This allows the VM to be re-configured for the first post-install boot up.

The components of this section of the domain XML are as follows:

⁠

Table 26.10. Event configuration elements

State
Description

<on_poweroff>
Specifies the action that is to be executed when the guest virtual machine requests a power off. Four arguments are possible:

destroy - This action terminates the domain completely and releases all resources.

restart - This action terminates the domain completely and restarts it with the same configuration.

preserve - This action terminates the domain completely but and its resources are preserved to allow for future analysis.

rename-restart - This action terminates the domain completely and then restarts it with a new name.

<on_reboot>
Specifies the action to be executed when the guest virtual machine requests a reboot. Four arguments are possible:

destroy - This action terminates the domain completely and releases all resources.

restart - This action terminates the domain completely and restarts it with the same configuration.

preserve - This action terminates the domain completely but and its resources are preserved to allow for future analysis.

rename-restart - This action terminates the domain completely and then restarts it with a new name.

<on_crash>
Specifies the action that is to be executed when the guest virtual machine crashes. In addition, it supports these additional actions:

coredump-destroy - The crashed domain's core is dumped, the domain is terminated completely, and all resources are released.

coredump-restart - The crashed domain's core is dumped, and the domain is restarted with the same configuration settings.

Four arguments are possible:

destroy - This action terminates the domain completely and releases all resources.

restart - This action terminates the domain completely and restarts it with the same configuration.

preserve - This action terminates the domain completely but and its resources are preserved to allow for future analysis.

rename-restart - This action terminates the domain completely and then restarts it with a new name.

<on_lockfailure>
Specifies the action to take when a lock manager loses resource locks. The following actions are recognized by libvirt, although not all of them need to be supported by individual lock managers. When no action is specified, each lock manager will take its default action. The following arguments are possible:

poweroff - Forcefully powers off the domain.

restart - Restarts the domain to reacquire its locks.

pause - Pauses the domain so that it can be manually resumed when lock issues are solved.

ignore - Keeps the domain running as if nothing happened.

⁠26.14. Power Management

It is possible to forcibly enable or disable BIOS advertisements to the guest virtual machine operating system using conventional management tools which affects the following section of the domain XML:

⁠

​
​
​  ...
​  <pm>
​    <suspend-to-disk enabled='no'/>
​    <suspend-to-mem enabled='yes'/>
​  </pm>
​  ...
​
​

Figure 26.23. Power Management

The <pm> element can be enabled using the argument yes or disabled using the argument no. BIOS support can be implemented for S3 using the suspend-to-disk argument and S4 using the suspend-to-mem argument for ACPI sleep states. If nothing is specified, the hypervisor will be left with its default value.

⁠26.15. Hypervisor features

Hypervisors may allow certain CPU or machine features to be enabled (state='on') or disabled (state='off').

⁠

​
​
​  ...
​  <features>
​    <pae/>
​    <acpi/>
​    <apic/>
​    <hap/>
​    <privnet/>
​    <hyperv>
​      <relaxed state='on'/>
​    </hyperv>
​  </features>
​  ...
​
​

Figure 26.24. Hypervisor features

All features are listed within the <features> element, if a <state> is not specified it is disabled. The available features can be found by calling the capabilities XML, but a common set for fully virtualized domains are:

⁠

Table 26.11. Hypervisor features elements

State | Description
<pae> | Physical address extension mode allows 32-bit guest virtual machines to address more than 4 GB of memory.
<acpi> | Useful for power management, for example, with KVM guest virtual machines it is required for graceful shutdown to work.
<apic> | Allows the use of programmable IRQ management. This element has an optional attribute eoi with values on and off, which sets the availability of EOI (End of Interrupt) for the guest virtual machine.
<hap> | Enables the use of hardware assisted paging if it is available in the hardware.

⁠26.16. Time keeping

The guest virtual machine clock is typically initialized from the host physical machine clock. Most operating systems expect the hardware clock to be kept in UTC, which is the default setting.

Accurate timekeeping on guest virtual machines is a key challenge for virtualization platforms. Different hypervisors attempt to handle the problem of timekeeping in a variety of ways. libvirt provides hypervisor-independent configuration settings for time management, using the <clock> and <timer> elements in the domain XML. The domain XML can be edited using the virsh edit command. See Section 23.11, “Editing a guest virtual machine's configuration file” for details.

⁠

​
​
​  ...
​  <clock offset='localtime'>
​    <timer name='rtc' tickpolicy='catchup' track='guest'>
​      <catchup threshold='123' slew='120' limit='10000'/>
​    </timer>
​    <timer name='pit' tickpolicy='delay'/>
​  </clock>
​  ...
​
​

Figure 26.25. Timekeeping

The components of this section of the domain XML are as follows:

⁠

Table 26.12. Timekeeping elements

State
Description

<clock>
The <clock> element is used to determine how the guest virtual machine clock is synchronized with the host physical machine clock. The offset attribute takes four possible values, allowing for fine grained control over how the guest virtual machine clock is synchronized to the host physical machine. Note that hypervisors are not required to support all policies across all time sources

utc - Synchronizes the clock to UTC when booted. utc mode can be converted to variable mode, which can be controlled by using the adjustment attribute. If the value is reset, the conversion is not done. A numeric value forces the conversion to variable mode using the value as the initial adjustment. The default adjustment is hypervisor specific.

localtime - Synchronizes the guest virtual machine clock with the host physical machine's configured timezone when booted. The adjustment attribute behaves the same as in utc mode.

timezone - Synchronizes the guest virtual machine clock to the requested time zone.

variable - Gives the guest virtual machine clock an arbitrary offset applied relative to UTC or localtime, depending on the basis attribute. The delta relative to UTC (or localtime) is specified in seconds, using the adjustment attribute. The guest virtual machine is free to adjust the RTC over time and expect that it will be honored at next reboot. This is in contrast to utc and localtime mode (with the optional attribute adjustment='reset'), where the RTC adjustments are lost at each reboot. In addition the basis attribute can be either utc (default) or localtime. The clock element may have zero or more <timer> elements.

<timer>
See Note

<present>
Specifies whether a particular timer is available to the guest virtual machine. Can be set to yes or no.

Additional information about the <timer> element

A <clock> element can have zero or more <timer> elements as children. The <timer> element specifies a time source used for guest virtual machine clock synchronization.

In each <timer> element only the name is required, and all other attributes are optional:

name - Selects which timer is being modified. The following values are acceptable: kvmclock, pit, or rtc.

track - Specifies the timer track. The following values are acceptable: boot, guest, or wall. track is only valid for name="rtc".

tickpolicy - Determines what happens when the deadline for injecting a tick to the guest virtual machine is missed. The following values can be assigned:

delay - Continues to deliver ticks at the normal rate. The guest virtual machine time will be delayed due to the late tick.

catchup - Delivers ticks at a higher rate in order to catch up with the missed tick. The guest virtual machine time is not displayed once catch up is complete. In addition, there can be three optional attributes, each a positive integer: threshold, slew, and limit.

merge - Merges the missed tick(s) into one tick and injects them. The guest virtual machine time may be delayed, depending on how the merge is done.

discard - Throws away the missed tick(s) and continues with future injection at its default interval setting. The guest virtual machine time may be delayed, unless there is an explicit statement for handling lost ticks.

Note

The value utc is set as the clock offset in a virtual machine by default. However, if the guest virtual machine clock is run with the localtime value, the clock offset needs to be changed to a different value in order to have the guest virtual machine clock synchronized with the host physical machine clock.

⁠

Example 26.1. Always synchronize to UTC

<clock offset="utc" />

⁠

Example 26.2. Always synchronize to the host physical machine timezone

<clock offset="localtime" />

⁠

Example 26.3. Synchronize to an arbitrary time zone

<clock offset="timezone" timezone="Europe/Paris" />

⁠

Example 26.4. Synchronize to UTC + arbitrary offset

<clock offset="variable" adjustment="123456" />

⁠26.17. Timer element attributes

The name element contains the name of the time source to be used. It can have any of the following values:

⁠

Table 26.13. Name attribute values

Value | Description
pit | Programmable Interval Timer - a timer with periodic interrupts. When using this attribute, the tickpolicy delay becomes the default setting.
rtc | Real Time Clock - a continuously running timer with periodic interrupts. This attribute supports the tickpolicy catchup sub-element.
kvmclock | KVM clock - the recommended clock source for KVM guest virtual machines. KVM pvclock, or kvm-clock allows guest virtual machines to read the host physical machine’s wall clock time.

The track attribute specifies what is tracked by the timer, and is only valid for a name value of rtc.

⁠

Table 26.14. track attribute values

Value | Description
boot | Corresponds to old host physical machine option, this is an unsupported tracking option.
guest | RTC always tracks the guest virtual machine time.
wall | RTC always tracks the host time.

The tickpolicy attribute and the values dictate the policy that is used to pass ticks on to the guest virtual machine.

⁠

Table 26.15. tickpolicy attribute values

Value | Description
delay | Continue to deliver at normal rate (ticks are delayed).
catchup | Deliver at a higher rate to catch up.
merge | Ticks merged into one single tick.
discard | All missed ticks are discarded.

The present attribute is used to override the default set of timers visible to the guest virtual machine. The present attribute can take the following values:

⁠

Table 26.16. present attribute values

Value | Description
yes | Force this timer to be visible to the guest virtual machine.
no | Force this timer to not be visible to the guest virtual machine.

⁠26.18. Devices

This set of XML elements are all used to describe devices provided to the guest virtual machine domain. All of the devices below are indicated as children of the main <devices> element.

The following virtual devices are supported:

virtio-scsi-pci - PCI bus storage device

virtio-9p-pci - PCI bus storage device

virtio-blk-pci - PCI bus storage device

virtio-net-pci - PCI bus network device also known as virtio-net

virtio-serial-pci - PCI bus input device

virtio-balloon-pci - PCI bus memory balloon device

virtio-rng-pci - PCI bus virtual random number generator device

Important

If a virtio device is created where the number of vectors is set to a value higher than 32, the device behaves as if it was set to a zero value on Red Hat Enterprise Linux 6, but not on Enterprise Linux 7. The resulting vector setting mismatch causes a migration error if the number of vectors on any virtio device on either platform is set to 33 or higher. It is therefore not reccomended to set the vector value to be greater than 32. All virtio devices with the exception of virtio-balloon-pci and virtio-rng-pci will accept a vector argument.

⁠

​
​
​  ...
​  <devices>
​    <emulator>/usr/lib/kvm/bin/kvm-dm</emulator>
​  </devices>
​  ...
​

Figure 26.26. Devices - child elements

The contents of the <emulator> element specify the fully qualified path to the device model emulator binary. The capabilities XML specifies the recommended default emulator to use for each particular domain type or architecture combination.

⁠26.18.1. Hard drives, floppy disks, CD-ROMs

This section of the domain XML specifies any device that looks like a disk, including any floppy disk, hard disk, CD-ROM, or para-virtualized driver that is specified in the <disk> element.

⁠

​
​
​...
​  <devices>
​    <disk type='file' snapshot='external'>
​      <driver name="tap" type="aio" cache="default"/>
​      <source file='/var/lib/xen/images/fv0' startupPolicy='optional'>
​        <seclabel relabel='no'/>
​      </source>
​      <target dev='hda' bus='ide'/>
​      <iotune>
​        <total_bytes_sec>10000000</total_bytes_sec>
​        <read_iops_sec>400000</read_iops_sec>
​        <write_iops_sec>100000</write_iops_sec>
​      </iotune>
​      <boot order='2'/>
​      <encryption type='...'>
​        ...
​      </encryption>
​      <shareable/>
​      <serial>
​        ...
​      </serial>
​    </disk>
​

Figure 26.27. Devices - Hard drives, floppy disks, CD-ROMs

⁠

​
​
​    <disk type='network'>
​      <driver name="qemu" type="raw" io="threads" ioeventfd="on" event_idx="off"/>
​      <source protocol="sheepdog" name="image_name">
​        <host name="hostname" port="7000"/>
​      </source>
​      <target dev="hdb" bus="ide"/>
​      <boot order='1'/>
​      <transient/>
​      <address type='drive' controller='0' bus='1' unit='0'/>
​    </disk>

Figure 26.28. Devices - Hard drives, floppy disks, CD-ROMs Example 2

⁠

​
​
​    <disk type='network'>
​      <driver name="qemu" type="raw"/>
​      <source protocol="rbd" name="image_name2">
​        <host name="hostname" port="7000"/>
​      </source>
​      <target dev="hdd" bus="ide"/>
​      <auth username='myuser'>
​        <secret type='ceph' usage='mypassid'/>
​      </auth>
​    </disk>

Figure 26.29. Devices - Hard drives, floppy disks, CD-ROMs Example 3

⁠

​
​
​    <disk type='block' device='cdrom'>
​      <driver name='qemu' type='raw'/>
​      <target dev='hdc' bus='ide' tray='open'/>
​      <readonly/>
​    </disk>
​    <disk type='network' device='cdrom'>
​      <driver name='qemu' type='raw'/>
​      <source protocol="http" name="url_path">
​        <host name="hostname" port="80"/>
​      </source>
​      <target dev='hdc' bus='ide' tray='open'/>
​      <readonly/>
​    </disk>

Figure 26.30. Devices - Hard drives, floppy disks, CD-ROMs Example 4

⁠

​
​
​    <disk type='network' device='cdrom'>
​      <driver name='qemu' type='raw'/>
​      <source protocol="https" name="url_path">
​        <host name="hostname" port="443"/>
​      </source>
​      <target dev='hdc' bus='ide' tray='open'/>
​      <readonly/>
​    </disk>
​    <disk type='network' device='cdrom'>
​      <driver name='qemu' type='raw'/>
​      <source protocol="ftp" name="url_path">
​        <host name="hostname" port="21"/>
​      </source>
​      <target dev='hdc' bus='ide' tray='open'/>
​      <readonly/>
​    </disk>

Figure 26.31. Devices - Hard drives, floppy disks, CD-ROMs Example 5

⁠

​
​
​    <disk type='network' device='cdrom'>
​      <driver name='qemu' type='raw'/>
​      <source protocol="ftps" name="url_path">
​        <host name="hostname" port="990"/>
​      </source>
​      <target dev='hdc' bus='ide' tray='open'/>
​      <readonly/>
​    </disk>
​    <disk type='network' device='cdrom'>
​      <driver name='qemu' type='raw'/>
​      <source protocol="tftp" name="url_path">
​        <host name="hostname" port="69"/>
​      </source>
​      <target dev='hdc' bus='ide' tray='open'/>
​      <readonly/>
​    </disk>
​    <disk type='block' device='lun'>
​      <driver name='qemu' type='raw'/>
​      <source dev='/dev/sda'/>
​      <target dev='sda' bus='scsi'/>
​      <address type='drive' controller='0' bus='0' target='3' unit='0'/>
​    </disk>

Figure 26.32. Devices - Hard drives, floppy disks, CD-ROMs Example 6

⁠

​
​
​    <disk type='block' device='disk'>
​      <driver name='qemu' type='raw'/>
​      <source dev='/dev/sda'/>
​      <geometry cyls='16383' heads='16' secs='63' trans='lba'/>
​      <blockio logical_block_size='512' physical_block_size='4096'/>
​      <target dev='hda' bus='ide'/>
​    </disk>
​    <disk type='volume' device='disk'>
​      <driver name='qemu' type='raw'/>
​      <source pool='blk-pool0' volume='blk-pool0-vol0'/>
​      <target dev='hda' bus='ide'/>
​    </disk>
​    <disk type='network' device='disk'>
​      <driver name='qemu' type='raw'/>
​      <source protocol='iscsi' name='iqn.2013-07.com.example:iscsi-nopool/2'>
​        <host name='example.com' port='3260'/>
​      </source>
​      <auth username='myuser'>
​        <secret type='chap' usage='libvirtiscsi'/>
​      </auth>
​      <target dev='vda' bus='virtio'/>
​    </disk>

Figure 26.33. Devices - Hard drives, floppy disks, CD-ROMs Example 7

⁠

​
​
​    <disk type='network' device='lun'>
​      <driver name='qemu' type='raw'/>
​      <source protocol='iscsi' name='iqn.2013-07.com.example:iscsi-nopool/1'>
​       iqn.2013-07.com.example:iscsi-pool
​        <host name='example.com' port='3260'/>
​      </source>
​      <auth username='myuser'>
​        <secret type='chap' usage='libvirtiscsi'/>
​      </auth>
​      <target dev='sda' bus='scsi'/>
​    </disk>
​    <disk type='volume' device='disk'>
​      <driver name='qemu' type='raw'/>
​      <source pool='iscsi-pool' volume='unit:0:0:1' mode='host'/>
​      <auth username='myuser'>
​        <secret type='chap' usage='libvirtiscsi'/>
​      </auth>
​      <target dev='vda' bus='virtio'/>
​    </disk>

Figure 26.34. Devices - Hard drives, floppy disks, CD-ROMs Example 8

⁠

​
​
​    <disk type='volume' device='disk'>
​      <driver name='qemu' type='raw'/>
​      <source pool='iscsi-pool' volume='unit:0:0:2' mode='direct'/>
​      <auth username='myuser'>
​        <secret type='chap' usage='libvirtiscsi'/>
​      </auth>
​      <target dev='vda' bus='virtio'/>
​    </disk>
​     <disk type='file' device='disk'>
​      <driver name='qemu' type='raw' cache='none'/>
​      <source file='/tmp/test.img' startupPolicy='optional'/>
​      <target dev='sdb' bus='scsi'/>
​      <readonly/>
​    </disk>
​    <disk type='file' device='disk'>
​      <driver name='qemu' type='raw' discard='unmap'/>
​      <source file='/var/lib/libvirt/images/discard1.img'/>
​      <target dev='vdb' bus='virtio'/>
​      <alias name='virtio-disk1'/>
​      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
​    </disk>
​  </devices>
​  ...
​

Figure 26.35. Devices - Hard drives, floppy disks, CD-ROMs Example 9

⁠26.18.1.1. Disk element

The <disk> element is the main container for describing disks. The attribute type can be used with the <disk> element. The following types are allowed:

file

block

dir

network

For more information, see Disk Elements.

⁠26.18.1.2. Source element

If <disk type='file'>, then the file attribute specifies the fully-qualified path to the file holding the disk. If <disk type='block'>, then the dev attribute specifies the path to the host physical machine device to serve as the disk. With both file and block, one or more optional sub-elements seclabel, described below, can be used to override the domain security labeling policy for just that source file. If the disk type is dir, then the dir attribute specifies the fully-qualified path to the directory to use as the disk. If the disk type is network, then the protocol attribute specifies the protocol to access to the requested image; possible values are nbd, rbd, sheepdog or gluster.

If the protocol attribute is rbd, sheepdog or gluster, an additional attribute name is mandatory to specify which volume and or image will be used. When the disk type is network, the source may have zero or more host sub-elements used to specify the host physical machines to connect, including: type='dir' and type='network'. For a file disk type which represents a CD-ROM or floppy (the device attribute), it is possible to define the policy for what to do with the disk if the source file is not accessible. This is done by setting the startupPolicy attribute with one of the following values:

mandatory causes a failure if missing for any reason. This is the default setting.

requisite causes a failure if missing on boot up, drops if missing on migrate, restore, or revert.

optional drops if missing at any start attempt.

⁠26.18.1.3. Mirror element

This element is present if the hypervisor has started a BlockCopy operation, where the <mirror> location in the attribute file will eventually have the same contents as the source, and with the file format in attribute format (which might differ from the format of the source). If an attribute ready is present, then it is known the disk is ready to pivot; otherwise, the disk is probably still copying. For now, this element only valid in output; it is ignored on input.

⁠26.18.1.4. Target element

The <target> element controls the bus or device under which the disk is exposed to the guest virtual machine operating system. The dev attribute indicates the logical device name. The actual device name specified is not guaranteed to map to the device name in the guest virtual machine operating system. The optional bus attribute specifies the type of disk device to emulate; possible values are driver specific, with typical values being ide, scsi, virtio, kvm, usb or sata. If omitted, the bus type is inferred from the style of the device name. For example, a device named 'sda' will typically be exported using a SCSI bus. The optional attribute tray indicates the tray status of the removable disks (for example, CD-ROM or Floppy disk), where the value can be either open or closed. The default setting is closed. For more information, see Target Elements.

⁠26.18.1.5. iotune element

The optional <iotune> element provides the ability to provide additional per-device I/O tuning, with values that can vary for each device (contrast this to the blkiotune element, which applies globally to the domain). This element has the following optional sub-elements (note that any sub-element not specified or at all or specified with a value of 0 implies no limit):

<total_bytes_sec> - The total throughput limit in bytes per second. This element cannot be used with <read_bytes_sec> or <write_bytes_sec>.

<read_bytes_sec> - The read throughput limit in bytes per second.

<write_bytes_sec> - The write throughput limit in bytes per second.

<total_iops_sec> - The total I/O operations per second. This element cannot be used with <read_iops_sec> or <write_iops_sec>.

<read_iops_sec> - The read I/O operations per second.

<write_iops_sec> - The write I/O operations per second.

⁠26.18.1.6. Driver element

The optional <driver> element allows specifying further details related to the hypervisor driver that is used to provide the disk. The following options may be used:

If the hypervisor supports multiple back-end drivers, the name attribute selects the primary back-end driver name, while the optional type attribute provides the sub-type. For a list of possible types, refer to Driver Elements.

The optional cache attribute controls the cache mechanism. Possible values are: default, none, writethrough, writeback, directsync (similar to writethrough, but it bypasses the host physical machine page cache) and unsafe (host physical machine may cache all disk I/O, and sync requests from guest virtual machines are ignored).

The optional error_policy attribute controls how the hypervisor behaves on a disk read or write error. Possible values are stop, report, ignore, and enospace. The default setting of error_policy is report. There is also an optional rerror_policy that controls behavior for read errors only. If no rerror_policy is given, error_policy is used for both read and write errors. If rerror_policy is given, it overrides the error_policy for read errors. Also note that enospace is not a valid policy for read errors, so if error_policy is set to enospace and no rerror_policy is given, the read error default setting, report will be used.

The optional io attribute controls specific policies on I/O; kvm guest virtual machines support threads and native. The optional ioeventfd attribute allows users to set domain I/O asynchronous handling for disk devices. The default is determined by the hypervisor. Accepted values are on and off. Enabling this allows the guest virtual machine to be executed while a separate thread handles I/O. Typically, guest virtual machines experiencing high system CPU utilization during I/O will benefit from this. On the other hand, an overloaded host physical machine can increase guest virtual machine I/O latency. However, it is recommended that you do not change the default setting, and allow the hypervisor to determine the setting.

Note

The ioeventfd attribute is included in the <driver> element of the disk XML section and also of the of the device XML section. In the former case, it influences the virtIO disk, and in the latter case the SCSI disk.

The optional event_idx attribute controls some aspects of device event processing and can be set to either on or off. If set to on, it will reduce the number of interrupts and exits for the guest virtual machine. The default is determined by the hypervisor and the default setting is on. When this behavior is not desired, setting off forces the feature off. However, it is highly recommended that you not change the default setting, and allow the hypervisor to dictate the setting.

The optional copy_on_read attribute controls whether to copy the read backing file into the image file. The accepted values can be either on or <off>. copy-on-read avoids accessing the same backing file sectors repeatedly, and is useful when the backing file is over a slow network. By default copy-on-read is off.

The discard='unmap' can be set to enable discard support. The same line can be replaced with discard='ignore' to disable. discard='ignore' is the default setting.

⁠26.18.1.7. Additional Device Elements

The following attributes may be used within the device element:

<boot> - Specifies that the disk is bootable.

⁠

Additional boot values

<order> - Determines the order in which devices will be tried during boot sequence.

<per-device> Boot elements cannot be used together with general boot elements in the BIOS boot loader section.

<encryption> - Specifies how the volume is encrypted.

<readonly> - Indicates the device cannot be modified by the guest virtual machine virtual machine. This setting is the default for disks with attribute <device='cdrom'>.

<shareable> Indicates the device is expected to be shared between domains (as long as hypervisor and operating system support this). If shareable is used, cache='no' should be used for that device.

<transient> - Indicates that changes to the device contents should be reverted automatically when the guest virtual machine exits. With some hypervisors, marking a disk transient prevents the domain from participating in migration or snapshots.

<serial>- Specifies the serial number of guest virtual machine's hard drive. For example, <serial>WD-WMAP9A966149</serial>.

<wwn> - Specifies the WWN (World Wide Name) of a virtual hard disk or CD-ROM drive. It must be composed of 16 hexadecimal digits.

<vendor> - Specifies the vendor of a virtual hard disk or CD-ROM device. It must not be longer than 8 printable characters.

<product> - Specifies the product of a virtual hard disk or CD-ROM device. It must not be longer than 16 printable characters

<host> - Supports 4 attributes: viz, name, port, transport and socket, which specify the host name, the port number, transport type, and path to socket, respectively. The meaning of this element and the number of the elements depend on the protocol attribute as shown here:

⁠

Additional host attributes

nbd - Specifies a server running nbd-server and may only be used for only one host physical machine.

rbd - Monitors servers of RBD type and may be used for one or more host physical machines.

sheepdog - Specifies one of the sheepdog servers (default is localhost:7000) and can be used with one or none of the host physical machines.

gluster - Specifies a server running a glusterd daemon and may be used for only only one host physical machine. The valid values for transport attribute are tcp, rdma or unix. If nothing is specified, tcp is assumed. If transport is unix, the socket attribute specifies path to unix socket.

<address> - Ties the disk to a given slot of a controller. The actual <controller> device can often be inferred but it can also be explicitly specified. The type attribute is mandatory, and is typically pci or drive. For a pci controller, additional attributes for bus, slot, and function must be present, as well as optional domain and multifunction. multifunction defaults to off. For a drive controller, additional attributes controller, bus, target, and unit are available, each with a default setting of 0.

auth - Provides the authentication credentials needed to access the source. It includes a mandatory attribute username, which identifies the user name to use during authentication, as well as a sub-element secret with mandatory attribute type. More information can be found here at Device Elements.

geometry - Provides the ability to override geometry settings. This mostly useful for S390 DASD-disks or older DOS-disks.

cyls - Specifies the number of cylinders.

heads - Specifies the number of heads.

secs - Specifies the number of sectors per track.

trans - Specifies the BIOS-Translation-Modes and can have the following values: none, lba or auto.

blockio - Allows the block device to be overridden with any of the block device properties listed below:

⁠

blockio options

logical_block_size - Reports to the guest virtual machine operating system and describes the smallest units for disk I/O.

physical_block_size - Reports to the guest virtual machine operating system and describes the disk's hardware sector size, which can be relevant for the alignment of disk data.

⁠26.18.2. Files ystems

The file systems directory on the host physical machine can be accessed directly from the guest virtual machine.

⁠

​
​
​ ...
​  <devices>
​    <filesystem type='template'>
​      <source name='my-vm-template'/>
​      <target dir='/'/>
​    </filesystem>
​    <filesystem type='mount' accessmode='passthrough'>
​      <driver type='path' wrpolicy='immediate'/>
​      <source dir='/export/to/guest'/>
​      <target dir='/import/from/host'/>
​      <readonly/>
​    </filesystem>
​    ...
​  </devices>
​  ...
​
​

Figure 26.36. Devices - files ystems

The filesystem attribute has the following possible values:

type='mount' - Specifies the host physical machine directory to mount in the guest virtual machine. This is the default type if one is not specified. This mode also has an optional sub-element driver, with an attribute type='path' or type='handle'. The driver block has an optional attribute wrpolicy that further controls interaction with the host physical machine page cache; omitting the attribute reverts to the default setting, while specifying a value immediate means that a host physical machine write back is immediately triggered for all pages touched during a guest virtual machine file write operation.

type='template' - Specifies the OpenVZ file system template and is only used by OpenVZ driver.

type='file' - Specifies that a host physical machine file will be treated as an image and mounted in the guest virtual machine. This file system format will be auto-detected and is only used by LXC driver.

type='block' - Specifies the host physical machine block device to mount in the guest virtual machine. The file system format will be auto-detected and is only used by the LXC driver.

type='ram' - Specifies that an in-memory file system, using memory from the host physical machine operating system will be used. The source element has a single attribute usage, which gives the memory usage limit in kibibytes and is only used by LXC driver.

type='bind' - Specifies a directory inside the guest virtual machine which will be bound to another directory inside the guest virtual machine. This element is only used by LXC driver.

accessmode - Specifies the security mode for accessing the source. Currently this only works with type='mount' for the KVM driver. The possible values are:

passthrough - Specifies that the source is accessed with the user's permission settings that are set from inside the guest virtual machine. This is the default accessmode if one is not specified.

mapped - Specifies that the source is accessed with the permission settings of the hypervisor.

squash - Similar to 'passthrough', the exception is that failure of privileged operations like chown are ignored. This makes a passthrough-like mode usable for people who run the hypervisor as non-root.

source - Specifies that the resource on the host physical machine that is being accessed in the guest virtual machine. The name attribute must be used with <type='template'>, and the dir attribute must be used with <type='mount'>. The usage attribute is used with <type='ram'> to set the memory limit in KB.

target - Dictates where the source drivers can be accessed in the guest virtual machine. For most drivers, this is an automatic mount point, but for KVM this is merely an arbitrary string tag that is exported to the guest virtual machine as a hint for where to mount.

readonly - Enables exporting the file system as a read-only mount for a guest virtual machine. By default read-write access is given.

space_hard_limit - Specifies the maximum space available to this guest virtual machine's file system.

space_soft_limit - Specifies the maximum space available to this guest virtual machine's file system. The container is permitted to exceed its soft limits for a grace period of time. Afterwards the hard limit is enforced.

⁠26.18.3. Device addresses

Many devices have an optional <address> sub-element to describe where the device placed on the virtual bus is presented to the guest virtual machine. If an address (or any optional attribute within an address) is omitted on input, libvirt will generate an appropriate address; but an explicit address is required if more control over layout is required. See below for device examples including an address element.

Every address has a mandatory attribute type that describes which bus the device is on. The choice of which address to use for a given device is constrained in part by the device and the architecture of the guest virtual machine. For example, a disk device uses type='disk', while a console device would use type='pci' on i686 or x86_64 guest virtual machines, or type='spapr-vio' on PowerPC64 pseries guest virtual machines. Each address <type> has additional optional attributes that control where on the bus the device will be placed. The additional attributes are as follows:

type='pci' - PCI addresses have the following additional attributes:

domain (a 2-byte hex integer, not currently used by KVM)

bus (a hex value between 0 and 0xff, inclusive)

slot (a hex value between 0x0 and 0x1f, inclusive)

function (a value between 0 and 7, inclusive)

Also available is the multi-function attribute, which controls turning on the multi-function bit for a particular slot or function in the PCI control register. This multi-function attribute defaults to 'off', but should be set to 'on' for function 0 of a slot that will have multiple functions used.

type='drive' - drive addresses have the following additional attributes:

controller- (a 2-digit controller number)

bus - (a 2-digit bus number)

target - (a 2-digit bus number)

unit - (a 2-digit unit number on the bus)

type='virtio-serial' - Each virtio-serial address has the following additional attributes:

controller - (a 2-digit controller number)

bus - (a 2-digit bus number)

slot - (a 2-digit slot within the bus)

type='ccid' - A CCID address, used for smart-cards, has the following additional attributes:

bus - (a 2-digit bus number)

slot - (a 2-digit slot within the bus)

type='usb' - USB addresses have the following additional attributes:

bus - (a hex value between 0 and 0xfff, inclusive)

port - (a dotted notation of up to four octets, such as 1.2 or 2.1.3.1)

type='spapr-vio - On PowerPC pseries guest virtual machines, devices can be assigned to the SPAPR-VIO bus. It has a flat 64-bit address space; by convention, devices are generally assigned at a non-zero multiple of 0x1000, but other addresses are valid and permitted by libvirt. The additional attribute: reg (the hex value address of the starting register) can be assigned to this attribute.

⁠26.18.4. Controllers

Depending on the guest virtual machine architecture, it is possible to assign many virtual devices to a single bus. Under normal circumstances libvirt can automatically infer which controller to use for the bus. However, it may be necessary to provide an explicit <controller> element in the guest virtual machine XML:

⁠

​
​
​  ...
​  <devices>
​    <controller type='ide' index='0'/>
​    <controller type='virtio-serial' index='0' ports='16' vectors='4'/>
​    <controller type='virtio-serial' index='1'>
​      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
​    <controller type='scsi' index='0' model='virtio-scsi' num_queues='8'/>
​    </controller>
​    ...
​  </devices>
​  ...
​

Figure 26.37. Controller Elements

Each controller has a mandatory attribute type, which must be one of "ide", "fdc", "scsi", "sata", "usb", "ccid", or "virtio-serial", and a mandatory attribute index which is the decimal integer describing in which order the bus controller is encountered (for use in controller attributes of address elements). The "virtio-serial" controller has two additional optional attributes, ports and vectors, which control how many devices can be connected through the controller.

A <controller type='scsi'> has an optional attribute model, which is one of "auto", "buslogic", "ibmvscsi", "lsilogic", "lsias1068", "virtio-scsi or "vmpvscsi". The <controller type='scsi'> also has an attribute num_queues which enables multi-queue support for the number of queues specified.

A "usb" controller has an optional attribute model, which is one of "piix3-uhci", "piix4-uhci", "ehci", "ich9-ehci1", "ich9-uhci1", "ich9-uhci2", "ich9-uhci3", "vt82c686b-uhci", "pci-ohci" or "nec-xhci". Additionally, if the USB bus needs to be explicitly disabled for the guest virtual machine, model='none' may be used. The PowerPC64 "spapr-vio" addresses do not have an associated controller.

For controllers that are themselves devices on a PCI or USB bus, an optional sub-element address can specify the exact relationship of the controller to its master bus, with semantics given above.

USB companion controllers have an optional sub-element master to specify the exact relationship of the companion to its master controller. A companion controller is on the same bus as its master, so the companion index value should be equal.

⁠

​
​
​  ...
​  <devices>
​    <controller type='usb' index='0' model='ich9-ehci1'>
​      <address type='pci' domain='0' bus='0' slot='4' function='7'/>
​    </controller>
​    <controller type='usb' index='0' model='ich9-uhci1'>
​      <master startport='0'/>
​      <address type='pci' domain='0' bus='0' slot='4' function='0' multifunction='on'/>
​    </controller>
​    ...
​  </devices>
​  ...
​

Figure 26.38. Devices - controllers - USB

⁠26.18.5. Device leases

When using a lock manager, you have the option to record device leases against a guest virtual machine. The lock manager will ensure that the guest virtual machine does not start unless the leases can be acquired. When configured using conventional management tools, the following section of the domain XML is affected:

⁠

​
​
​  ...
​  <devices>
​    ...
​    <lease>
​      <lockspace>somearea</lockspace>
​      <key>somekey</key>
​      <target path='/some/lease/path' offset='1024'/>
​    </lease>
​    ...
​  </devices>
​  ...
​

Figure 26.39. Devices - device leases

The lease section can have the following arguments:

lockspace - An arbitrary string that identifies lockspace within which the key is held. Lock managers may impose extra restrictions on the format, or length of the lockspace name.

key - An arbitrary string that uniquely identifies the lease to be acquired. Lock managers may impose extra restrictions on the format, or length of the key.

target - The fully qualified path of the file associated with the lockspace. The offset specifies where the lease is stored within the file. If the lock manager does not require a offset, set this value to 0.

⁠26.18.6. Host physical machine device assignment

⁠26.18.6.1. USB / PCI devices

The host physical machine's USB and PCI devices can be passed through to the guest virtual machine using the hostdev element, by modifying the host physical machine using a management tool, configure the following section of the domain XML file:

⁠

​
​
​  ...
​  <devices>
​    <hostdev mode='subsystem' type='usb'>
​      <source startupPolicy='optional'>
​        <vendor id='0x1234'/>
​        <product id='0xbeef'/>
​      </source>
​      <boot order='2'/>
​    </hostdev>
​  </devices>
​  ...
​

Figure 26.40. Devices - Host physical machine device assignment

Alternatively the following can also be done:

⁠

​
​
​  ...
​  <devices>
​    <hostdev mode='subsystem' type='pci' managed='yes'>
​      <source>
​        <address bus='0x06' slot='0x02' function='0x0'/>
​      </source>
​      <boot order='1'/>
​      <rom bar='on' file='/etc/fake/boot.bin'/>
​    </hostdev>
​  </devices>
​  ...
​

Figure 26.41. Devices - Host physical machine device assignment alternative

Alternatively the following can also be done:

⁠

​
​
​  ...
​  <devices>
​    <hostdev mode='subsystem' type='scsi'>
​      <source>
​        <adapter name='scsi_host0'/>
​        <address type='scsi' bus='0' target='0' unit='0'/>
​      </source>
​      <readonly/>
​      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
​    </hostdev>
​  </devices>
​  ..
​
​

Figure 26.42. Devices - host physical machine scsi device assignment

The components of this section of the domain XML are as follows:

⁠

Table 26.17. Host physical machine device assignment elements

Parameter
Description

hostdev
This is the main container for describing host physical machine devices. For USB device passthrough mode is always subsystem and type is usb for a USB device and pci for a PCI device. When managed is yes for a PCI device, it is detached from the host physical machine before being passed on to the guest virtual machine, and reattached to the host physical machine after the guest virtual machine exits. If managed is omitted or no for PCI and for USB devices, the user is responsible to use the argument virNodeDeviceDettach (or virsh nodedev-dettach) before starting the guest virtual machine or hot-plugging the device, and virNodeDeviceReAttach (or virsh nodedev-reattach) after hot-unplug or stopping the guest virtual machine.

source
Describes the device as seen from the host physical machine. The USB device can be addressed by vendor or product ID using the vendor and product elements or by the device's address on the host physical machines using the address element. PCI devices on the other hand can only be described by their address. Note that the source element of USB devices may contain a startupPolicy attribute which can be used to define a rule for what to do if the specified host physical machine USB device is not found. The attribute accepts the following values:

mandatory - Fails if missing for any reason (the default).

requisite - Fails if missing on boot up, drops if missing on migrate/restore/revert.

optional - Drops if missing at any start attempt.

vendor, product
These elements each have an id attribute that specifies the USB vendor and product ID. The IDs can be given in decimal, hexadecimal (starting with 0x) or octal (starting with 0) form.

boot
Specifies that the device is bootable. The attribute's order determines the order in which devices will be tried during boot sequence. The per-device boot elements cannot be used together with general boot elements in BIOS boot loader section.

rom
Used to change how a PCI device's ROM is presented to the guest virtual machine. The optional bar attribute can be set to on or off, and determines whether or not the device's ROM will be visible in the guest virtual machine's memory map. (In PCI documentation, the rom bar setting controls the presence of the Base Address Register for the ROM). If no rom bar is specified, the default setting will be used. The optional file attribute is used to point to a binary file to be presented to the guest virtual machine as the device's ROM BIOS. This can be useful, for example, to provide a PXE boot ROM for a virtual function of an SR-IOV capable ethernet device (which has no boot ROMs for the VFs).

address
Also has a bus and device attribute to specify the USB bus and device number the device appears at on the host physical machine. The values of these attributes can be given in decimal, hexadecimal (starting with 0x) or octal (starting with 0) form. For PCI devices, the element carries 3 attributes allowing to designate the device as can be found with lspci or with virsh nodedev-list.

⁠26.18.6.2. Block / character devices

The host physical machine's block / character devices can be passed through to the guest virtual machine by using management tools to modify the domain XML hostdev element. Note that this is only possible with container based virtualization.

⁠

​
​
​...
​<hostdev mode='capabilities' type='storage'>
​  <source>
​    <block>/dev/sdf1</block>
​  </source>
​</hostdev>
​...
​
​

Figure 26.43. Devices - Host physical machine device assignment block character devices

An alternative approach is this:

⁠

​
​
​...
​<hostdev mode='capabilities' type='misc'>
​  <source>
​    <char>/dev/input/event3</char>
​  </source>
​</hostdev>
​...
​
​

Figure 26.44. Devices - Host physical machine device assignment block character devices alternative 1

Another alternative approach is this:

⁠

​
​
​...
​<hostdev mode='capabilities' type='net'>
​  <source>
​    <interface>eth0</interface>
​  </source>
​</hostdev>
​...
​
​
​

Figure 26.45. Devices - Host physical machine device assignment block character devices alternative 2

The components of this section of the domain XML are as follows:

⁠

Table 26.18. Block / character device elements

Parameter | Description
hostdev | This is the main container for describing host physical machine devices. For block/character devices, passthrough mode is always capabilities, and type is block for a block device and char for a character device.
source | This describes the device as seen from the host physical machine. For block devices, the path to the block device in the host physical machine operating system is provided in the nested block element, while for character devices, the char element is used.

⁠26.18.7. Redirected devices

USB device redirection through a character device is configured by modifying the following section of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <redirdev bus='usb' type='tcp'>
​      <source mode='connect' host='localhost' service='4000'/>
​      <boot order='1'/>
​    </redirdev>
​    <redirfilter>
​      <usbdev class='0x08' vendor='0x1234' product='0xbeef' version='2.00' allow='yes'/>
​      <usbdev allow='no'/>
​    </redirfilter>
​  </devices>
​  ...
​

Figure 26.46. Devices - redirected devices

The components of this section of the domain XML are as follows:

⁠

Table 26.19. Redirected device elements

Parameter
Description

redirdev
This is the main container for describing redirected devices. bus must be usb for a USB device. An additional attribute type is required, matching one of the supported serial device types, to describe the host physical machine side of the tunnel: type='tcp' or type='spicevmc' (which uses the usbredir channel of a SPICE graphics device) are typical. The redirdev element has an optional sub-element, address, which can tie the device to a particular controller. Further sub-elements, such as source, may be required according to the given type, although a target sub-element is not required (since the consumer of the character device is the hypervisor itself, rather than a device visible in the guest virtual machine).

boot
Specifies that the device is bootable. The order attribute determines the order in which devices will be tried during boot sequence. The per-device boot elements cannot be used together with general boot elements in BIOS boot loader section.

redirfilter
This is used for creating the filter rule to filter out certain devices from redirection. It uses sub-element usbdev to define each filter rule. The class attribute is the USB Class code.

⁠26.18.8. Smartcard devices

A virtual smartcard device can be supplied to the guest virtual machine via the smartcard element. A USB smartcard reader device on the host physical machine cannot be used on a guest virtual machine with device passthrough. This is because it cannot be made available to both the host physical machine and guest virtual machine, and can lock the host physical machine computer when it is removed from the guest virtual machine. Therefore, some hypervisors provide a specialized virtual device that can present a smartcard interface to the guest virtual machine, with several modes for describing how the credentials are obtained from the host physical machine or even a from a channel created to a third-party smartcard provider.

Configure USB device redirection through a character device with management tools to modify the following section of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <smartcard mode='host'/>
​    <smartcard mode='host-certificates'>
​      <certificate>cert1</certificate>
​      <certificate>cert2</certificate>
​      <certificate>cert3</certificate>
​      <database>/etc/pki/nssdb/</database>
​    </smartcard>
​    <smartcard mode='passthrough' type='tcp'>
​      <source mode='bind' host='127.0.0.1' service='2001'/>
​      <protocol type='raw'/>
​      <address type='ccid' controller='0' slot='0'/>
​    </smartcard>
​    <smartcard mode='passthrough' type='spicevmc'/>
​  </devices>
​  ...
​

Figure 26.47. Devices - smartcard devices

The smartcard element has a mandatory attribute mode. In each mode, the guest virtual machine sees a device on its USB bus that behaves like a physical USB CCID (Chip/Smart Card Interface Device) card.

The mode attributes are as follows:

⁠

Table 26.20. Smartcard mode elements

Parameter
Description

mode='host'
In this mode, the hypervisor relays all requests from the guest virtual machine into direct access to the host physical machine's smartcard via NSS. No other attributes or sub-elements are required. See below about the use of an optional address sub-element.

mode='host-certificates'
This mode allows you to provide three NSS certificate names residing in a database on the host physical machine, rather than requiring a smartcard to be plugged into the host physical machine. These certificates can be generated via the command certutil -d /etc/pki/nssdb -x -t CT,CT,CT -S -s CN=cert1 -n cert1, and the resulting three certificate names must be supplied as the content of each of three certificate sub-elements. An additional sub-element database can specify the absolute path to an alternate directory (matching the -d flag of the certutil command when creating the certificates); if not present, it defaults to /etc/pki/nssdb.

mode='passthrough'
Using this mode allows you to tunnel all requests through a secondary character device to a third-party provider (which may in turn be communicating to a smartcard or using three certificate files, rather than having the hypervisor directly communicate with the host physical machine. In this mode of operation, an additional attribute type is required, matching one of the supported serial device types, to describe the host physical machine side of the tunnel; type='tcp' or type='spicevmc' (which uses the smartcard channel of a SPICE graphics device) are typical. Further sub-elements, such as source, may be required according to the given type, although a target sub-element is not required (since the consumer of the character device is the hypervisor itself, rather than a device visible in the guest virtual machine).

Each mode supports an optional sub-element address, which fine-tunes the correlation between the smartcard and a ccid bus controller (Refer to Section 26.18.3, “Device addresses”).

⁠26.18.9. Network interfaces

Modify the network interface devices using management tools to configure the following part of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='direct' trustGuestRxFilters='yes'>
​      <source dev='eth0'/>
​      <mac address='52:54:00:5d:c7:9e'/>
​      <boot order='1'/>
​      <rom bar='off'/>
​    </interface>
​  </devices>
​  ...
​

Figure 26.48. Devices - network interfaces

There are several possibilities for configuring the network interface for the guest virtual machine. This is done by setting a value to the interface element's type attribute. The following values may be used:

"direct" - Attaches the guest virtual machine's NIC to the physical NIC on the host physical machine. Refer to Section 26.18.9.6, “Direct attachment to physical interfaces” for more details and an example.

"network" - This is the recommended configuration for general guest virtual machine connectivity on host physical machines with dynamic or wireless networking configurations. Refer to Section 26.18.9.1, “Virtual networks” for more details and an example.

"bridge" - This is the recommended configuration setting for guest virtual machine connectivity on host physical machines with static wired networking configurations. Refer to Section 26.18.9.2, “Bridge to LAN” for more details and an example.

"ethernet" - Provides a means for the administrator to execute an arbitrary script to connect the guest virtual machine's network to the LAN. Refer to Section 26.18.9.5, “Generic Ethernet connection” for more details and an example.

"hostdev" - Allows a PCI network device to be directly assigned to the guest virtual machine using generic device passthrough. Refer to Section 26.18.9.7, “PCI passthrough” for more details and an example.

"mcast" - A multicast group can be used to represent a virtual network. Refer to Section 26.18.9.8, “Multicast tunnel” for more details and an example.

"user" - Using the user option sets the user space SLIRP stack parameters provides a virtual LAN with NAT to the outside world. Refer to Section 26.18.9.4, “User space SLIRP stack” for more details and an example.

"server" - Using the server option creates a TCP client/server architecture in order to provide a virtual network where one guest virtual machine provides the server end of the network and all other guest virtual machines are configured as clients. Refer to Section 26.18.9.9, “TCP tunnel” for more details and an example.

Each of these options has a link to give more details. Additionally, each <interface> element can be defined with an optional <trustGuestRxFilters> attribute which allows host physical machine to detect and trust reports received from the guest virtual machine. These reports are sent each time the interface receives changes to the filter. This includes changes to the primary MAC address, the device address filter, or the vlan configuration. The <trustGuestRxFilters> attribute is disabled by default for security reasons. It should also be noted that support for this attribute depends on the guest network device model as well as on the host physical machine's connection type. Currently it is only supported for the virtio device models and for macvtap connections on the host physical machine. OA simple case use where it is recommended to set the optional parameter <trustGuestRxFilters> is if you want to give your guest virtual machines the permission to control host physical machine side filters, as any filters that are set by the guest will also be mirrored on the host.

In addition to the attributes listed above, each <interface> element can take an optional <address> sub-element that can tie the interface to a particular PCI slot, with attribute type='pci'. Refer to Section 26.18.3, “Device addresses” for more information.

⁠26.18.9.1. Virtual networks

This is the recommended configuration for general guest virtual machine connectivity on host physical machines with dynamic or wireless networking configurations (or multi-host physical machine environments where the host physical machine hardware details, which are described separately in a <network> definition). In addition, it provides a connection with details that are described by the named network definition. Depending on the virtual network's forward mode configuration, the network may be totally isolated (no <forward> element given), using NAT to connect to an explicit network device or to the default route (forward mode='nat'), routed with no NAT (forward mode='route'), or connected directly to one of the host physical machine's network interfaces (using macvtap) or bridge devices (forward mode='bridge|private|vepa|passthrough')

For networks with a forward mode of bridge, private, vepa, and passthrough, it is assumed that the host physical machine has any necessary DNS and DHCP services already set up outside the scope of libvirt. In the case of isolated, nat, and routed networks, DHCP and DNS are provided on the virtual network by libvirt, and the IP range can be determined by examining the virtual network configuration with virsh net-dumpxml [networkname]. The 'default' virtual network, which is set up out of the box, uses NAT to connect to the default route and has an IP range of 192.168.122.0/255.255.255.0. Each guest virtual machine will have an associated tun device created with a name of vnetN, which can also be overridden with the <target> element (refer to Section 26.18.9.11, “Overriding the target element”).

When the source of an interface is a network, a port group can be specified along with the name of the network; one network may have multiple portgroups defined, with each portgroup containing slightly different configuration information for different classes of network connections. Also, similar to <direct> network connections (described below), a connection of type network may specify a <virtualport> element, with configuration data to be forwarded to a 802.1Qbg or 802.1Qbh compliant Virtual Ethernet Port Aggregator (VEPA)switch, or to an Open vSwitch virtual switch.

Since the type of switch is dependent on the configuration setting in the <network> element on the host physical machine, it is acceptable to omit the <virtualport type> attribute. You will need to specify the <virtualport type> either once or many times. When the domain starts up a complete <virtualport> element is constructed by merging together the type and attributes defined. This results in a newly-constructed virtual port. Note that the attributes from lower virtual ports cannot make changes on the attributes defined in higher virtual ports. Interfaces take the highest priority, while port group is lowest priority.

For example, in order to work properly with both an 802.1Qbh switch and an Open vSwitch switch, you may choose to specify no type, but both a profileid (in case the switch is 802.1Qbh) and an interfaceid (in case the switch is Open vSwitch) You may also omit the other attributes, such as managerid, typeid, or profileid, to be filled in from the network's virtualport. If you want to limit a guest virtual machine to connect only to certain types of switches, you can specify the virtual port type, but still omit some or all of the parameters. In this case, if the host physical machine's network has a different type of virtual port, connection of the interface will fail. Define the virtual network parameters using management tools that modify the following part of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='network'>
​      <source network='default'/>
​    </interface>
​    ...
​    <interface type='network'>
​      <source network='default' portgroup='engineering'/>
​      <target dev='vnet7'/>
​      <mac address="00:11:22:33:44:55"/>
​      <virtualport>
​        <parameters instanceid='09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f'/>
​      </virtualport>
​
​    </interface>
​  </devices>
​  ...
​

Figure 26.49. Devices - network interfaces- virtual networks

⁠26.18.9.2. Bridge to LAN

As mentioned in, Section 26.18.9, “Network interfaces”, this is the recommended configuration setting for guest virtual machine connectivity on host physical machines with static wired networking configurations.

Bridge to LAN provides a bridge from the guest virtual machine directly onto the LAN. This assumes there is a bridge device on the host physical machine which has one or more of the host physical machines physical NICs enslaved. The guest virtual machine will have an associated tun device created with a name of <vnetN>, which can also be overridden with the <target> element (refer to Section 26.18.9.11, “Overriding the target element”). The <tun> device will be enslaved to the bridge. The IP range or network configuration is the same as what is used on the LAN. This provides the guest virtual machine full incoming and outgoing network access, just like a physical machine.

On Linux systems, the bridge device is normally a standard Linux host physical machine bridge. On host physical machines that support Open vSwitch, it is also possible to connect to an Open vSwitch bridge device by adding virtualport type='openvswitch'/ to the interface definition. The Open vSwitch type virtualport accepts two parameters in its parameters element: an interfaceid which is a standard UUID used to uniquely identify this particular interface to Open vSwitch (if you do no specify one, a random interfaceid will be generated when first defining the interface), and an optional profileid which is sent to Open vSwitch as the interfaces <port-profile>. To set the bridge to LAN settings, use a management tool that will configure the following part of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    ...
​    <interface type='bridge'>
​      <source bridge='br0'/>
​    </interface>
​    <interface type='bridge'>
​      <source bridge='br1'/>
​      <target dev='vnet7'/>
​      <mac address="00:11:22:33:44:55"/>
​    </interface>
​    <interface type='bridge'>
​      <source bridge='ovsbr'/>
​      <virtualport type='openvswitch'>
​        <parameters profileid='menial' interfaceid='09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f'/>
​      </virtualport>
​    </interface>
​    ...
​  </devices>
​

Figure 26.50. Devices - network interfaces- bridge to LAN

⁠26.18.9.3. Setting a port masquerading range

In cases where you want to set the port masquerading range, set the port as follows:

⁠

​
​
​<forward mode='nat'>
​   <address start='1.2.3.4' end='1.2.3.10'/>
​</forward>  ...
​

Figure 26.51. Port Masquerading Range

These values should be set using the iptables commands as shown in Section 20.3, “Network Address Translation”

⁠26.18.9.4. User space SLIRP stack

Setting the user space SLIRP stack parameters provides a virtual LAN with NAT to the outside world. The virtual network has DHCP and DNS services and will give the guest virtual machine an IP addresses starting from 10.0.2.15. The default router is 10.0.2.2 and the DNS server is 10.0.2.3. This networking is the only option for unprivileged users who need their guest virtual machines to have outgoing access.

The user space SLIRP stack parameters are defined in the following part of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='user'/>
​    ...
​    <interface type='user'>
​      <mac address="00:11:22:33:44:55"/>
​    </interface>
​  </devices>
​  ...
​

Figure 26.52. Devices - network interfaces- User space SLIRP stack

⁠26.18.9.5. Generic Ethernet connection

This provides a means for the administrator to execute an arbitrary script to connect the guest virtual machine's network to the LAN. The guest virtual machine will have a <tun> device created with a name of vnetN, which can also be overridden with the <target> element. After creating the tun device a shell script will be run and complete the required host physical machine network integration. By default, this script is called /etc/kvm-ifup but can be overridden (refer to Section 26.18.9.11, “Overriding the target element”).

The generic ethernet connection parameters are defined in the following part of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='ethernet'/>
​    ...
​    <interface type='ethernet'>
​      <target dev='vnet7'/>
​      <script path='/etc/kvm-ifup-mynet'/>
​    </interface>
​  </devices>
​  ...
​

Figure 26.53. Devices - network interfaces- generic ethernet connection

⁠26.18.9.6. Direct attachment to physical interfaces

This directly attaches the guest virtual machine's NIC to the physical interface of the host physical machine, if the physical interface is specified.

This requires the Linux macvtap driver to be available. One of the following mode attribute values vepa ( 'Virtual Ethernet Port Aggregator'), bridge or private can be chosen for the operation mode of the macvtap device. vepa is the default mode.

Manipulating direct attachment to physical interfaces involves setting the following parameters in this section of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    ...
​    <interface type='direct'>
​      <source dev='eth0' mode='vepa'/>
​    </interface>
​  </devices>
​  ...
​

Figure 26.54. Devices - network interfaces- direct attachment to physical interfaces

The individual modes cause the delivery of packets to behave as shown in Table 26.21, “Direct attachment to physical interface elements”:

⁠

Table 26.21. Direct attachment to physical interface elements

Element
Description

vepa
All of the guest virtual machines' packets are sent to the external bridge. Packets whose destination is a guest virtual machine on the same host physical machine as where the packet originates from are sent back to the host physical machine by the VEPA capable bridge (today's bridges are typically not VEPA capable).

bridge
Packets whose destination is on the same host physical machine as where they originate from are directly delivered to the target macvtap device. Both origin and destination devices need to be in bridge mode for direct delivery. If either one of them is in vepa mode, a VEPA capable bridge is required.

private
All packets are sent to the external bridge and will only be delivered to a target virtual machine on the same host physical machine if they are sent through an external router or gateway and that device sends them back to the host physical machine. This procedure is followed if either the source or destination device is in private mode.

passthrough
This feature attaches a virtual function of a SR-IOV capable NIC directly to a guest virtual machine without losing the migration capability. All packets are sent to the VF/IF of the configured network device. Depending on the capabilities of the device, additional prerequisites or limitations may apply; for example, this requires kernel 2.6.38 or later.

The network access of directly attached virtual machines can be managed by the hardware switch to which the physical interface of the host physical machine is connected to.

The interface can have additional parameters as shown below, if the switch conforms to the IEEE 802.1Qbg standard. The parameters of the virtualport element are documented in more detail in the IEEE 802.1Qbg standard. The values are network specific and should be provided by the network administrator. In 802.1Qbg terms, the Virtual Station Interface (VSI) represents the virtual interface of a virtual machine.

Note that IEEE 802.1Qbg requires a non-zero value for the VLAN ID.

Additional elements that can be manipulated are described in Table 26.22, “Direct attachment to physical interface additional elements”:

⁠

Table 26.22. Direct attachment to physical interface additional elements

Element | Description
managerid | The VSI Manager ID identifies the database containing the VSI type and instance definitions. This is an integer value and the value 0 is reserved.
typeid | The VSI Type ID identifies a VSI type characterizing the network access. VSI types are typically managed by network administrator. This is an integer value.
typeidversion | The VSI Type Version allows multiple versions of a VSI Type. This is an integer value.
instanceid | The VSI Instance ID Identifier is generated when a VSI instance (i.e. a virtual interface of a virtual machine) is created. This is a globally unique identifier.
profileid | The profile ID contains the name of the port profile that is to be applied onto this interface. This name is resolved by the port profile database into the network parameters from the port profile, and those network parameters will be applied to this interface.

Additional parameters in the domain XML include:

⁠

​
​
​  ...
​  <devices>
​    ...
​    <interface type='direct'>
​      <source dev='eth0.2' mode='vepa'/>
​      <virtualport type="802.1Qbg">
​        <parameters managerid="11" typeid="1193047" typeidversion="2" instanceid="09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f"/>
​      </virtualport>
​    </interface>
​  </devices>
​  ...
​

Figure 26.55. Devices - network interfaces- direct attachment to physical interfaces additional parameters

The interface can have additional parameters as shown below if the switch conforms to the IEEE 802.1Qbh standard. The values are network specific and should be provided by the network administrator.

Additional parameters in the domain XML include:

⁠

​
​
​  ...
​  <devices>
​    ...
​    <interface type='direct'>
​      <source dev='eth0' mode='private'/>
​      <virtualport type='802.1Qbh'>
​        <parameters profileid='finance'/>
​      </virtualport>
​    </interface>
​  </devices>
​  ...
​

Figure 26.56. Devices - network interfaces - direct attachment to physical interfaces more additional parameters

The profileid attribute contains the name of the port profile to be applied to this interface. This name is resolved by the port profile database into the network parameters from the port profile, and those network parameters will be applied to this interface.

⁠26.18.9.7. PCI passthrough

A PCI network device (specified by the source element) is directly assigned to the guest virtual machine using generic device passthrough, after first optionally setting the device's MAC address to the configured value, and associating the device with an 802.1Qbh capable switch using an optionally specified virtualport element (see the examples of virtualport given above for type='direct' network devices). Note that due to limitations in standard single-port PCI ethernet card driver design, only SR-IOV (Single Root I/O Virtualization) virtual function (VF) devices can be assigned in this manner. To assign a standard single-port PCI or PCIe ethernet card to a guest virtual machine, use the traditional hostdev device definition.

Note that this "intelligent passthrough" of network devices is very similar to the functionality of a standard hostdev device, the difference being that this method allows specifying a MAC address and virtualport for the passed-through device. If these capabilities are not required, if you have a standard single-port PCI, PCIe, or USB network card that does not support SR-IOV (and hence would anyway lose the configured MAC address during reset after being assigned to the guest virtual machine domain), or if you are using libvirt version older than 0.9.11, use standard hostdev definition to assign the device to the guest virtual machine instead of interface type='hostdev'.

⁠

​
​
​  ...
​  <devices>
​    <interface type='hostdev'>
​      <driver name='vfio'/>
​      <source>
​        <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
​      </source>
​      <mac address='52:54:00:6d:90:02'>
​      <virtualport type='802.1Qbh'>
​        <parameters profileid='finance'/>
​      </virtualport>
​    </interface>
​  </devices>
​  ...
​

Figure 26.57. Devices - network interfaces- PCI passthrough

⁠26.18.9.8. Multicast tunnel

A multicast group can be used to represent a virtual network. Any guest virtual machine with network devices within the same multicast group will communicate with each other, even if they reside across multiple physical host physical machines. This mode may be used as an unprivileged user. There is no default DNS or DHCP support and no outgoing network access. To provide outgoing network access, one of the guest virtual machines should have a second NIC which is connected to one of the first 4 network types in order to provide appropriate routing. The multicast protocol is compatible with protocols used by user mode Linux guest virtual machines as well. Note that the source address used must be from the multicast address block. A multicast tunnel is created by manipulating the interface type using a management tool and setting it to mcast, and providing a mac address and source address, for example:

⁠

​
​
​  ...
​  <devices>
​    <interface type='mcast'>
​      <mac address='52:54:00:6d:90:01'>
​      <source address='230.0.0.1' port='5558'/>
​    </interface>
​  </devices>
​  ...
​

Figure 26.58. Devices - network interfaces- multicast tunnel

⁠26.18.9.9. TCP tunnel

Creating a TCP client/server architecture is another way to provide a virtual network where one guest virtual machine provides the server end of the network and all other guest virtual machines are configured as clients. All network traffic between the guest virtual machines is routed through the guest virtual machine that is configured as the server. This model is also available for use to unprivileged users. There is no default DNS or DHCP support and no outgoing network access. To provide outgoing network access, one of the guest virtual machines should have a second NIC which is connected to one of the first 4 network types thereby providing the appropriate routing. A TCP tunnel is created by manipulating the interface type using a management tool and setting it to mcast, and providing a mac address and source address, for example:

⁠

​
​
​  ...
​  <devices>
​    <interface type='server'>
​      <mac address='52:54:00:22:c9:42'>
​      <source address='192.168.0.1' port='5558'/>
​    </interface>
​    ...
​    <interface type='client'>
​      <mac address='52:54:00:8b:c9:51'>
​      <source address='192.168.0.1' port='5558'/>
​    </interface>
​  </devices>
​  ...
​

Figure 26.59. Devices - network interfaces- TCP tunnel

⁠26.18.9.10. Setting NIC driver-specific options

Some NICs may have tunable driver-specific options. These options are set as attributes of the driver sub-element of the interface definition. These options are set by using management tools to configure the following sections of the domain XML:

⁠

​
​
​  <devices>
​    <interface type='network'>
​      <source network='default'/>
​      <target dev='vnet1'/>
​      <model type='virtio'/>
​      <driver name='vhost' txmode='iothread' ioeventfd='on' event_idx='off'/>
​    </interface>
​  </devices>
​  ...
​

Figure 26.60. Devices - network interfaces- setting NIC driver-specific options

The following attributes are available for the "virtio" NIC driver:

⁠

Table 26.23. virtio NIC driver elements

Parameter
Description

name
The optional name attribute forces which type of back-end driver to use. The value can be either kvm (a user-space back-end) or vhost (a kernel back-end, which requires the vhost module to be provided by the kernel); an attempt to require the vhost driver without kernel support will be rejected. The default setting is vhost if the vhost driver is present, but will silently fall back to kvm if not.

txmode
Specifies how to handle transmission of packets when the transmit buffer is full. The value can be either iothread or timer. If set to iothread, packet tx is all done in an iothread in the bottom half of the driver (this option translates into adding "tx=bh" to the kvm commandline -device virtio-net-pci option). If set to timer, tx work is done in KVM, and if there is more tx data than can be sent at the present time, a timer is set before KVM moves on to do other things; when the timer fires, another attempt is made to send more data. It is not recommended to change this value.

ioeventfd
Sets domain I/O asynchronous handling for the interface device. The default is left to the discretion of the hypervisor. Accepted values are on and off . Enabling this option allows KVM to execute a guest virtual machine while a separate thread handles I/O. Typically, guest virtual machines experiencing high system CPU utilization during I/O will benefit from this. On the other hand, overloading the physical host machine may also increase guest virtual machine I/O latency. It is not recommended to change this value.

event_idx
The event_idx attribute controls some aspects of device event processing. The value can be either on or off. on is the default, which reduces the number of interrupts and exits for the guest virtual machine. In situations where this behavior is sub-optimal, this attribute provides a way to force the feature off. It is not recommended to change this value.

⁠26.18.9.11. Overriding the target element

To override the target element, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='network'>
​      <source network='default'/>
​      <target dev='vnet1'/>
​    </interface>
​  </devices>
​  ...
​

Figure 26.61. Devices - network interfaces- overriding the target element

If no target is specified, certain hypervisors will automatically generate a name for the created tun device. This name can be manually specified, however the name must not start with either vnet or vif, which are prefixes reserved by libvirt and certain hypervisors. Manually-specified targets using these prefixes will be ignored.

⁠26.18.9.12. Specifying boot order

To specify the boot order, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='network'>
​      <source network='default'/>
​      <target dev='vnet1'/>
​      <boot order='1'/>
​    </interface>
​  </devices>
​  ...

Figure 26.62. Specifying boot order

In hypervisors which support it, you can set a specific NIC to be used for the network boot. The order of attributes determine the order in which devices will be tried during boot sequence. Note that the per-device boot elements cannot be used together with general boot elements in BIOS bootloader section.

⁠26.18.9.13. Interface ROM BIOS configuration

To specify the ROM BIOS configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='network'>
​      <source network='default'/>
​      <target dev='vnet1'/>
​      <rom bar='on' file='/etc/fake/boot.bin'/>
​    </interface>
​  </devices>
​  ...

Figure 26.63. Interface ROM BIOS configuration

For hypervisors which support it, you can change how a PCI Network device's ROM is presented to the guest virtual machine. The bar attribute can be set to on or off, and determines whether or not the device's ROM will be visible in the guest virtual machine's memory map. (In PCI documentation, the rom bar setting controls the presence of the Base Address Register for the ROM). If no rom baris specified, the KVM default will be used (older versions of KVM used off for the default, while newer KVM hypervisors default to on). The optional file attribute is used to point to a binary file to be presented to the guest virtual machine as the device's ROM BIOS. This can be useful to provide an alternative boot ROM for a network device.

⁠26.18.9.14. Quality of service (QoS)

Incoming and outgoing traffic can be shaped independently to set Quality of Service (QoS). The bandwidth element can have at most one inbound and one outbound child elements. Leaving any of these child elements out results in no QoS being applied on that traffic direction. Therefore, to shape only a domain's incoming traffic, use inbound only, and vice versa.

Each of these elements has one mandatory attribute average (or floor as described below). Average specifies the average bit rate on the interface being shaped. Then there are two optional attributes: peak, This attribute specifies the maximum rate at which the bridge can send data, in kilobytes a second. A limitation of this implementation is this attribute in the outbound element is ignored, as Linux ingress filters don't know it yet. burst, specifies the amount of bytes that can be burst at peak speed. Accepted values for attributes are integer numbers.

The units for average and peak attributes are kilobytes per second, whereas burst is only set in kilobytes. In addition, inbound traffic can optionally have a floor attribute. This guarantees minimal throughput for shaped interfaces. Using the floor requires that all traffic goes through one point where QoS decisions can take place. As such, it may only be used in cases where the interface type='network'/ with a forward type of route, nat, or no forward at all). It should be noted that within a virtual network, all connected interfaces are required to have at least the inbound QoS set (average at least) but the floor attribute does not require specifying average. However, peak and burst attributes still require average. At the present time, ingress qdiscs may not have any classes, and therefore floor may only be applied only on inbound and not outbound traffic.

To specify the QoS configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='network'>
​      <source network='default'/>
​      <target dev='vnet0'/>
​      <bandwidth>
​        <inbound average='1000' peak='5000' floor='200' burst='1024'/>
​        <outbound average='128' peak='256' burst='256'/>
​      </bandwidth>
​    </interface>
​  <devices>
​  ...

Figure 26.64. Quality of service

⁠26.18.9.15. Setting VLAN tag (on supported network types only)

To specify the VLAN tag configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='bridge'>
​      <vlan>
​        <tag id='42'/>
​      </vlan>
​      <source bridge='ovsbr0'/>
​      <virtualport type='openvswitch'>
​        <parameters interfaceid='09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f'/>
​      </virtualport>
​    </interface>
​  <devices>
​  ...

Figure 26.65. Setting VLAN tag (on supported network types only)

If the network connection used by the guest virtual machine supports VLAN tagging transparent to the guest virtual machine, an optional vlan element can specify one or more VLAN tags to apply to the guest virtual machine's network traffic. Only OpenvSwitch and type='hostdev' SR-IOV interfaces support transparent VLAN tagging of guest virtual machine traffic; other interfaces, including standard Linux bridges and libvirt's own virtual networks, do not support it. 802.1Qbh (vn-link) and 802.1Qbg (VEPA) switches provide their own methods (outside of libvirt) to tag guest virtual machine traffic onto specific VLANs. To allow for specification of multiple tags (in the case of VLAN trunking), the tag subelement specifies which VLAN tag to use (for example, tag id='42'/). If an interface has more than one vlan element defined, it is assumed that the user wants to do VLAN trunking using all the specified tags. In the case that VLAN trunking with a single tag is desired, the optional attribute trunk='yes' can be added to the top-level vlan element.

⁠26.18.9.16. Modifying virtual link state

This element sets the virtual network link state. Possible values for attribute state are up and down. If down is specified as the value, the interface behaves as the network cable is disconnected. Default behavior if this element is unspecified is up.

To specify the virtual link state configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <interface type='network'>
​      <source network='default'/>
​      <target dev='vnet0'/>
​      <link state='down'/>
​    </interface>
​  <devices>
​  ...

Figure 26.66. Modifying virtual link state

⁠26.18.10. Input devices

Input devices allow interaction with the graphical framebuffer in the guest virtual machine. When enabling the framebuffer, an input device is automatically provided. It may be possible to add additional devices explicitly, for example, to provide a graphics tablet for absolute cursor movement.

To specify the input device configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <input type='mouse' bus='usb'/>
​  </devices>
​  ...

Figure 26.67. Input devices

The <input> element has one mandatory attribute: type, which can be set to mouse or tablet. tablet provides absolute cursor movement, while mouse uses relative movement. The optional bus attribute can be used to refine the exact device type and can be set to kvm (para-virtualized), ps2, and usb.

The input element has an optional sub-element <address>, which can tie the device to a particular PCI slot, as documented above.

⁠26.18.11. Hub devices

A hub is a device that expands a single port into several so that there are more ports available to connect devices to a host physical machine system.

To specify the hub device configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <hub type='usb'/>
​  </devices>
​  ...

Figure 26.68. Hub devices

The hub element has one mandatory attribute, type, which can only be set to usb. The hub element has an optional sub-element, address, with type='usb', which can tie the device to a particular controller.

⁠26.18.12. Graphical framebuffers

A graphics device allows for graphical interaction with the guest virtual machine operating system. A guest virtual machine will typically have either a framebuffer or a text console configured to allow interaction with the admin.

To specify the graphical framebuffer device configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <graphics type='sdl' display=':0.0'/>
​    <graphics type='vnc' port='5904'>
​      <listen type='address' address='1.2.3.4'/>
​    </graphics>
​    <graphics type='rdp' autoport='yes' multiUser='yes' />
​    <graphics type='desktop' fullscreen='yes'/>
​    <graphics type='spice'>
​      <listen type='network' network='rednet'/>
​    </graphics>
​  </devices>
​  ...

Figure 26.69. Graphical framebuffers

The graphics element has a mandatory type attribute which takes the value sdl, vnc, rdp or desktop as explained below:

⁠

Table 26.24. Graphical framebuffer elements

Parameter
Description

sdl
This displays a window on the host physical machine desktop. It accepts 3 optional arguments: a display attribute for the display to use, an xauth attribute for the authentication identifier, and an optional fullscreen attribute accepting values yes or no.

vnc
Starts a VNC server. The port attribute specifies the TCP port number (with -1 as legacy syntax indicating that it should be auto-allocated). The autoport attribute is the preferred syntax for indicating auto-allocation of the TCP port to use. The listen attribute is an IP address for the server to listen on. The passwd attribute provides a VNC password in clear text. The keymap attribute specifies the keymap to use. It is possible to set a limit on the validity of the password be giving an timestamp passwdValidTo='2010-04-09T15:51:00' assumed to be in UTC. The connected attribute allows control of connected client during password changes. VNC accepts the keep value only; note that it may not be supported by all hypervisors. Rather than using listen/port, KVM supports a socket attribute for listening on a UNIX domain socket path.

spice
Starts a SPICE server. The port attribute specifies the TCP port number (with -1 as legacy syntax indicating that it should be auto-allocated), while tlsPort gives an alternative secure port number. The autoport attribute is the new preferred syntax for indicating auto-allocation of both port numbers. The listen attribute is an IP address for the server to listen on. The passwd attribute provides a SPICE password in clear text. The keymap attribute specifies the keymap to use. It is possible to set a limit on the validity of the password be giving an timestamp passwdValidTo='2010-04-09T15:51:00' assumed to be in UTC. The connected attribute allows control of a connected client during password changes. SPICE accepts keep to keep a client connected, disconnect to disconnect the client and fail to fail changing password. Note, this is not supported by all hypervisors. The defaultMode attribute sets the default channel security policy; valid values are secure, insecure and the default any (which is secure if possible, but falls back to insecure rather than erroring out if no secure path is available).

When SPICE has both a normal and TLS secured TCP port configured, it may be desirable to restrict what channels can be run on each port. This is achieved by adding one or more channel elements inside the main graphics element. Valid channel names include main, display, inputs, cursor, playback, record, smartcard, and usbredir.

To specify the SPICE configuration settings, use a mangement tool to make the following changes to the domain XML:

⁠

​
​
​  <graphics type='spice' port='-1' tlsPort='-1' autoport='yes'>
​    <channel name='main' mode='secure'/>
​    <channel name='record' mode='insecure'/>
​    <image compression='auto_glz'/>
​    <streaming mode='filter'/>
​    <clipboard copypaste='no'/>
​    <mouse mode='client'/>
​  </graphics>

Figure 26.70. SPICE configuration

SPICE supports variable compression settings for audio, images and streaming. These settings are configured using the compression attribute in all following elements: image to set image compression (accepts auto_glz, auto_lz, quic, glz, lz, off), jpeg for JPEG compression for images over WAN (accepts auto, never, always), zlib for configuring WAN image compression (accepts auto, never, always) and playback for enabling audio stream compression (accepts on or off).

The streaming element sets streaming mode. The mode attribute can be set to filter, all or off.

In addition, copy and paste functionality (through the SPICE agent) is set by the clipboard element. It is enabled by default, and can be disabled by setting the copypaste property to no.

The mouse element sets mouse mode. The mode attribute can be set to server or client. If no mode is specified, the KVM default will be used (client mode).

Additional elements include:

⁠

Table 26.25. Additional graphical framebuffer elements

Parameter
Description

rdp
Starts a RDP server. The port attribute specifies the TCP port number (with -1 as legacy syntax indicating that it should be auto-allocated). The autoport attribute is the preferred syntax for indicating auto-allocation of the TCP port to use. The replaceUser attribute is a boolean deciding whether multiple simultaneous connections to the virtual machine are permitted. The multiUser decides whether the existing connection must be dropped and a new connection must be established by the VRDP server, when a new client connects in single connection mode.

desktop
This value is reserved for VirtualBox domains for the moment. It displays a window on the host physical machine desktop, similarly to "sdl", but uses the VirtualBox viewer. Just like "sdl", it accepts the optional attributes display and fullscreen.

listen
Rather than putting the address information used to set up the listening socket for graphics types vnc and spice in the graphics, the listen attribute, a separate sub-element of graphics, can be specified (see the examples above). listen accepts the following attributes:

type - Set to either address or network. This tells whether this listen element is specifying the address to be used directly, or by naming a network (which will then be used to determine an appropriate address for listening).

address - This attribute will contain either an IP address or hostname (which will be resolved to an IP address via a DNS query) to listen on. In the "live" XML of a running domain, this attribute will be set to the IP address used for listening, even if type='network'.

network - If type='network', the network attribute will contain the name of a network in libvirt's list of configured networks. The named network configuration will be examined to determine an appropriate listen address. For example, if the network has an IPv4 address in its configuration (for example, if it has a forward type of route, NAT, or an isolated type), the first IPv4 address listed in the network's configuration will be used. If the network is describing a host physical machine bridge, the first IPv4 address associated with that bridge device will be used. If the network is describing one of the 'direct' (macvtap) modes, the first IPv4 address of the first forward dev will be used.

⁠26.18.13. Video devices

To specify the video device configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <video>
​      <model type='vga' vram='8192' heads='1'>
​        <acceleration accel3d='yes' accel2d='yes'/>
​      </model>
​    </video>
​  </devices>
​  ...

Figure 26.71. Video devices

The graphics element has a mandatory type attribute which takes the value "sdl", "vnc", "rdp" or "desktop" as explained below:

⁠

Table 26.26. Graphical framebuffer elements

Parameter
Description

video
The video element is the container for describing video devices. For backwards compatibility, if no video is set but there is a graphics element in the domain XML, then libvirt will add a default video according to the guest virtual machine type. If "ram" or "vram" are not supplied, a default value is used.

model
This has a mandatory type attribute which takes the value vga, cirrus, vmvga, kvm, vbox, or qxl depending on the hypervisor features available. You can also provide the amount of video memory in kibibytes (blocks of 1024 bytes) using vram and the number of figure with heads.

acceleration
If acceleration is supported it should be enabled using the accel3d and accel2d attributes in the acceleration element.

address
The optional address sub-element can be used to tie the video device to a particular PCI slot.

⁠26.18.14. Consoles, serial, and channel devices

A character device provides a way to interact with the virtual machine. Para-virtualized consoles, serial ports, and channels are all classed as character devices and are represented using the same syntax.

To specify the consoles, channel and other device configuration settings, use a management tool to make the following changes to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <serial type='pty'>
​      <source path='/dev/pts/3'/>
​      <target port='0'/>
​    </serial>
​    <console type='pty'>
​      <source path='/dev/pts/4'/>
​      <target port='0'/>
​    </console>
​    <channel type='unix'>
​      <source mode='bind' path='/tmp/guestfwd'/>
​      <target type='guestfwd' address='10.0.2.1' port='4600'/>
​    </channel>
​  </devices>
​  ...

Figure 26.72. Consoles, serial, and channel devices

In each of these directives, the top-level element name (serial, console, channel) describes how the device is presented to the guest virtual machine. The guest virtual machine interface is configured by the target element. The interface presented to the host physical machine is given in the type attribute of the top-level element. The host physical machine interface is configured by the source element. The source element may contain an optional seclabel to override the way that labeling is done on the socket path. If this element is not present, the security label is inherited from the per-domain setting. Each character device element has an optional sub-element address which can tie the device to a particular controller or PCI slot.

Note

Parallel ports, as well as the isa-parallel device, are no longer supported.

⁠26.18.15. Guest virtual machine interfaces

A character device presents itself to the guest virtual machine as one of the following types.

To set the serial port, use a management tool to make the following change to the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <serial type='pty'>
​      <source path='/dev/pts/3'/>
​      <target port='0'/>
​    </serial>
​  </devices>
​  ...

Figure 26.73. Guest virtual machine interface serial port

<target> can have a port attribute, which specifies the port number. Ports are numbered starting from 0. There are usually 0, 1 or 2 serial ports. There is also an optional type attribute, which has two choices for its value, isa-serial or usb-serial. If type is missing, isa-serial will be used by default. For usb-serial, an optional sub-element <address> with type='usb' can tie the device to a particular controller, documented above.

The <console> element is used to represent interactive consoles. Depending on the type of guest virtual machine in use, the consoles might be para-virtualized devices, or they might be a clone of a serial device, according to the following rules:

If no targetType attribute is set, then the default device type is according to the hypervisor's rules. The default type will be added when re-querying the XML fed into libvirt. For fully virtualized guest virtual machines, the default device type will usually be a serial port.

If the targetType attribute is serial, and if no <serial> element exists, the console element will be copied to the <serial> element. If a <serial> element does already exist, the console element will be ignored.

If the targetType attribute is not serial, it will be treated normally.

Only the first <console> element may use a targetType of serial. Secondary consoles must all be para-virtualized.

On s390, the console element may use a targetType of sclp or sclplm (line mode). SCLP is the native console type for s390. There is no controller associated to SCLP consoles.

In the example below, a virtio console device is exposed in the guest virtual machine as /dev/hvc[0-7] (for more information, see the Fedora project's virtio-serial page):

⁠

​
​
​  ...
​  <devices>
​    <console type='pty'>
​      <source path='/dev/pts/4'/>
​      <target port='0'/>
​    </console>
​
​    <!-- KVM virtio console -->
​    <console type='pty'>
​      <source path='/dev/pts/5'/>
​      <target type='virtio' port='0'/>
​    </console>
​  </devices>
​  ...
​
​  ...
​  <devices>
​    <!-- KVM s390 sclp console -->
​    <console type='pty'>
​      <source path='/dev/pts/1'/>
​      <target type='sclp' port='0'/>
​    </console>
​  </devices>
​  ...

Figure 26.74. Guest virtual machine interface - virtio console device

If the console is presented as a serial port, the <target> element has the same attributes as for a serial port. There is usually only one console.

⁠26.18.16. Channel

This represents a private communication channel between the host physical machine and the guest virtual machine. It is manipulated by making changes to a guest virtual machine using a management tool to edit following section of the domain XML:

⁠

​
​
​  ...
​  <devices>
​    <channel type='unix'>
​      <source mode='bind' path='/tmp/guestfwd'/>
​      <target type='guestfwd' address='10.0.2.1' port='4600'/>
​    </channel>
​
​    <!-- KVM virtio channel -->
​    <channel type='pty'>
​      <target type='virtio' name='arbitrary.virtio.serial.port.name'/>
​    </channel>
​    <channel type='unix'>
​      <source mode='bind' path='/var/lib/libvirt/kvm/f16x86_64.agent'/>
​      <target type='virtio' name='org.kvm.guest_agent.0'/>
​    </channel>
​    <channel type='spicevmc'>
​      <target type='virtio' name='com.redhat.spice.0'/>
​    </channel>
​  </devices>
​  ...

Figure 26.75. Channel

This can be implemented in a variety of ways. The specific type of <channel> is given in the type attribute of the <target> element. Different channel types have different target attributes as follows:

guestfwd - Dictates that TCP traffic sent by the guest virtual machine to a given IP address and port is forwarded to the channel device on the host physical machine. The target element must have address and port attributes.

virtio - para-virtualized virtio channel. <channel> is exposed in the guest virtual machine under /dev/vport*, and if the optional element name is specified, /dev/virtio-ports/$name (for more information, see the Fedora project's virtio-serial page). The optional element address can tie the channel to a particular type='virtio-serial' controller, documented above. With KVM, if name is "org.kvm.guest_agent.0", then libvirt can interact with a guest agent installed in the guest virtual machine, for actions such as guest virtual machine shutdown or file system quiescing.

spicevmc - Para-virtualized SPICE channel. The domain must also have a SPICE server as a graphics device, at which point the host physical machine piggy-backs messages across the main channel. The target element must be present, with attribute type='virtio'; an optional attribute name controls how the guest virtual machine will have access to the channel, and defaults to name='com.redhat.spice.0'. The optional <address> element can tie the channel to a particular type='virtio-serial' controller.

⁠26.18.17. Host physical machine interface

A character device presents itself to the host physical machine as one of the following types:

⁠

Table 26.27. Character device elements

Parameter
Description
XML snippet

Domain logfile
Disables all input on the character device, and sends output into the virtual machine's logfile.

<devices>
   <console type='stdio'>
     <target port='1'/>
   </console>
</devices>

Device logfile
A file is opened and all data sent to the character device is written to the file.

<devices>
   <serial type="file">
      <source path="/var/log/vm/vm-serial.log"/>
      <target port="1"/>
   </serial>
</devices>

Virtual console
Connects the character device to the graphical framebuffer in a virtual console. This is typically accessed via a special hotkey sequence such as "ctrl+alt+3".

<devices>
   <serial type='vc'>
      <target port="1"/>
   </serial>
</devices>

Null device
Connects the character device to the void. No data is ever provided to the input. All data written is discarded.

<devices>
   <serial type='null'>
      <target port="1"/>
   </serial>
</devices>

Pseudo TTY
A Pseudo TTY is allocated using /dev/ptmx. A suitable client such as virsh console can connect to interact with the serial port locally.

<devices>
   <serial type="pty">
      <source path="/dev/pts/3"/>
      <target port="1"/>
   </serial>
</devices>

NB Special case
NB special case if <console type='pty'>, then the TTY path is also duplicated as an attribute tty='/dev/pts/3' on the top level <console> tag. This provides compat with existing syntax for <console> tags.

Host physical machine device proxy
The character device is passed through to the underlying physical character device. The device types must match, eg the emulated serial port should only be connected to a host physical machine serial port - don't connect a serial port to a parallel port.

<devices>
   <serial type="dev">
      <source path="/dev/ttyS0"/>
      <target port="1"/>
   </serial>
</devices>

Named pipe
The character device writes output to a named pipe. See the pipe(7) man page for more info.

<devices>
   <serial type="pipe">
      <source path="/tmp/mypipe"/>
      <target port="1"/>
   </serial>
</devices>

TCP client/server
The character device acts as a TCP client connecting to a remote server.

<devices>
   <serial type="tcp">
      <source mode="connect" host="0.0.0.0" service="2445"/>
      <protocol type="raw"/>
      <target port="1"/>
   </serial>
</devices>

Or as a TCP server waiting for a client connection.

<devices>
   <serial type="tcp">
      <source mode="bind" host="127.0.0.1" service="2445"/>
      <protocol type="raw"/>
      <target port="1"/>
   </serial>
</devices>

Alternatively you can use telnet instead of raw TCP. In addition, you can also use telnets (secure telnet) and tls.

<devices>
   <serial type="tcp">
      <source mode="connect" host="0.0.0.0" service="2445"/>
      <protocol type="telnet"/>
      <target port="1"/>
   </serial>
      <serial type="tcp">
      <source mode="bind" host="127.0.0.1" service="2445"/>
      <protocol type="telnet"/>
      <target port="1"/>
   </serial>
</devices>

UDP network console
The character device acts as a UDP netconsole service, sending and receiving packets. This is a lossy service.

<devices>
   <serial type="udp">
      <source mode="bind" host="0.0.0.0" service="2445"/>
      <source mode="connect" host="0.0.0.0" service="2445"/>
      <target port="1"/>
   </serial>
</devices>

UNIX domain socket client/server
The character device acts as a UNIX domain socket server, accepting connections from local clients.

<devices>
   <serial type="unix">
      <source mode="bind" path="/tmp/foo"/>
      <target port="1"/>
   </serial>
</devices>

⁠26.18.18. Sound devices

A virtual sound card can be attached to the host physical machine via the sound element.

⁠

​
​
​  ...
​  <devices>
​    <sound model='sb16'/>
​  </devices>
​  ...

Figure 26.76. Virtual sound card

The sound element has one mandatory attribute, model, which specifies what real sound device is emulated. Valid values are specific to the underlying hypervisor, though typical choices are 'sb16', 'ac97', and 'ich6'. In addition, a sound element with 'ich6' model set can have optional codec sub-elements to attach various audio codecs to the audio device. If not specified, a default codec will be attached to allow playback and recording. Valid values are 'duplex' (advertises a line-in and a line-out) and 'micro' (advertises a speaker and a microphone).

⁠

​
​
​  ...
​  <devices>
​    <sound model='ich6'>
​      <codec type='micro'/>
​    <sound/>
​  </devices>
​  ...

Figure 26.77. Sound devices

Each sound element has an optional sub-element <address> which can tie the device to a particular PCI slot, documented above.

⁠26.18.19. Watchdog device

A virtual hardware watchdog device can be added to the guest virtual machine using the <watchdog> element. The watchdog device requires an additional driver and management daemon in the guest virtual machine. Currently there is no support notification when the watchdog fires.

⁠

​
​
​  ...
​  <devices>
​    <watchdog model='i6300esb'/>
​  </devices>
​  ...
​
​  ...
​  <devices>
​    <watchdog model='i6300esb' action='poweroff'/>
​  </devices>
​...

Figure 26.78. Watchdog device

The following attributes are declared in this XML:

model - The required model attribute specifies what real watchdog device is emulated. Valid values are specific to the underlying hypervisor.

The model attribute may take the following values:

i6300esb — the recommended device, emulating a PCI Intel 6300ESB

ib700 — emulates an ISA iBase IB700

action - The optional action attribute describes what action to take when the watchdog expires. Valid values are specific to the underlying hypervisor. The action attribute can have the following values:

reset — default setting, forcefully resets the guest virtual machine

shutdown — gracefully shuts down the guest virtual machine (not recommended)

poweroff — forcefully powers off the guest virtual machine

pause — pauses the guest virtual machine

none — does nothing

dump — automatically dumps the guest virtual machine.

Note that the 'shutdown' action requires that the guest virtual machine is responsive to ACPI signals. In the sort of situations where the watchdog has expired, guest virtual machines are usually unable to respond to ACPI signals. Therefore using 'shutdown' is not recommended. In addition, the directory to save dump files can be configured by auto_dump_path in file /etc/libvirt/kvm.conf.

⁠26.18.20. Setting a panic device

Red Hat Enterpise Linux now has a way to detect Linux guest virtual machine kernel panics . To do so, you need to have a pvpanic device enabled, and then to add a <panic> element as a child of the <device> parent element. libvirt exposes the device to the guest virtual machine, and if it is running a new enough Linux kernel that knows how to drive the device, will use the device to inform libvirt any time the guest kernel panics. This panic is fed to libvirt as an event, and the <on_crash> element of the domain XML determines what libvirt will do as a result of the crash, which includes the possibility of capturing a guest virtual machine core dump, rebooting the guest virtual machine, or merely halting the guest virtual machine to await further action.

To configure the panic mechanism, place the following snippet into the domain XML devices element, by running virsh edit to open and edit the XML:

⁠

​
​
​           <devices>
​              <panic>
​                 <address type='isa' iobase='0x505'/>
​              </panic>
​           </devices>
​
​

Figure 26.79. Panic element contents

The element <address> specifies the address of panic. The default ioport is 0x505. In most cases specifying an address is not needed.

⁠26.18.21. Memory balloon device

A virtual memory balloon device is added to all KVM guest virtual machines. It will be seen as <memballoon> element. It will be automatically added when appropriate, so there is no need to explicitly add this element in the guest virtual machine XML unless a specific PCI slot needs to be assigned. Note that if the <memballoon> device needs to be explicitly disabled, model='none' may be used.

In the following example, KVM has automatically added the memballoon device:

⁠

​
​
​  ...
​  <devices>
​    <memballoon model='virtio'/>
​  </devices>
​  ...

Figure 26.80. Memory balloon device

The following example shows the device has been added manually with static PCI slot 2 requested:

⁠

​
​
​  ...
​  <devices>
​    <memballoon model='virtio'>
​      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
​    </memballoon>
​  </devices>
​...

Figure 26.81. Memory balloon device added manually

The required model attribute specifies what type of balloon device is provided. Valid values are specific to the virtualization platform; in the KVM hypervisor, 'virtio' is the default setting.

⁠26.18.22. TPM devices

The TPM device enables the guest virtual machine to have access to TPM functionality. The TPM passthrough device type provides access to the host physical machine's TPM for one guest virtual machine. No other software may use the TPM device (typically /dev/tpm0) at the time the guest virtual machine is started. The following domain XML example shows the usage of the TPM passthrough device:

⁠

​
​
​  ...
​  <devices>
​    <tpm model='tpm-tis'>
​      <backend type='passthrough'>
​        <backend path='/dev/tpm0'/>
​      </backend>
​    </tpm>
​  </devices>
​  ...

Figure 26.82. TPM devices

The model attribute specifies what device model KVM provides to the guest virtual machine. If no model name is provided, tpm-tis will automatically be chosen. The <backend> element specifies the type of TPM device. The following types are supported: 'passthrough' — uses the host physical machine's TPM device and 'passthrough'. This backend type requires exclusive access to a TPM device on the host physical machine. An example for such a device is /dev/tpm0. The filename is specified as path attribute of the source element. If no file name is specified, /dev/tpm0 is automatically used.

⁠26.19. Storage pools

Although all storage pool back-ends share the same public APIs and XML format, they have varying levels of capabilities. Some may allow creation of volumes, others may only allow use of pre-existing volumes. Some may have constraints on volume size, or placement.

The top level element for a storage pool document is <pool>. It has a single attribute type, which can take the following values: dir, fs, netfs, disk, iscsi, logical, scsi, mpath, rbd, sheepdog, or gluster.

⁠26.19.1. Providing metadata for the storage pool

The following XML example, shows the metadata tags that can be added to a storage pool. In this example, the pool is an iSCSI storage pool.

⁠

​
​
​     <pool type="iscsi">
​        <name>virtimages</name>
​        <uuid>3e3fce45-4f53-4fa7-bb32-11f34168b82b</uuid>
​        <allocation>10000000</allocation>
​        <capacity>50000000</capacity>
​        <available>40000000</available>
​        ...
​     </pool>
​
​

Figure 26.83. General metadata tags

The elements that are used in this example are explained in the Table 26.28, “virt-sysprep commands”.

⁠

Table 26.28. virt-sysprep commands

Element | Description
<name> | Provides a name for the storage pool which must be unique to the host physical machine. This is mandatory when defining a storage pool.
<uuid> | Provides an identifier for the storage pool which must be globally unique. Although supplying the UUID is optional, if the UUID is not provided at the time the storage pool is created, a UUID will be automatically generated.
<allocation> | Provides the total storage allocation for the storage pool. This may be larger than the sum of the total allocation across all storage volumes due to the metadata overhead. This value is expressed in bytes. This element is read-only and the value should not be changed.
<capacity> | Provides the total storage capacity for the pool. Due to underlying device constraints it may not be possible to use the full capacity for storage volumes. This value is in bytes. This element is read-only and the value should not be changed.
<available> | Provides the free space available for allocating new storage volumes in the storage pool. Due to underlying device constraints it may not be possible to allocate the entire free space to a single storage volume. This value is in bytes. This element is read-only and the value should not be changed.

⁠26.19.2. Source elements

Within the <pool> element there can be a single <source> element defined (only one). The child elements of <source> depend on the storage pool type. Some examples of the XML that can be used are as follows:

⁠

​
​
​           ...
​        <source>
​          <host name="iscsi.example.com"/>
​          <device path="demo-target"/>
​          <auth type='chap' username='myname'>
​            <secret type='iscsi' usage='mycluster_myname'/>
​          </auth>
​          <vendor name="Acme"/>
​          <product name="model"/>
​        </source>
​        ...
​
​

Figure 26.84. Source element option 1

⁠

​
​
​         ...
​        <source>
​          <adapter type='fc_host' parent='scsi_host5' wwnn='20000000c9831b4b' wwpn='10000000c9831b4b'/>
​        </source>
​        ...
​
​

Figure 26.85. Source element option 2

The child elements that are accepted by <source> are explained in Table 26.29, “Source child elements commands”.

⁠

Table 26.29. Source child elements commands

Element
Description

<device>
Provides the source for storage pools backed by host physical machine devices (based on <pool type=> (as shown in Section 26.19, “Storage pools” )). May be repeated multiple times depending on backend driver. Contains a single attribute path which is the fully qualified path to the block device node.

<dir>
Provides the source for storage pools backed by directories (<pool type='dir'>), or optionally to select a subdirectory within a storage pool that is based on a filesystem (<pool type='gluster'>). This element may only occur once per (<pool>). This element accepts a single attribute (<path>) which is the full path to the backing directory.

<adapter>
Provides the source for storage pools backed by SCSI adapters (<pool type='scsi'>). This element may only occur once per (<pool>). Attribute name is the SCSI adapter name (ex. "scsi_host1". Although "host1" is still supported for backwards compatibility, it is not recommended. Attribute type specifies the adapter type. Valid values are 'fc_host'| 'scsi_host'. If omitted and the name attribute is specified, then it defaults to type='scsi_host'. To keep backwards compatibility, the attribute type is optional for the type='scsi_host' adapter, but mandatory for the type='fc_host' adapter. Attributes wwnn (Word Wide Node Name) and wwpn (Word Wide Port Name) are used by the type='fc_host' adapter to uniquely identify the device in the Fibre Channel storage fabric (the device can be either a HBA or vHBA). Both wwnn and wwpn should be specified (Refer to Section 23.12.11, “Dump a Device” for instructions on how to get wwnn/wwpn of a (v)HBA). The optional attribute parent specifies the parent device for the type='fc_host' adapter.

<host>
Provides the source for storage pools backed by storage from a remote server (type='netfs'|'iscsi'|'rbd'|'sheepdog'|'gluster'). This element should be used in combination with a <directory> or <device> element. Contains an attribute name which is the hostname or IP address of the server. May optionally contain a port attribute for the protocol specific port number.

<auth>
If present, the <auth> element provides the authentication credentials needed to access the source by the setting of the type attribute (pool type='iscsi'|'rbd'). The type must be either type='chap' or type='ceph'. Use "ceph" for Ceph RBD (Rados Block Device) network sources and use "iscsi" for CHAP (Challenge-Handshake Authentication Protocol) iSCSI targets. Additionally a mandatory attribute username identifies the username to use during authentication as well as a sub-element secret with a mandatory attribute type, to tie back to a libvirt secret object that holds the actual password or other credentials. The domain XML intentionally does not expose the password, only the reference to the object that manages the password. The secret element requires either a uuid attribute with the UUID of the secret object or a usage attribute matching the key that was specified in the secret object. Refer to ceph for more information.

<name>
Provides the source for storage pools backed by a storage device from a named element <type> which can take the values: (type='logical'|'rbd'|'sheepdog','gluster').

<format>
Provides information about the format of the storage pool <type> which can take the following values: type='logical'|'disk'|'fs'|'netfs'). It should be noted that this value is backend specific. This is typically used to indicate a filesystem type, or a network filesystem type, or a partition table type, or an LVM metadata type. As all drivers are required to have a default value for this, the element is optional.

<vendor>
Provides optional information about the vendor of the storage device. This contains a single attribute <name> whose value is backend specific.

<product>
Provides optional information about the product name of the storage device. This contains a single attribute <name> whose value is back-end specific.

ceph

In all cases where type='ceph' is listed take note that ceph is not supported with Red Hat Enterprise Linux 7 but you can create storage pools with type='chap'. type='ceph' is available on Red Hat Enterprise Virtualization . Call your service representative for details.

⁠26.19.3. Creating target elements

A single <target> element is contained within the top level <pool> element for the following types: (type='dir'|'fs'|'netfs'|'logical'|'disk'|'iscsi'|'scsi'|'mpath'). This tag is used to describe the mapping of the storage pool into the host filesystem. It can contain the following child elements:

⁠

​
​
​     <pool>
​        ...
​        <target>
​          <path>/dev/disk/by-path</path>
​          <permissions>
​            <owner>107</owner>
​            <group>107</group>
​            <mode>0744</mode>
​            <label>virt_image_t</label>
​          </permissions>
​          <timestamps>
​            <atime>1341933637.273190990</atime>
​            <mtime>1341930622.047245868</mtime>
​            <ctime>1341930622.047245868</ctime>
​          </timestamps>
​          <encryption type='...'>
​            ...
​          </encryption>
​        </target>
​      </pool>
​
​

Figure 26.86. Target elements XML example

The table (Table 26.30, “Target child elements”) explains the child elements that are valid for the parent <target> element:

⁠

Table 26.30. Target child elements

Element
Description

<path>
Provides the location at which the storage pool will be mapped into the local filesystem namespace. For a filesystem/directory based storage pool it will be the name of the directory in which storage volumes will be created. For device based storage pools it will be the name of the directory in which the device's nodes exist. For the latter, /dev/may seem like the logical choice, however, the device's nodes there are not guaranteed to be stable across reboots, since they are allocated on demand. It is preferable to use a stable location such as one of the /dev/disk/by-{path,id,uuid,label} locations.

<permissions>
This is currently only useful for directory or filesystem based storage pools, which are mapped as a directory into the local filesystem namespace. It provides information about the permissions to use for the final directory when the storage pool is built. The <mode> element contains the octal permission set. The <owner> element contains the numeric user ID. The <group> element contains the numeric group ID. The <label> element contains the MAC (for example, SELinux) label string.

<timestamps>
Provides timing information about the storage volume. Up to four sub-elements are present, where timestamps='atime'|'btime|'ctime'|'mtime' holds the access, birth, change, and modification time of the storage volume, where known. The used time format is <seconds>.<nanoseconds> since the beginning of the epoch (1 Jan 1970). If nanosecond resolution is 0 or otherwise unsupported by the host operating system or filesystem, then the nanoseconds part is omitted. This is a read-only attribute and is ignored when creating a storage volume.

<encryption>
If present, specifies how the storage volume is encrypted. Refer to Storage Encryption page for more information.

⁠26.19.4. Setting device extents

If a storage pool exposes information about its underlying placement or allocation scheme, the <device> element within the <source> element may contain information about its available extents. Some storage pools have a constraint that a storage volume must be allocated entirely within a single constraint (such as disk partition pools). Thus, the extent information allows an application to determine the maximum possible size for a new storage volume.

For storage pools supporting extent information, within each <device> element there will be zero or more <freeExtent> elements. Each of these elements contains two attributes, <start> and <end> which provide the boundaries of the extent on the device, measured in bytes.

⁠26.20. Storage Volumes

A storage volume will generally be either a file or a device node; since 1.2.0, an optional output-only attribute type lists the actual type (file, block, dir, network, or netdir),

⁠26.20.1. General metadata

The top section of the <volume> element contains information known as metadata as shown in this XML example:

⁠

​
​
​      ...
​      <volume type='file'>
​        <name>sparse.img</name>
​        <key>/var/lib/xen/images/sparse.img</key>
​        <allocation>0</allocation>
​        <capacity unit="T">1</capacity>
​        ...
​      </volume>
​
​

Figure 26.87. General metadata for storage volumes

The table (Table 26.31, “Volume child elements”) explains the child elements that are valid for the parent <volume> element:

⁠

Table 26.31. Volume child elements

Element
Description

<name>
Provides a name for the storage volume which is unique to the storage pool. This is mandatory when defining a storage volume.

<key>
Provides an identifier for the storage volume which identifies a single storage volume. In some cases it's possible to have two distinct keys identifying a single storage volume. This field cannot be set when creating a storage volume as it is always generated.

<allocation>
Provides the total storage allocation for the storage volume. This may be smaller than the logical capacity if the storage volume is sparsely allocated. It may also be larger than the logical capacity if the storage volume has substantial metadata overhead. This value is in bytes. If omitted when creating a storage volume, the storage volume will be fully allocated at time of creation. If set to a value smaller than the capacity, the storage pool has the option of deciding to sparsely allocate a storage volume or not. Different types of storage pools may treat sparse storage volumes differently. For example, a logical pool will not automatically expand a storage volume's allocation when it gets full; the user is responsible for configuring it or configuring dmeventd to do so automatically. By default this is specified in bytes. Refer to A note about unit

<capacity>
Provides the logical capacity for the storage volume. This value is in bytes by default, but a <unit> attribute can be specified with the same semantics as for <allocation> described in A note about unit. This is compulsory when creating a storage volume.

<source>
Provides information about the underlying storage allocation of the storage volume. This may not be available for some storage pool types.

<target>
Provides information about the representation of the storage volume on the local host physical machine.

A note about unit

When necessary, an optional attribute unit can be specified to adjust the passed value. This attribute can be used with the elements <allocation> and <capacity>. Accepted values for the attribute unit include:

B or bytes for bytes

KB for kilobytes

K or KiB for kibibytes

MB for megabytes

M or MiB for mebibytes

GB for gigabytes

G or GiB for gibibytes

TB for terabytes

T or TiB for tebibytes

PB for petabytes

P or PiB for pebibytes

EB for exabytes

E or EiB for exbibytes

⁠26.20.2. Setting target elements

The <target> element can be placed in the <volume> top level element. It is used to describe the mapping that is done on the storage volume into the host physical machine filesystem. This element can take the following child elements:

⁠

​
​
​       <target>
​          <path>/var/lib/virt/images/sparse.img</path>
​          <format type='qcow2'/>
​          <permissions>
​            <owner>107</owner>
​            <group>107</group>
​            <mode>0744</mode>
​            <label>virt_image_t</label>
​          </permissions>
​          <compat>1.1</compat>
​          <features>
​            <lazy_refcounts/>
​          </features>
​        </target>
​
​

Figure 26.88. Target child elements

The specific child elements for <target> are explained in Table 26.32, “Target child elements”:

⁠

Table 26.32. Target child elements

Element
Description

<path>
Provides the location at which the storage volume can be accessed on the local filesystem, as an absolute path. This is a read-only attribute, and should not be specified when creating a volume.

<format>
Provides information about the pool specific volume format. For disk based storage pools, it will provide the partition type. For filesystem or directory based storage pools, it will provide the file format type, (such as cow, qcow, vmdk, raw). If omitted when creating a storage volume, the storage pool's default format will be used. The actual format is specified via the type attribute. Refer to the sections on the specific storage pools in Chapter 14, Storage pools for the list of valid values.

<permissions>
Provides information about the default permissions to use when creating storage volumes. This is currently only useful for directory or filesystem based storage pools, where the storage volumes allocated are simple files. For storage pools where the storage volumes are device nodes, the hotplug scripts determine permissions. It contains four child elements. The <mode> element contains the octal permission set. The <owner> element contains the numeric user ID. The <group> element contains the numeric group ID. The <label> element contains the MAC (for example, SELinux) label string.

<compat>
Specify compatibility level. So far, this is only used for <type='qcow2'> volumes. Valid values are <compat>0.10</compat> for qcow2 (version 2) and <compat>1.1</compat> for qcow2 (version 3) so far for specifying the QEMU version the images should be compatible with. If the <feature> element is present, <compat>1.1</compat> is used. If omitted, qemu-img default is used.

<features>
Format-specific features. Presently is only used with <format type='qcow2'/> (version 3). Valid sub-elements include <lazy_refcounts/>. This reduces the amount of metadata writes and flushes, and therefore improves initial write performance. This improvement is seen especially for writethrough cache modes, at the cost of having to repair the image after a crash, and allows delayed reference counter updates. It is recommended to use this feature with qcow2 (version 3), as it is faster when this is implemented.

⁠26.20.3. Setting backing store elements

A single <backingStore> element is contained within the top level <volume> element. This tag is used to describe the optional copy-on-write backing store for the storage volume. It can contain the following child elements:

⁠

​
​
​        <backingStore>
​          <path>/var/lib/virt/images/master.img</path>
​          <format type='raw'/>
​          <permissions>
​            <owner>107</owner>
​            <group>107</group>
​            <mode>0744</mode>
​            <label>virt_image_t</label>
​          </permissions>
​        </backingStore>
​
​

Figure 26.89. Backing store child elements

⁠

Table 26.33. Backing store child elements

Element
Description

<path>
Provides the location at which the backing store can be accessed on the local filesystem, as an absolute path. If omitted, there is no backing store for this storage volume.

<format>
Provides information about the pool specific backing store format. For disk based storage pools it will provide the partition type. For filesystem or directory based storage pools it will provide the file format type (such as cow, qcow, vmdk, raw). The actual format is specified via the <type> attribute. Consult the pool-specific docs for the list of valid values. Most file formats require a backing store of the same format, however, the qcow2 format allows a different backing store format.

<permissions>
Provides information about the permissions of the backing file. It contains four child elements. The <owner> element contains the numeric user ID. The <group> element contains the numeric group ID. The <label> element contains the MAC (for example, SELinux) label string.

⁠26.21. Security label

The <seclabel> element allows control over the operation of the security drivers. There are three basic modes of operation, 'dynamic' where libvirt automatically generates a unique security label, 'static' where the application/administrator chooses the labels, or 'none' where confinement is disabled. With dynamic label generation, libvirt will always automatically relabel any resources associated with the virtual machine. With static label assignment, by default, the administrator or application must ensure labels are set correctly on any resources, however, automatic relabeling can be enabled if desired.

If more than one security driver is used by libvirt, multiple seclabel tags can be used, one for each driver and the security driver referenced by each tag can be defined using the attribute model. Valid input XML configurations for the top-level security label are:

⁠

​
​
​  <seclabel type='dynamic' model='selinux'/>
​
​  <seclabel type='dynamic' model='selinux'>
​    <baselabel>system_u:system_r:my_svirt_t:s0</baselabel>
​  </seclabel>
​
​  <seclabel type='static' model='selinux' relabel='no'>
​    <label>system_u:system_r:svirt_t:s0:c392,c662</label>
​  </seclabel>
​
​  <seclabel type='static' model='selinux' relabel='yes'>
​    <label>system_u:system_r:svirt_t:s0:c392,c662</label>
​  </seclabel>
​
​  <seclabel type='none'/>

Figure 26.90. Security label

If no 'type' attribute is provided in the input XML, then the security driver default setting will be used, which may be either 'none' or 'dynamic'. If a <baselabel> is set but no 'type' is set, then the type is presumed to be 'dynamic'. When viewing the XML for a running guest virtual machine with automatic resource relabeling active, an additional XML element, imagelabel, will be included. This is an output-only element, so will be ignored in user supplied XML documents.

The following elements can be manipulated with the following values:

type - Either static, dynamic or none to determine whether libvirt automatically generates a unique security label or not.

model - A valid security model name, matching the currently activated security model.

relabel - Either yes or no. This must always be yes if dynamic label assignment is used. With static label assignment it will default to no.

<label> - If static labelling is used, this must specify the full security label to assign to the virtual domain. The format of the content depends on the security driver in use:

SELinux: a SELinux context.

AppArmor: an AppArmor profile.

DAC: owner and group separated by colon. They can be defined both as user/group names or UID/GID. The driver will first try to parse these values as names, but a leading plus sign can used to force the driver to parse them as UID or GID.

<baselabel> - If dynamic labelling is used, this can optionally be used to specify the base security label. The format of the content depends on the security driver in use.

<imagelabel> - This is an output only element, which shows the security label used on resources associated with the virtual domain. The format of the content depends on the security driver in use. When relabeling is in effect, it is also possible to fine-tune the labeling done for specific source file names, by either disabling the labeling (useful if the file exists on NFS or other file system that lacks security labeling) or requesting an alternate label (useful when a management application creates a special label to allow sharing of some, but not all, resources between domains). When a seclabel element is attached to a specific path rather than the top-level domain assignment, only the attribute relabel or the sub-element label are supported.

⁠26.22. A Sample configuration file

KVM hardware accelerated guest virtual machine on i686:

⁠

​
​
​<domain type='kvm'>
​  <name>demo2</name>
​  <uuid>4dea24b3-1d52-d8f3-2516-782e98a23fa0</uuid>
​  <memory>131072</memory>
​  <vcpu>1</vcpu>
​  <os>
​    <type arch="i686">hvm</type>
​  </os>
​  <clock sync="localtime"/>
​  <devices>
​    <emulator>/usr/bin/kvm-kvm</emulator>
​    <disk type='file' device='disk'>
​      <source file='/var/lib/libvirt/images/demo2.img'/>
​      <target dev='hda'/>
​    </disk>
​    <interface type='network'>
​      <source network='default'/>
​      <mac address='24:42:53:21:52:45'/>
​    </interface>
​    <graphics type='vnc' port='-1' keymap='de'/>
​  </devices>
​</domain>

Figure 26.91. Example domain XML configuration

⁠Part III. Appendices

⁠Appendix A. Troubleshooting

This chapter covers common problems and solutions for Red Hat Enterprise Linux 7 virtualization issues.

Read this chapter to develop an understanding of some of the common problems associated with virtualization technologies. It is recommended that you experiment and test virtualization on Red Hat Enterprise Linux 7 to develop your troubleshooting skills.

If you cannot find the answer in this document, there may be an answer online from the virtualization community. Refer to Section C.1, “Online resources” for a list of Linux virtualization websites.

⁠A.1. Debugging and troubleshooting tools

This section summarizes the system administrator applications, the networking utilities, and debugging tools. You can use these standard system administration tools and logs to assist with troubleshooting:

kvm_stat - refer to Section A.5, “kvm_stat”

trace-cmd

ftrace - Refer to the Red Hat Enterprise Linux Developer Guide.

vmstat

iostat

lsof

systemtap

crash

sysrq

sysrq t

sysrq w

These networking tools can assist with troubleshooting virtualization networking problems:

ifconfig

tcpdump

The tcpdump command 'sniffs' network packets. tcpdump is useful for finding network abnormalities and problems with network authentication. There is a graphical version of tcpdump named wireshark.

brctl

brctl is a networking tool that inspects and configures the Ethernet bridge configuration in the Linux kernel. You must have root access before performing these example commands:

# brctl show
bridge-name    bridge-id          STP  enabled  interfaces
-----------------------------------------------------------------------------
virtbr0             8000.feffffff       yes        eth0

# brctl showmacs virtbr0
port-no           mac-addr                  local?       aging timer
1                 fe:ff:ff:ff:ff:           yes            0.00
2                 fe:ff:ff:fe:ff:           yes            0.00
# brctl showstp virtbr0
virtbr0
bridge-id              8000.fefffffffff
designated-root        8000.fefffffffff
root-port              0                   path-cost             0
max-age                20.00               bridge-max-age        20.00
hello-time             2.00                bridge-hello-time     2.00
forward-delay          0.00                bridge-forward-delay  0.00
aging-time            300.01
hello-timer            1.43                tcn-timer             0.00
topology-change-timer  0.00                gc-timer              0.02

Listed below are some other useful commands for troubleshooting virtualization:

strace is a command which traces system calls and events received and used by another process.

vncviewer connects to a VNC server running on your server or a virtual machine. Install vncviewer using the yum install vnc command.

vncserver starts a remote desktop on your server. Gives you the ability to run graphical user interfaces such as virt-manager via a remote session. Install vncserver using the yum install vnc-server command.

In addition to all the commands listed above, examining the following log files can be quite helpful:

/var/log/messages (From Red Hat Enterprise Linux 7, the libvirtd log is recorded here by default.)

/var/log/audit/audit.log

⁠A.2. Preparing for disaster recovery

If at all possible, it is best to prepare for situations where your equipment will be compromised due to weather, or other reasons. It is highly recommended that you perform a backup of the following files and directories on your host physical machine:

From the /etc/libvirt directory, all files.

From the /var/lib/libvirt directory, back up the following items:

The current dnsmasq DHCP leases found in /var/lib/libvirt/dnsmasq

The running virtual network configuration files found in /var/lib/libvirt/network

The guest virtual machine core dump files found in /var/lib/libvirt/qemu/dump

The saved guest virtual machine files (using the virsh save command save or similar), found in /var/lib/libvirt/qemu/save

The guest virtual machine snapshot files which are found in */var/lib/libvirt/qemu/snapsho*

The guest virtual machine disk images can be found in the /var/lib/libvirt/images directory. For instructions on how to back up the guest image files, use the steps described in Procedure A.1, “Creating a backup of the guest virtual machine's disk image for disaster recovery purposes”.

If you are using bridges, you will also need to back up the files located in /etc/sysconfig/network-scripts/ifcfg-<bridge_name>

⁠

Procedure A.1. Creating a backup of the guest virtual machine's disk image for disaster recovery purposes

This procedure will cover how to back up several different disk image types.

To back up only the guest virtual machine disk image, back up the files located in /var/lib/libvirt/images. To back up guest virtual machine disk images with LVM logical volumes, run the following command:

# lvcreate --snapshot --name snap --size 8G /dev/vg0/data

This command creates a snapshot volume named snap with a size of 8G as part of a 64G volume.

Create a file for the snapshots using a command similar to this one:

# mkdir /mnt/virt.snapshot

Mount the directory you created and the snapshot volume using the following command:

# mount /dev/vg0/snap /mnt/virt.snapshot

Use one of the following commands to back up the volume:

# tar -pzc -f /mnt/backup/virt-snapshot-MM-DD-YYYY.tgz /mnt/virt.snapshot++++++++++++

# rsync -a /mnt/virt.snapshot/ /mnt/backup/virt-snapshot.MM-DD-YYYY/

⁠A.3. Creating virsh dump files

Executing a virsh dump command sends a request to dump the core of a guest virtual machine to a file so errors in the virtual machine can be diagnosed. Running this command may require you to manually ensure proper permissions on file and path specified by the argument corefilepath. The virsh dump command is similar to a coredump (or the crash utility). To create the virsh dump file, run:

#virsh dump <domain> <corefilepath> [--bypass-cache] { [--live] | [--crash] | [--reset] } [--verbose] [--memory-only]

While the domain (guest virtual machine domain name) and corefilepath (location of the newly created core dump file) are mandatory, the following arguments are optional:

--live creates a dump file on a running machine and doesn't pause it.

--crash stops the guest virtual machine and generates the dump file. The guest virtual machine will be listed as Shut off, with the reason as Crashed. Note that in virt-manager the status will be listed as Shut off also.

--reset will reset the guest virtual machine following a successful dump. Note, these three switches are mutually exclusive.

--bypass-cache uses O_DIRECT to bypass the file system cache.

--memory-only the dump file will be saved as an elf file, and will only include domain’s memory and cpu common register value. This option is very useful if the domain uses host devices directly.

--verbose displays the progress of the dump

The entire dump process may be monitored using virsh domjobinfo command and can be canceled by running virsh domjobabort.

⁠A.4. Capturing trace data on a constant basis using the Systemtap flight recorder

You can capture QEMU trace data all the time using a systemtap initscript provided in the qemu-kvm package. This package uses SystemTap's flight recorder mode to trace all running guest virtual machines and to save the results to a fixed-size buffer on the host. Old trace entries are overwritten by new entries when the buffer is filled.

⁠

Procedure A.2. Configuring and running systemtap

Install the package

Install the systemtap-initscript package by running the following command:

# yum install systemtap-initscript

Copy the configuration file

Copy the systemtap scripts and the configuration files to the systemtap directory by running the following commands:

# cp /usr/share/qemu-kvm/systemtap/script.d/qemu_kvm.stp /etc/systemtap/script.d/
# cp /usr/share/qemu-kvm/systemtap/conf.d/qemu_kvm.conf /etc/systemtap/conf.d/

The set of trace events to enable is given in qemu_kvm.stp. This SystemTap script can be customized to add or remove trace events provided in /usr/share/systemtap/tapset/qemu-kvm-simpletrace.stp.

SystemTap customizations can be made to qemu_kvm.conf to control the flight recorder buffer size and whether to store traces in memory only or in the disk as well.

Start the service

Start the systemtap service by runnuing the following command:

# systemctl start systemtap qemu_kvm

Make systemtap enabled to run at boot time

Enable the systemtap service to run at bootime by running the following command:

# systemctl enable systemtap qemu_kvm

Confirmation the service is running

Confirm that the service is working by running the following command:

# systemctl status systemtap qemu_kvm
qemu_kvm is running...

⁠

Procedure A.3. Inspecting the trace buffer

Create a trace buffer dump file

Create a trace buffer dump file called trace.log and place it in the tmp directory by running the following command:

# staprun -A qemu_kvm >/tmp/trace.log

You can change the file name and location to something else.

Start the service

As the previous step stops the service, start it again by running the following command:

# systemctl start systemtap qemu_kvm

Convert the trace contents into a readable format

To convert the trace file contents into a more readable format, run the following command:

# /usr/share/qemu-kvm/simpletrace.py --no-header /usr/share/qemu-kvm/trace-events /tmp/trace.log

Note

The following notes and limitations should be noted:

The systemtap service is disabled by default.

There is a small performance penalty when this service is enabled, but it depends on which events are enabled in total.

There is a README file located in /usr/share/doc/qemu-kvm-*/README.systemtap.

⁠A.5. kvm_stat

The kvm_stat command is a python script which retrieves runtime statistics from the kvm kernel module. The kvm_stat command can be used to diagnose guest behavior visible to kvm. In particular, performance related issues with guests. Currently, the reported statistics are for the entire system; the behavior of all running guests is reported. To run this script you need to install the qemu-kvm-tools package. Refer to Section 2.2, “Installing virtualization packages on an existing Red Hat Enterprise Linux system”.

The kvm_stat command requires that the kvm kernel module is loaded and debugfs is mounted. If either of these features are not enabled, the command will output the required steps to enable debugfs or the kvm module. For example:

# kvm_stat
Please mount debugfs ('mount -t debugfs debugfs /sys/kernel/debug')
and ensure the kvm modules are loaded

Mount debugfs if required:

# mount -t debugfs debugfs /sys/kernel/debug

⁠

kvm_stat output

The kvm_stat command outputs statistics for all guests and the host. The output is updated until the command is terminated (using Ctrl+c or the q key). Note that the output you see on your screen may differ. For an explanation of the output elements, click on any of the terms to link to the defintion.

#kvm_stat -l
kvm_ack_irq 				0         0
kvm_age_page				0         0
kvm_apic				44        44
kvm_apic_accept_irq			12        12
kvm_apic_ipi				4         4
kvm_async_pf_completed			0         0
kvm_async_pf_doublefault		0         0
kvm_async_pf_not_present		0         0
kvm_async_pf_ready			0         0
kvm_cpuid				0         0
kvm_cr					0         0
kvm_emulate_insn			2         2
kvm_entry				56        56
kvm_eoi					12        12
kvm_exit				56        56
Individual exit reasons follow, refer to kvm_exit (NAME) for more information.
kvm_exit(CLGI)				0         0
kvm_exit(CPUID)				0         0
kvm_exit(CR0_SEL_WRITE)			0         0
kvm_exit(EXCP_BASE)			0         0
kvm_exit(FERR_FREEZE)			0         0
kvm_exit(GDTR_READ)			0         0
kvm_exit(GDTR_WRITE)			0         0
kvm_exit(HLT)				11        11
kvm_exit(ICEBP)				0         0
kvm_exit(IDTR_READ)			0         0
kvm_exit(IDTR_WRITE)			0         0
kvm_exit(INIT)				0         0
kvm_exit(INTR)				0         0
kvm_exit(INVD)				0         0
kvm_exit(INVLPG)			0         0
kvm_exit(INVLPGA)			0         0
kvm_exit(IOIO)				0         0
kvm_exit(IRET)				0         0
kvm_exit(LDTR_READ)			0         0
kvm_exit(LDTR_WRITE)			0         0
kvm_exit(MONITOR)			0         0
kvm_exit(MSR)				40        40
kvm_exit(MWAIT)				0         0
kvm_exit(MWAIT_COND)			0         0
kvm_exit(NMI)				0         0
kvm_exit(NPF)				0         0
kvm_exit(PAUSE)				0         0
kvm_exit(POPF)				0         0
kvm_exit(PUSHF)				0         0
kvm_exit(RDPMC)				0         0
kvm_exit(RDTSC)				0         0
kvm_exit(RDTSCP)			0         0
kvm_exit(READ_CR0)			0         0
kvm_exit(READ_CR3)			0         0
kvm_exit(READ_CR4)			0         0
kvm_exit(READ_CR8)			0         0
kvm_exit(READ_DR0)			0         0
kvm_exit(READ_DR1)			0         0
kvm_exit(READ_DR2)			0         0
kvm_exit(READ_DR3)			0         0
kvm_exit(READ_DR4)			0         0
kvm_exit(READ_DR5)			0         0
kvm_exit(READ_DR6)			0         0
kvm_exit(READ_DR7)			0         0
kvm_exit(RSM)				0         0
kvm_exit(SHUTDOWN)			0         0
kvm_exit(SKINIT)			0         0
kvm_exit(SMI)				0         0
kvm_exit(STGI)				0         0
kvm_exit(SWINT)				0         0
kvm_exit(TASK_SWITCH)			0         0
kvm_exit(TR_READ)			0         0
kvm_exit(TR_WRITE)			0         0
kvm_exit(VINTR)				1         1
kvm_exit(VMLOAD)			0         0
kvm_exit(VMMCALL)			0         0
kvm_exit(VMRUN)				0         0
kvm_exit(VMSAVE)			0         0
kvm_exit(WBINVD)			0         0
kvm_exit(WRITE_CR0)			2         2
kvm_exit(WRITE_CR3)			0         0
kvm_exit(WRITE_CR4)			0         0
kvm_exit(WRITE_CR8)			0         0
kvm_exit(WRITE_DR0)			0         0
kvm_exit(WRITE_DR1)			0         0
kvm_exit(WRITE_DR2)			0         0
kvm_exit(WRITE_DR3)			0         0
kvm_exit(WRITE_DR4)			0         0
kvm_exit(WRITE_DR5)			0         0
kvm_exit(WRITE_DR6)			0         0
kvm_exit(WRITE_DR7)			0         0
kvm_fpu					4         4
kvm_hv_hypercall			0         0
kvm_hypercall				0         0
kvm_inj_exception			0         0
kvm_inj_virq				12        12
kvm_invlpga				0         0
kvm_ioapic_set_irq			0         0
kvm_mmio				0         0
kvm_msi_set_irq				0         0
kvm_msr					40        40
kvm_nested_intercepts			0         0
kvm_nested_intr_vmexit			0         0
kvm_nested_vmexit			0         0
kvm_nested_vmexit_inject      		0         0
kvm_nested_vmrun              		0         0
kvm_page_fault                		0         0
kvm_pic_set_irq                		0         0
kvm_pio                        		0         0
kvm_pv_eoi				12        12
kvm_set_irq                    		0         0
kvm_skinit                     		0         0
kvm_track_tsc                  		0         0
kvm_try_async_get_page         		0         0
kvm_update_master_clock			0         0
kvm_userspace_exit            		0         0
kvm_write_tsc_offset			0         0
vcpu_match_mmio               		0         0

⁠

Explanation of variables:

⁠kvm_ack_irq

Number of interrupt controller (PIC/IOAPIC) interrupt acknowledgements.

⁠kvm_age_page

Number of page age iterations by MMU notifiers.

⁠kvm_apic

Number of APIC register accesses.

⁠kvm_apic_accept_irq

Number of interrupts accepted into local APIC.

⁠kvm_apic_ipi

Number of inter processor interrupts.

⁠kvm_async_pf_completed

Number of completions of asynchronous page faults.

⁠kvm_async_pf_doublefault

Number of asynchronous page fault halts.

⁠kvm_async_pf_not_present

Number of initializations of asynchronous page faults.

⁠kvm_async_pf_ready

Number of completions of asynchronous page faults.

⁠kvm_cpuid

Number of CPUID instructions executed.

⁠kvm_cr

Number of trapped and emulated CR register accesses (CR0, CR3, CR4, CR8).

⁠kvm_emulate_insn

Number of emulated instructions.

⁠kvm_entry

Number of emulated instructions.

⁠kvm_eoi

Number of APIC EOI's (end of interrupt) notifications.

⁠kvm_exit

Number of VM-exits.

⁠kvm_exit (NAME)

Individual exits which are processor specific. Please refer to your processor's documentation for more specific information.

⁠kvm_fpu

Number of KVM FPU reloads.

⁠kvm_hv_hypercall

Number of Hyper-V hypercalls.

⁠kvm_hypercall

Number of non-Hyper-V hypercalls.

⁠kvm_inj_exception

Number of exceptions injected into guest.

⁠kvm_inj_virq

Number of interrupts injected into guest.

⁠kvm_invlpga

Number of INVLPGA instructions intercepted.

⁠kvm_ioapic_set_irq

Number of interrupts level changes to the virtual IOAPIC controller.

⁠kvm_mmio

Number of emulated MMIO operations.

⁠kvm_msi_set_irq

Number of MSI interrupts.

⁠kvm_msr

Number of MSR accesses.

⁠kvm_nested_intercepts

Number of L1 -> L2 nested SVM switches.

⁠kvm_nested_vmrun

Number of L1 -> L2 nested SVM switches.

⁠kvm_nested_intr_vmexit

Number of nested VM-exit injections due to interrupt window.

⁠kvm_nested_vmexit

Exits to hypervisor while executing nested (L2) guest.

⁠kvm_nested_vmexit_inject

Number of L2 -> L1 nested switches.

⁠kvm_page_fault

Number of page faults handled by hypervisor.

⁠kvm_pic_set_irq

Number of interrupts level changes to the virtual PIC controller.

⁠kvm_pio

Number of emulated PIO operations.

⁠kvm_pv_eoi

Number of para-virtual EOI events.

⁠kvm_set_irq

Number of interrupt level changes at the generic IRQ controller level (counts PIC, IOAPIC and MSI).

⁠kvm_skinit

Number of SVM SKINIT exits.

⁠kvm_track_tsc

Number of TSC writes.

⁠kvm_try_async_get_page

Number of asynchronous page fault attempts.

⁠kvm_update_master_clock

Number of pvclock masterclock updates.

⁠kvm_userspace_exit

Number of exits to userspace.

⁠kvm_write_tsc_offset

Number of TSC offset writes.

⁠vcpu_match_mmio

Number of SPTE cached MMIO hits.

The output information from the kvm_stat command is exported by the KVM hypervisor as pseudo files which are located in the /sys/kernel/debug/tracing/events/kvm/ directory.

⁠A.6. Troubleshooting with serial consoles

Linux kernels can output information to serial ports. This is useful for debugging kernel panics and hardware issues with video devices or headless servers. The subsections in this section cover setting up serial console output for host physical machines using the KVM hypervisor.

This section covers how to enable serial console output for fully virtualized guests.

Fully virtualized guest serial console output can be viewed with the virsh console command.

Be aware fully virtualized guest serial consoles have some limitations. Present limitations include:

output data may be dropped or scrambled.

The serial port is called ttyS0 on Linux .

You must configure the virtualized operating system to output information to the virtual serial port.

To output kernel information from a fully virtualized Linux guest into the domain, modify the etc/default/grub file. Append the following to the kernel line: console=tty0 console=ttyS0,115200.

title Red Hat Enterprise Linux Server (2.6.32-36.x86-64)
	root (hd0,0)
	kernel /vmlinuz-2.6.32-36.x86-64 ro root=/dev/volgroup00/logvol00 \
	console=tty0 console=ttyS0,115200
	initrd /initrd-2.6.32-36.x86-64.img

Run the following command in the guest:

# grub2-mkconfig -o /boot/grub2/grub.cfg

Reboot the guest.

On the host, access the serial console with the following command:

# virsh console

You can also use virt-manager to display the virtual text console. In the guest console window, select Serial 1 in Text Consoles from the View menu.

⁠A.7. Virtualization log files

Each fully virtualized guest log is in the /var/log/libvirt/qemu/ directory. Each guest log is named as GuestName.log and will be periodically compressed once a size limit is reached.

If you encounter any errors with the Virtual Machine Manager, you can review the generated data in the virt-manager.log file that resides in the $HOME/.virt-manager directory.

⁠A.8. Loop device errors

If file-based guest images are used you may have to increase the number of configured loop devices. The default configuration allows up to eight active loop devices. If more than eight file-based guests or loop devices are needed the number of loop devices configured can be adjusted in the /etc/modprobe.d/directory. Add the following line:

options loop max_loop=64

This example uses 64 but you can specify another number to set the maximum loop value. You may also have to implement loop device backed guests on your system. To use a loop device backed guests for a full virtualized system, use the phy: device or file: file commands.

⁠A.9. Live Migration Errors

There may be cases where a guest changes memory too fast, and the live migration process has to transfer it over and over again, and fails to finish (converge). This issue is not scheduled to be resolved at the moment for Red Hat Enterprise Linux 6, and is scheduled to be fixed in Red Hat Enterprise Linux 7.1.

The current live-migration implementation has a default migration time configured to 30ms. This value determines the guest pause time at the end of the migration in order to transfer the leftovers. Higher values increase the odds that live migration will converge

⁠A.10. Enabling Intel VT-x and AMD-V virtualization hardware extensions in BIOS

Note

To expand your expertise, you might also be interested in the Red Hat Enterprise Virtualization (RH318) training course.

This section describes how to identify hardware virtualization extensions and enable them in your BIOS if they are disabled.

The Intel VT-x extensions can be disabled in the BIOS. Certain laptop vendors have disabled the Intel VT-x extensions by default in their CPUs.

The virtualization extensions cannot be disabled in the BIOS for AMD-V.

Refer to the following section for instructions on enabling disabled virtualization extensions.

Verify the virtualization extensions are enabled in BIOS. The BIOS settings for Intel VT or AMD-V are usually in the Chipset or Processor menus. The menu names may vary from this guide, the virtualization extension settings may be found in Security Settings or other non standard menu names.

⁠

Procedure A.4. Enabling virtualization extensions in BIOS

Reboot the computer and open the system's BIOS menu. This can usually be done by pressing the delete key, the F1 key or Alt and F4 keys depending on the system.

Enabling the virtualization extensions in BIOS

Note

Many of the steps below may vary depending on your motherboard, processor type, chipset and OEM. Refer to your system's accompanying documentation for the correct information on configuring your system.

Open the Processor submenu The processor settings menu may be hidden in the Chipset, Advanced CPU Configuration or Northbridge.

Enable Intel Virtualization Technology (also known as Intel VT-x). AMD-V extensions cannot be disabled in the BIOS and should already be enabled. The virtualization extensions may be labeled Virtualization Extensions, Vanderpool or various other names depending on the OEM and system BIOS.

Enable Intel VT-d or AMD IOMMU, if the options are available. Intel VT-d and AMD IOMMU are used for PCI device assignment.

Select Save & Exit.

Reboot the machine.

When the machine has booted, run grep -E "vmx|svm" /proc/cpuinfo. Specifying --color is optional, but useful if you want the search term highlighted. If the command outputs, the virtualization extensions are now enabled. If there is no output your system may not have the virtualization extensions or the correct BIOS setting enabled.

⁠A.11. Generating a new unique MAC address

In some cases you will need to generate a new and unique MAC address for a guest virtual machine. There is no command line tool available to generate a new MAC address at the time of writing. The script provided below can generate a new MAC address for your guest virtual machines. Save the script on your guest virtual machine as macgen.py. Now from that directory you can run the script using ./macgen.py and it will generate a new MAC address. A sample output would look like the following:

$ ./macgen.py
00:16:3e:20:b0:11

#!/usr/bin/python
# macgen.py script to generate a MAC address for guest virtual machines
#
import random
#
def randomMAC():
	mac = [ 0x00, 0x16, 0x3e,
		random.randint(0x00, 0x7f),
		random.randint(0x00, 0xff),
		random.randint(0x00, 0xff) ]
	return ':'.join(map(lambda x: "%02x" % x, mac))
#
print randomMAC()

⁠

Another method to generate a new MAC for your guest virtual machine

You can also use the built-in modules of python-virtinst to generate a new MAC address and UUID for use in a guest virtual machine configuration file:

# echo  'import virtinst.util ; print\
 virtinst.util.uuidToString(virtinst.util.randomUUID())' | python
# echo  'import virtinst.util ; print virtinst.util.randomMAC()' | python

The script above can also be implemented as a script file as seen below.

#!/usr/bin/env python
#  -*- mode: python; -*-
print ""
print "New UUID:"
import virtinst.util ; print virtinst.util.uuidToString(virtinst.util.randomUUID())
print "New MAC:"
import virtinst.util ; print virtinst.util.randomMAC()
print ""

⁠A.12. KVM networking performance

By default, KVM virtual machines are assigned a virtual Realtek 8139 (rtl8139) NIC (network interface controller).

The rtl8139 virtualized NIC works fine in most environments,but this device can suffer from performance degradation problems on some networks, such as a 10 Gigabit Ethernet.

To improve performance, you can switch to the para-virtualized network driver.

Note

Note that the virtualized Intel PRO/1000 (e1000) driver is also supported as an emulated driver choice. To use the e1000 driver, replace virtio in the procedure below with e1000. For the best performance it is recommended to use the virtio driver.

⁠

Procedure A.5. Switching to the virtio driver

Shutdown the guest operating system.

Edit the guest's configuration file with the virsh command (where GUEST is the guest's name):

# virsh edit GUEST

The virsh edit command uses the $EDITOR shell variable to determine which editor to use.

Find the network interface section of the configuration. This section resembles the snippet below:

<interface type='network'>
  [output truncated]
  <model type='rtl8139' />
</interface>

Change the type attribute of the model element from 'rtl8139' to 'virtio'. This will change the driver from the rtl8139 driver to the e1000 driver.

<interface type='network'>
  [output truncated]
  <model type='virtio' />
</interface>

Save the changes and exit the text editor

Restart the guest operating system.

⁠

Creating new guests using other network drivers

Alternatively, new guests can be created with a different network driver. This may be required if you are having difficulty installing guests over a network connection. This method requires you to have at least one guest already created (possibly installed from CD or DVD) to use as a template.

Create an XML template from an existing guest (in this example, named Guest1):

# virsh dumpxml Guest1 > /tmp/guest-template.xml

Copy and edit the XML file and update the unique fields: virtual machine name, UUID, disk image, MAC address, and any other unique parameters. Note that you can delete the UUID and MAC address lines and virsh will generate a UUID and MAC address.

# cp /tmp/guest-template.xml /tmp/new-guest.xml
# vi /tmp/new-guest.xml

Add the model line in the network interface section:

 <interface type='network'>
  [output truncated]
  <model type='virtio' />
</interface>

Create the new virtual machine:

# virsh define /tmp/new-guest.xml
# virsh start new-guest

⁠A.13. Workaround for creating external snapshots with libvirt

There are two classes of snapshots for QEMU guests. Internal snapshots are contained completely within a qcow2 file, and fully supported by libvirt, allowing for creating, deleting, and reverting of snapshots. This is the default setting used by libvirt when creating a snapshot, especially when no option is specified. Although this file type takes a bit longer than others in creating the the snapshot, it is required by libvirt to use qcow2 disks. Another drawback to this file type is that qcow2 disks are not subject to receive improvements from QEMU.

External snapshots, on the other hand work with any type of original disk image, can be taken with no guest downtime, and are able to receive active improvements from QEMU. In libvirt, they are created when using the --disk-only or --memspec option to snapshot-create-as (or when specifying an explicit XML file to snapshot-create that does the same). At the moment external snapshots are a one-way operation as libvirt can create them but can't do anything further with them. A workaround is described here.

⁠A.14. Missing characters on guest console with Japanese keyboard

On a Red Hat Enterprise Linux 7 host, connecting a Japanese keyboard locally to a machine may result in typed characters such as the underscore (the _ character) not being displayed correctly in guest consoles. This occurs because the required keymap is not set correctly by default.

With Red Hat Enterprise Linux 6 and Red Hat Enterprise Linux  7 guests, there is usually no error message produced when pressing the associated key. However, Red Hat Enterprise Linux 4 and Red Hat Enterprise Linux 5 guests may display an error similar to the following:

atkdb.c: Unknown key pressed (translated set 2, code 0x0 on isa0060/serio0).
atkbd.c: Use 'setkeycodes 00 <keycode>' to make it known.

To fix this issue in virt-manager, perform the following steps:

Open the affected guest in virt-manager.

Click View → Details.

Select Display VNC in the list.

Change Auto to ja in the Keymap pull-down menu.

Click the Apply button.

Alternatively, to fix this issue using the virsh edit command on the target guest:

Run virsh edit <target guest>

Add the following attribute to the <graphics> tag: keymap='ja'. For example:

 <graphics type='vnc' port='-1' autoport='yes' keymap='ja'/>

⁠A.15. Guest virtual machine fails to shutdown

Traditionally, executing a virsh shutdown command causes a power button ACPI event to be sent, thus copying the same action as when someone presses a power button on a physical machine. Within every physical machine, it is up to the OS to handle this event. In the past operating systems would just silently shutdown. Today, the most usual action is to show a dialog asking what should be done. Some operating systems even ignore this event completely, especially when no users are logged in. When such operating systems are installed on a guest virtual machine, running virsh shutdown just does not work (it is either ignored or a dialog is shown on a virtual display). However, if a qemu-guest-agent channel is added to a guest virtual machine and this agent is running inside the guest virtual machine's OS, the virsh shutdown command will ask the agent to shutdown the guest OS instead of sending the ACPI event. The agent will call for a shutdown from inside the guest virtual machine OS and everything works as expected.

⁠

Procedure A.6. Configuring the guest agent channel in a guest virtual machine

Stop the guest virtual machine.

Open the Domain XML for the guest virtual machine and add the following snippet:

⁠

​
​
​<channel type='unix'>
​    <source mode='bind'/>
​    <target type='virtio' name='org.qemu.guest_agent.0'/>
​</channel>

Figure A.1. Configuring the guest agent channel

Start the guest virtual machine, by running virsh start [domain].

Install qemu-guest-agent on the guest virtual machine (yum install qemu-guest-agent) and make it run automatically at every boot as a service (qemu-guest-agent.service).

⁠A.16. Disable SMART disk monitoring for guest virtual machines

SMART disk monitoring can be safely disabled as virtual disks and the physical storage devices are managed by the host physical machine.

# service smartd stop
# systemctl --del smartd

⁠A.17. libguestfs troubleshooting

A test tool is available to check that libguestfs is working. Run the following command after installing libguestfs (root access not required) to test for normal operation:

$ libguestfs-test-tool

This tool prints a large amount of text to test the operation of libguestfs. If the test is successful, the following text will appear near the end of the output:

===== TEST FINISHED OK =====

⁠A.18. Common libvirt errors and troubleshooting

This appendix documents common libvirt-related problems and errors along with instructions for dealing with them.

Locate the error on the table below and follow the corresponding link under Solution for detailed troubleshooting information.

⁠

Table A.1. Common libvirt errors

Error
Description of problem
Solution

libvirtd failed to start
The libvirt daemon failed to start. However, there is no information about this error in /var/log/messages.
Section A.18.1, “libvirtd failed to start”

Cannot read CA certificate
This is one of several errors that occur when the URI fails to connect to the hypervisor.
Section A.18.2, “The URI failed to connect to the hypervisor”

Failed to connect socket ... : Permission denied
This is one of several errors that occur when the URI fails to connect to the hypervisor.
Section A.18.2, “The URI failed to connect to the hypervisor”

Other connectivity errors
These are other errors that occur when the URI fails to connect to the hypervisor.
Section A.18.2, “The URI failed to connect to the hypervisor”

Internal error guest CPU is not compatible with host CPU
The guest virtual machine cannot be started because the host and guest processors are different.
Section A.18.3, “The guest virtual machine cannot be started: internal error guest CPU is not compatible with host CPU”

Failed to create domain from vm.xml error: monitor socket did not show up.: Connection refused
The guest virtual machine (or domain) starting fails and returns this error or similar.
Section A.18.4, “Guest starting fails with error: monitor socket did not show up”

Internal error cannot find character device (null)
This error can occur when attempting to connect a guest's console. It reports that there is no serial console configured for the guest virtual machine.
Section A.18.5, “Internal error cannot find character device (null)”

No boot device
After building a guest virtual machine from an existing disk image, the guest booting stalls. However, the guest can start successfully using the QEMU command directly.
Section A.18.6, “Guest virtual machine booting stalls with error: No boot device”

The virtual network "default" has not been started

If the default network (or other locally-created network) is unable to start, any virtual machine configured to use that network for its connectivity will also fail to start.

Section A.18.7, “Virtual network default has not been started”

PXE boot (or DHCP) on guest failed
A guest virtual machine starts successfully, but is unable to acquire an IP address from DHCP, boot using the PXE protocol, or both. This is often a result of a long forward delay time set for the bridge, or when the iptables package and kernel do not support checksum mangling rules.
Section A.18.8, “PXE boot (or DHCP) on guest failed”

Guest can reach outside network, but cannot reach host when using macvtap interface

A guest can communicate with other guests, but cannot connect to the host machine after being configured to use a macvtap (or type='direct') network interface.

This is actually not an error — it is the defined behavior of macvtap.

Section A.18.9, “Guest can reach outside network, but cannot reach host when using macvtap interface”

Could not add rule to fixup DHCP response checksums on network 'default'
This warning message is almost always harmless, but is often mistakenly seen as evidence of a problem.
Section A.18.10, “Could not add rule to fixup DHCP response checksums on network 'default'”

Unable to add bridge br0 port vnet0: No such device
This error message or the similar Failed to add tap interface to bridge 'br0': No such device reveal that the bridge device specified in the guest's (or domain's) <interface> definition does not exist.
Section A.18.11, “Unable to add bridge br0 port vnet0: No such device”

Warning: could not open /dev/net/tun: no virtual network emulation qemu-kvm: -netdev tap,script=/etc/my-qemu-ifup,id=hostnet0: Device 'tap' could not be initialized
The guest virtual machine does not start after configuring a type='ethernet' (or 'generic ethernet') interface in the host system. This error or similar appears either in libvirtd.log, /var/log/libvirt/qemu/name_of_guest.log, or in both.
Section A.18.12, “Guest is unable to start with error: warning: could not open /dev/net/tun”

Unable to resolve address name_of_host service '49155': Name or service not known
QEMU guest migration fails and this error message appears with an unfamiliar hostname.
Section A.18.13, “Migration fails with Error: unable to resolve address”

Unable to allow access for disk path /var/lib/libvirt/images/qemu.img: No such file or directory
A guest virtual machine cannot be migrated because libvirt cannot access the disk image(s).
Section A.18.14, “Migration fails with Unable to allow access for disk path: No such file or directory”

No guest virtual machines are present when libvirtd is started
The libvirt daemon is successfully started, but no guest virtual machines appear to be present when running virsh list --all.
Section A.18.15, “No guest virtual machines are present when libvirtd is started”

Unable to connect to server at 'host:16509': Connection refused ... error: failed to connect to the hypervisor
While libvirtd should listen on TCP ports for connections, the connection to the hypervisor fails.
Section A.18.16, “Unable to connect to server at 'host:16509': Connection refused ... error: failed to connect to the hypervisor”

Common XML errors
libvirt uses XML documents to store structured data. Several common errors occur with XML documents when they are passed to libvirt through the API. This entry provides instructions for editing guest XML definitions, and details common errors in XML syntax and configuration.
Section A.18.17, “Common XML errors”

⁠A.18.1. libvirtd failed to start

⁠Symptom

The libvirt daemon does not start automatically. Starting the libvirt daemon manually fails as well:

# /etc/init.d/libvirtd start
* Caching service dependencies ...                                                                                             [ ok ]
* Starting libvirtd ...
/usr/sbin/libvirtd: error: Unable to initialize network sockets. Check /var/log/messages or run without --daemon for more info.
* start-stop-daemon: failed to start `/usr/sbin/libvirtd'                                                                      [ !! ]
* ERROR: libvirtd failed to start

Moreover, there is not 'more info' about this error in /var/log/messages.

⁠Investigation

Change libvirt's logging in /etc/libvirt/libvirtd.conf by enabling the line below. To enable the setting the line, open the /etc/libvirt/libvirtd.conf file in a text editor, remove the hash (or #) symbol from the beginning of the following line, and save the change:

log_outputs="3:syslog:libvirtd"

Note

This line is commented out by default to prevent libvirt from producing excessive log messages. After diagnosing the problem, it is recommended to comment this line again in the /etc/libvirt/libvirtd.conf file.

Restart libvirt to determine if this has solved the problem.

If libvirtd still does not start successfully, an error similar to the following will be shown in the /var/log/messages file:

Feb  6 17:22:09 bart libvirtd: 17576: info : libvirt version: 0.9.9
Feb  6 17:22:09 bart libvirtd: 17576: error : virNetTLSContextCheckCertFile:92: Cannot read CA certificate '/etc/pki/CA/cacert.pem': No such file or directory
Feb  6 17:22:09 bart /etc/init.d/libvirtd[17573]: start-stop-daemon: failed to start `/usr/sbin/libvirtd'
Feb  6 17:22:09 bart /etc/init.d/libvirtd[17565]: ERROR: libvirtd failed to start

The libvirtd man page shows that the missing cacert.pem file is used as TLS authority when libvirt is run in Listen for TCP/IP connections mode. This means the --listen parameter is being passed.

⁠Solution

Configure the libvirt daemon's settings with one of the following methods:

Install a CA certificate.

Note

For more information on CA certificates and configuring system authentication, refer to the Configuring Authentication chapter in the Red Hat Enterprise Linux 7 Deployment Guide.

Do not use TLS; use bare TCP instead. In /etc/libvirt/libvirtd.conf set listen_tls = 0 and listen_tcp = 1. The default values are listen_tls = 1 and listen_tcp = 0.

Do not pass the --listen parameter. In /etc/sysconfig/libvirtd.conf change the LIBVIRTD_ARGS variable.

⁠A.18.2. The URI failed to connect to the hypervisor

Several different errors can occur when connecting to the server (for example, when running virsh).

⁠A.18.2.1. Cannot read CA certificate

⁠Symptom

When running a command, the following error (or similar) appears:

$ virsh -c name_of_uri list
error: Cannot read CA certificate '/etc/pki/CA/cacert.pem': No such file or directory
error: failed to connect to the hypervisor

⁠Investigation

The error message is misleading about the actual cause. This error can be caused by a variety of factors, such as an incorrectly specified URI, or a connection that is not configured.

⁠Solution

⁠Incorrectly specified URI

When specifying qemu://system or qemu://session as a connection URI, virsh attempts to connect to hostnames system or session respectively. This is because virsh recognizes the text after the second forward slash as the host.

Use three forward slashes to connect to the local host. For example, specifying qemu:///system instructs virsh connect to the system instance of libvirtd on the local host.

When a host name is specified, the QEMU transport defaults to TLS. This results in certificates.

⁠Connection is not configured

The URI is correct (for example, qemu[+tls]://server/system) but the certificates are not set up properly on your machine. For information on configuring TLS, see Setting up libvirt for TLS available from the libvirt website.

⁠A.18.2.2. Failed to connect socket ... : Permission denied

⁠Symptom

When running a virsh command, the following error (or similar) appears:

$ virsh -c qemu:///system list
error: Failed to connect socket to '/var/run/libvirt/libvirt-sock': Permission denied
error: failed to connect to the hypervisor

⁠Investigation

Without any hostname specified, the connection to QEMU uses UNIX sockets by default. If there is no error running this command as root, the UNIX socket options in /etc/libvirt/libvirtd.conf are likely misconfigured.

⁠Solution

To connect as a non-root user using UNIX sockets, configure the following options in /etc/libvirt/libvirtd.conf:

unix_sock_group = <group>
unix_sock_ro_perms = <perms>
unix_sock_rw_perms = <perms>

Note

The user running virsh must be a member of the group specified in the unix_sock_group option.

⁠A.18.2.3. Other connectivity errors

⁠Unable to connect to server at server:port: Connection refused

The daemon is not running on the server or is configured not to listen, using configuration option listen_tcp or listen_tls.

⁠End of file while reading data: nc: using stream socket: Input/output error

If you specified ssh transport, the daemon is likely not running on the server. Solve this error by verifying that the daemon is running on the server.

⁠A.18.3. The guest virtual machine cannot be started: internal error guest CPU is not compatible with host CPU

⁠Symptom

Running on an Intel Core i7 processor (which virt-manager refers to as Nehalem, or the older Core 2 Duo, referred to as Penryn), a KVM guest (or domain) is created using virt-manager. After installation, the guest's processor is changed to match the host's CPU. The guest is then unable to start and reports this error:

2012-02-06 17:49:15.985+0000: 20757: error : qemuBuildCpuArgStr:3565 : internal error guest CPU is not compatible with host CPU

Additionally, clicking Copy host CPU configuration in virt-manager shows Pentium III instead of Nehalem or Penryn.

⁠Investigation

The /usr/share/libvirt/cpu_map.xml file lists the flags that define each CPU model. The Nehalem and Penryn definitions contain this:

<feature name='nx'/>

As a result, the NX (or No eXecute) flag needs to be presented to identify the CPU as Nehalem or Penryn. However, in /proc/cpuinfo, this flag is missing.

⁠Solution

Nearly all new BIOSes allow enabling or disabling of the No eXecute bit. However, if disabled, some CPUs do not report this flag and thus libvirt detects a different CPU. Enabling this functionality instructs libvirt to report the correct CPU. Refer to your hardware documentation for further instructions on this subject.

⁠A.18.4. Guest starting fails with error: monitor socket did not show up

⁠Symptom

The guest virtual machine (or domain) starting fails with this error (or similar):

# virsh -c qemu:///system create name_of_guest.xml error: Failed to create domain from name_of_guest.xml error: monitor socket did not show up.: Connection refused

⁠Investigation

This error message shows:

libvirt is working;

The QEMU process failed to start up; and

libvirt quits when trying to connect QEMU or the QEMU agent monitor socket.

To understand the error details, examine the guest log:

# cat /var/log/libvirt/qemu/name_of_guest.log
LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin QEMU_AUDIO_DRV=none /usr/bin/qemu-kvm -S -M pc -enable-kvm -m 768 -smp 1,sockets=1,cores=1,threads=1 -name name_of_guest -uuid ebfaadbe-e908-ba92-fdb8-3fa2db557a42 -nodefaults -chardev socket,id=monitor,path=/var/lib/libvirt/qemu/name_of_guest.monitor,server,nowait -mon chardev=monitor,mode=readline -no-reboot -boot c -kernel /var/lib/libvirt/boot/vmlinuz -initrd /var/lib/libvirt/boot/initrd.img -append method=http://www.example.com/pub/product/release/version/x86_64/os/ -drive file=/var/lib/libvirt/images/name_of_guest.img,if=none,id=drive-ide0-0-0,boot=on -device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0 -device virtio-net-pci,vlan=0,id=net0,mac=52:40:00:f4:f1:0a,bus=pci.0,addr=0x4 -net tap,fd=42,vlan=0,name=hostnet0 -chardev pty,id=serial0 -device isa-serial,chardev=serial0 -usb -vnc 127.0.0.1:0 -k en-gb -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,
addr=0x3
char device redirected to /dev/pts/1
qemu: could not load kernel '/var/lib/libvirt/boot/vmlinuz':
Permission denied

⁠Solution

The guest log contains the details needed to fix the error.

If a host physical machine is shut down while the guest is still running a libvirt version prior to 0.9.5, the libvirt-guest's init script attempts to perform a managed save of the guest. If the managed save was incomplete (for example, due to loss of power before the managed save image was flushed to disk), the save image is corrupted and will not be loaded by QEMU. The older version of libvirt does not recognize the corruption, making the problem perpetual. In this case, the guest log will show an attempt to use -incoming as one of its arguments, meaning that libvirt is trying to start QEMU by migrating in the saved state file.

This problem can be fixed by running virsh managedsave-remove name_of_guest to remove the corrupted managed save image. Newer versions of libvirt take steps to avoid the corruption in the first place, as well as adding virsh start --force-boot name_of_guest to bypass any managed save image.

⁠A.18.5. Internal error cannot find character device (null)

⁠Symptom

This error message appears when attempting to connect to a guest virtual machine's console:

# virsh console test2 Connected to domain test2 Escape character is ^] error: internal error cannot find character device (null)

⁠Investigation

This error message shows that there is no serial console configured for the guest virtual machine.

⁠Solution

Set up a serial console in the guest's XML file.

⁠

Procedure A.7. Setting up a serial console in the guest's XML

Add the following XML to the guest virtual machine's XML using virsh edit:

<serial type='pty'>
  <target port='0'/>
</serial>
<console type='pty'>
  <target type='serial' port='0'/>
</console>

Set up the console in the guest kernel command line.

To do this, either log in to the guest virtual machine to edit the /boot/grub/grub.conf file directly, or use the virt-edit command line tool. Add the following to the guest kernel command line:

console=ttyS0,115200

Run the followings command:

# virsh start vm && virsh console vm

⁠A.18.6. Guest virtual machine booting stalls with error: No boot device

⁠Symptom

After building a guest virtual machine from an existing disk image, the guest booting stalls with the error message No boot device. However, the guest virtual machine can start successfully using the QEMU command directly.

⁠Investigation

The disk's bus type is not specified in the command for importing the existing disk image:

# virt-install \
--connect qemu:///system \
--ram 2048 -n rhel_64 \
--os-type=linux --os-variant=rhel5 \
--disk  path=/root/RHEL-Server-5.8-64-virtio.qcow2,device=disk,format=qcow2 \
--vcpus=2 --graphics spice --noautoconsole --import

However, the command line used to boot up the guest virtual machine using QEMU directly shows that it uses virtio for its bus type:

# ps -ef | grep qemu
/usr/libexec/qemu-kvm -monitor stdio -drive file=/root/RHEL-Server-5.8-32-virtio.qcow2,index=0,if=virtio,media=disk,cache=none,format=qcow2 -net nic,vlan=0,model=rtl8139,macaddr=00:30:91:aa:04:74 -net tap,vlan=0,script=/etc/qemu-ifup,downscript=no -m 2048 -smp 2,cores=1,threads=1,sockets=2 -cpu qemu64,+sse2 -soundhw ac97 -rtc-td-hack -M rhel5.6.0 -usbdevice tablet -vnc :10 -boot c -no-kvm-pit-reinjection

Note the bus= in the guest's XML generated by libvirt for the imported guest:

<domain type='qemu'>
 <name>rhel_64</name>
 <uuid>6cd34d52-59e3-5a42-29e4-1d173759f3e7</uuid>
 <memory>2097152</memory>
 <currentMemory>2097152</currentMemory>
 <vcpu>2</vcpu>
 <os>
   <type arch='x86_64' machine='rhel5.4.0'>hvm</type>
   <boot dev='hd'/>
 </os>
 <features>
   <acpi/>
   <apic/>
   <pae/>
 </features>
 <clock offset='utc'>
   <timer name='pit' tickpolicy='delay'/>
 </clock>
 <on_poweroff>destroy</on_poweroff>
 <on_reboot>restart</on_reboot>
 <on_crash>restart</on_crash>
 <devices>
   <emulator>/usr/libexec/qemu-kvm</emulator>
   <disk type='file' device='disk'>
     <driver name='qemu' type='qcow2' cache='none'/>
     <source file='/root/RHEL-Server-5.8-64-virtio.qcow2'/>
     <emphasis role="bold"><target dev='hda' bus='ide'/></emphasis>
     <address type='drive' controller='0' bus='0' unit='0'/>
   </disk>
   <controller type='ide' index='0'/>
   <interface type='bridge'>
     <mac address='54:52:00:08:3e:8c'/>
     <source bridge='br0'/>
   </interface>
   <serial type='pty'>
     <target port='0'/>
   </serial>
   <console type='pty'>
     <target port='0'/>
   </console>
   <input type='mouse' bus='ps2'/>
   <graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
   <video>
     <model type='cirrus' vram='9216' heads='1'/>
   </video>
 </devices>
 </domain>

The bus type for the disk is set as ide, which is the default value set by libvirt. This is the incorrect bus type, and has caused the unsuccessful boot for the imported guest.

⁠Solution

⁠

Procedure A.8. Correcting the disk bus type

Undefine the imported guest virtual machine, then re-import it with bus=virtio and the following:

# virsh destroy rhel_64
# virsh undefine rhel_64
# virt-install \
--connect qemu:///system \
--ram 1024 -n rhel_64 -r 2048 \
--os-type=linux --os-variant=rhel5  \
--disk path=/root/RHEL-Server-5.8-64-virtio.qcow2,device=disk,bus=virtio,format=qcow2 \
--vcpus=2 --graphics spice --noautoconsole --import

Edit the imported guest's XML using virsh edit and correct the disk bus type.

⁠A.18.7. Virtual network default has not been started

⁠Symptom

Normally, the configuration for a virtual network named default is installed as part of the libvirt package, and is configured to autostart when libvirtd is started.

If the default network (or any other locally-created network) is unable to start, any virtual machine configured to use that network for its connectivity will also fail to start, resulting in this error message:

Virtual network default has not been started

⁠Investigation

One of the most common reasons for a libvirt virtual network's failure to start is that the dnsmasq instance required to serve DHCP and DNS requests from clients on that network has failed to start.

To determine if this is the cause, run virsh net-start default from a root shell to start the default virtual network.

If this action does not successfully start the virtual network, open /var/log/libvirt/libvirtd.log to view the complete error log message.

If a message similar to the following appears, the problem is likely a systemwide dnsmasq instance that is already listening on libvirt's bridge, and is preventing libvirt's own dnsmasq instance from doing so. The most important parts to note in the error message are dnsmasq and exit status 2:

Could not start virtual network default: internal error
Child process (/usr/sbin/dnsmasq --strict-order --bind-interfaces
--pid-file=/var/run/libvirt/network/default.pid --conf-file=
--except-interface lo --listen-address 192.168.122.1
--dhcp-range 192.168.122.2,192.168.122.254
--dhcp-leasefile=/var/lib/libvirt/dnsmasq/default.leases
--dhcp-lease-max=253 --dhcp-no-override) status unexpected: exit status 2

⁠Solution

If the machine is not using dnsmasq to serve DHCP for the physical network, disable dnsmasq completely.

If it is necessary to run dnsmasq to serve DHCP for the physical network, edit the /etc/dnsmasq.conf file. Add or remove the comment mark the first line, as well as one of the two lines following that line. Do not add or remove the comment from all three lines:

bind-interfaces
interface=name_of_physical_interface
listen-address=chosen_IP_address

After making this change and saving the file, restart the system wide dnsmasq service.

Next, start the default network with the virsh net-start default command.

Start the virtual machines.

⁠A.18.8. PXE boot (or DHCP) on guest failed

⁠Symptom

A guest virtual machine starts successfully, but is then either unable to acquire an IP address from DHCP or boot using the PXE protocol, or both. There are two common causes of this error: having a long forward delay time set for the bridge, and when the iptables package and kernel do not support checksum mangling rules.

⁠Long forward delay time on bridge

⁠Investigation

This is the most common cause of this error. If the guest network interface is connecting to a bridge device that has STP (Spanning Tree Protocol) enabled, as well as a long forward delay set, the bridge will not forward network packets from the guest virtual machine onto the bridge until at least that number of forward delay seconds have elapsed since the guest connected to the bridge. This delay allows the bridge time to watch traffic from the interface and determine the MAC addresses behind it, and prevent forwarding loops in the network topology.

If the forward delay is longer than the timeout of the guest's PXE or DHCP client, then the client's operation will fail, and the guest will either fail to boot (in the case of PXE) or fail to acquire an IP address (in the case of DHCP).

⁠Solution

If this is the case, change the forward delay on the bridge to 0, disable STP on the bridge, or both.

Note

This solution applies only if the bridge is not used to connect multiple networks, but just to connect multiple endpoints to a single network (the most common use case for bridges used by libvirt).

If the guest has interfaces connecting to a libvirt-managed virtual network, edit the definition for the network, and restart it. For example, edit the default network with the following command:

# virsh net-edit default

Add the following attributes to the <bridge> element:

<name_of_bridge='virbr0' delay='0' stp='on'/>

Note

delay='0' and stp='on' are the default settings for virtual networks, so this step is only necessary if the configuration has been modified from the default.

If the guest interface is connected to a host bridge that was configured outside of libvirt, change the delay setting.

Add or edit the following lines in the /etc/sysconfig/network-scripts/ifcfg-name_of_bridge file to turn STP on with a 0 second delay:

STP=on
DELAY=0

After changing the configuration file, restart the bridge device:

/usr/sbin/ifdown name_of_bridge
/usr/sbin/ifup name_of_bridge

Note

If name_of_bridge is not the root bridge in the network, that bridge's delay will eventually reset to the delay time configured for the root bridge. In this case, the only solution is to disable STP completely on name_of_bridge.

⁠The iptables package and kernel do not support checksum mangling rules

⁠Investigation

This message is only a problem if all four of the following conditions are true:

The guest is using virtio network devices.

If so, the configuration file will contain model type='virtio'

The host has the vhost-net module loaded.

This is true if ls /dev/vhost-net does not return an empty result.

The guest is attempting to get an IP address from a DHCP server that is running directly on the host.

The iptables version on the host is older than 1.4.10.

iptables 1.4.10 was the first version to add the libxt_CHECKSUM extension. This is the case if the following message appears in the libvirtd logs:

warning: Could not add rule to fixup DHCP response checksums on network default
warning: May need to update iptables package and kernel to support CHECKSUM rule.

Important

Unless all of the other three conditions in this list are also true, the above warning message can be disregarded, and is not an indicator of any other problems.

When these conditions occur, UDP packets sent from the host to the guest have uncomputed checksums. This makes the host's UDP packets seem invalid to the guest's network stack.

⁠Solution

To solve this problem, invalidate any of the four points above. The best solution is to update the host iptables and kernel to iptables-1.4.10 or newer where possible. Otherwise, the most specific fix is to disable the vhost-net driver for this particular guest. To do this, edit the guest configuration with this command:

virsh edit name_of_guest

Change or add a <driver> line to the <interface> section:

<interface type='network'>
  <model type='virtio'/>
  <driver name='qemu'/>
  ...
</interface>

Save the changes, shut down the guest, and then restart it.

If this problem is still not resolved, the issue may be due to a conflict between firewalld and the default libvirt network.

To fix this, stop firewalld with the service firewalld stop command, then restart libvirt with the service libvirtd restart command.

⁠A.18.9. Guest can reach outside network, but cannot reach host when using macvtap interface

⁠Symptom

A guest virtual machine can communicate with other guests, but cannot connect to the host machine after being configured to use a macvtap (also known as type='direct') network interface.

⁠Investigation

Even when not connecting to a Virtual Ethernet Port Aggregator (VEPA) or VN-Link capable switch, macvtap interfaces can be useful. Setting the mode of such an interface to bridge allows the guest to be directly connected to the physical network in a very simple manner without the setup issues (or NetworkManager incompatibility) that can accompany the use of a traditional host bridge device.

However, when a guest virtual machine is configured to use a type='direct' network interface such as macvtap, despite having the ability to communicate with other guests and other external hosts on the network, the guest cannot communicate with its own host.

This situation is actually not an error — it is the defined behavior of macvtap. Due to the way in which the host's physical Ethernet is attached to the macvtap bridge, traffic into that bridge from the guests that is forwarded to the physical interface cannot be bounced back up to the host's IP stack. Additionally, traffic from the host's IP stack that is sent to the physical interface cannot be bounced back up to the macvtap bridge for forwarding to the guests.

⁠Solution

Use libvirt to create an isolated network, and create a second interface for each guest virtual machine that is connected to this network. The host and guests can then directly communicate over this isolated network, while also maintaining compatibility with NetworkManager.

⁠

Procedure A.9. Creating an isolated network with libvirt

Add and save the following XML in the /tmp/isolated.xml file. If the 192.168.254.0/24 network is already in use elsewhere on your network, you can choose a different network.

⁠

​
​
​...
​<network>
​  <name>isolated</name>
​  <ip address='192.168.254.1' netmask='255.255.255.0'>
​    <dhcp>
​      <range start='192.168.254.2' end='192.168.254.254'/>
​    </dhcp>
​  </ip>
​</network>
​...

Figure A.2. Isolated Network XML

Create the network with this command: virsh net-define /tmp/isolated.xml

Set the network to autostart with the virsh net-autostart isolated command.

Start the network with the virsh net-start isolated command.

Using virsh edit name_of_guest, edit the configuration of each guest that uses macvtap for its network connection and add a new <interface> in the <devices> section similar to the following (note the <model type='virtio'/> line is optional to include):

⁠

​
​
​...
​<interface type='network' trustGuestRxFilters='yes'>
​  <source network='isolated'/>
​  <model type='virtio'/>
​</interface>

Figure A.3. Interface Device XML

Shut down, then restart each of these guests.

The guests are now able to reach the host at the address 192.168.254.1, and the host will be able to reach the guests at the IP address they acquired from DHCP (alternatively, you can manually configure the IP addresses for the guests). Since this new network is isolated to only the host and guests, all other communication from the guests will use the macvtap interface. Refer to Section 26.18.9, “Network interfaces” for more information.

⁠A.18.10. Could not add rule to fixup DHCP response checksums on network 'default'

⁠Symptom

This message appears:

Could not add rule to fixup DHCP response checksums on network 'default'

⁠Investigation

Although this message appears to be evidence of an error, it is almost always harmless.

⁠Solution

Unless the problem you are experiencing is that the guest virtual machines are unable to acquire IP addresses through DHCP, this message can be ignored.

If this is the case, refer to Section A.18.8, “PXE boot (or DHCP) on guest failed” for further details on this situation.

⁠A.18.11. Unable to add bridge br0 port vnet0: No such device

⁠Symptom

The following error message appears:

Unable to add bridge name_of_bridge port vnet0: No such device

For example, if the bridge name is br0, the error message will appear as:

Unable to add bridge br0 port vnet0: No such device

In libvirt versions 0.9.6 and earlier, the same error appears as:

Failed to add tap interface to bridge name_of_bridge: No such device

Or for example, if the bridge is named br0:

Failed to add tap interface to bridge 'br0': No such device

⁠Investigation

Both error messages reveal that the bridge device specified in the guest's (or domain's) <interface> definition does not exist.

To verify the bridge device listed in the error message does not exist, use the shell command ifconfig br0.

A message similar to this confirms the host has no bridge by that name:

br0: error fetching interface information: Device not found

If this is the case, continue to the solution.

However, if the resulting message is similar to the following, the issue exists elsewhere:

br0        Link encap:Ethernet  HWaddr 00:00:5A:11:70:48
           inet addr:10.22.1.5  Bcast:10.255.255.255  Mask:255.0.0.0
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           RX packets:249841 errors:0 dropped:0 overruns:0 frame:0
           TX packets:281948 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:0
	   RX bytes:106327234 (101.4 MiB)  TX bytes:21182634 (20.2 MiB)

⁠Solution

⁠Edit the existing bridge or create a new bridge with virsh

Use virsh to either edit the settings of an existing bridge or network, or to add the bridge device to the host system configuration.

⁠Edit the existing bridge settings using virsh

Use virsh edit name_of_guest to change the <interface> definition to use a bridge or network that already exists.

For example, change type='bridge' to type='network', and <source bridge='br0'/> to <source network='default'/>.

⁠Create a host bridge using virsh

For libvirt version 0.9.8 and later, a bridge device can be created with the virsh iface-bridge command. This will create a bridge device br0 with eth0, the physical network interface which is set as part of a bridge, attached:

virsh iface-bridge eth0 br0

Optional: If desired, remove this bridge and restore the original eth0 configuration with this command:

virsh iface-unbridge br0

⁠Create a host bridge manually

For older versions of libvirt, it is possible to manually create a bridge device on the host. Refer to Section 7.4.3, “Bridged networking with libvirt” for instructions.

⁠A.18.12. Guest is unable to start with error: warning: could not open /dev/net/tun

⁠Symptom

The guest virtual machine does not start after configuring a type='ethernet' (also known as 'generic ethernet') interface in the host system. An error appears either in libvirtd.log, /var/log/libvirt/qemu/name_of_guest.log, or in both, similar to the below message:

warning: could not open /dev/net/tun: no virtual network emulation qemu-kvm: -netdev tap,script=/etc/my-qemu-ifup,id=hostnet0: Device 'tap' could not be initialized

⁠Investigation

Use of the generic ethernet interface type (<interface type='ethernet'>) is discouraged, because using it requires lowering the level of host protection against potential security flaws in QEMU and its guests. However, it is sometimes necessary to use this type of interface to take advantage of some other facility that is not yet supported directly in libvirt. For example, openvswitch was not supported in libvirt until libvirt-0.9.11, so in older versions of libvirt, <interface type='ethernet'> was the only way to connect a guest to an openvswitch bridge.

However, if you configure a <interface type='ethernet'> interface without making any other changes to the host system, the guest virtual machine will not start successfully.

The reason for this failure is that for this type of interface, a script called by QEMU needs to manipulate the tap device. However, with type='ethernet' configured, in an attempt to lock down QEMU, libvirt and SELinux have put in place several checks to prevent this. (Normally, libvirt performs all of the tap device creation and manipulation, and passes an open file descriptor for the tap device to QEMU.)

⁠Solution

Reconfigure the host system to be compatible with the generic ethernet interface.

⁠

Procedure A.10. Reconfiguring the host system to use the generic ethernet interface

Set SELinux to permissive by configuring SELINUX=permissive in /etc/selinux/config:

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#       enforcing - SELinux security policy is enforced.
#       permissive - SELinux prints warnings instead of enforcing.
#       disabled - No SELinux policy is loaded.
SELINUX=permissive
# SELINUXTYPE= can take one of these two values:
#       targeted - Targeted processes are protected,
#       mls - Multi Level Security protection.
SELINUXTYPE=targeted

From a root shell, run the command setenforce permissive.

In /etc/libvirt/qemu.conf add or edit the following lines:

clear_emulator_capabilities = 0

user = "root"

group = "root"

cgroup_device_acl = [
        "/dev/null", "/dev/full", "/dev/zero",
        "/dev/random", "/dev/urandom",
        "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
	"/dev/rtc", "/dev/hpet", "/dev/net/tun",

Restart libvirtd.

Important

Since each of these steps significantly decreases the host's security protections against QEMU guest domains, this configuration should only be used if there is no alternative to using <interface type='ethernet'>.

Note

For more information on SELinux, refer to the Red Hat Enterprise Linux 7 Security-Enhanced Linux User Guide.

⁠A.18.13. Migration fails with Error: unable to resolve address

⁠Symptom

QEMU guest migration fails and this error message appears:

# virsh migrate qemu qemu+tcp://192.168.122.12/system
  error: Unable to resolve address name_of_host service '49155': Name or service not known

For example, if the destination hostname is "newyork", the error message will appear as:

# virsh migrate qemu qemu+tcp://192.168.122.12/system
error: Unable to resolve address 'newyork' service '49155': Name or service not known

However, this error looks strange as we did not use "newyork" hostname anywhere.

⁠Investigation

During migration, libvirtd running on the destination host creates a URI from an address and port where it expects to receive migration data and sends it back to libvirtd running on the source host.

In this case, the destination host (192.168.122.12) has its name set to 'newyork'. For some reason, libvirtd running on that host is unable to resolve the name to an IP address that could be sent back and still be useful. For this reason, it returned the 'newyork' hostname hoping the source libvirtd would be more successful with resolving the name. This can happen if DNS is not properly configured or /etc/hosts has the hostname associated with local loopback address (127.0.0.1).

Note that the address used for migration data cannot be automatically determined from the address used for connecting to destination libvirtd (for example, from qemu+tcp://192.168.122.12/system). This is because to communicate with the destination libvirtd, the source libvirtd may need to use network infrastructure different from that which virsh (possibly running on a separate machine) requires.

⁠Solution

The best solution is to configure DNS correctly so that all hosts involved in migration are able to resolve all host names.

If DNS cannot be configured to do this, a list of every host used for migration can be added manually to the /etc/hosts file on each of the hosts. However, it is difficult to keep such lists consistent in a dynamic environment.

If the host names cannot be made resolvable by any means, virsh migrate supports specifying the migration host:

# virsh migrate qemu qemu+tcp://192.168.122.12/system tcp://192.168.122.12

Destination libvirtd will take the tcp://192.168.122.12 URI and append an automatically generated port number. If this is not desirable (because of firewall configuration, for example), the port number can be specified in this command:

# virsh migrate qemu qemu+tcp://192.168.122.12/system tcp://192.168.122.12:12345

Another option is to use tunneled migration. Tunneled migration does not create a separate connection for migration data, but instead tunnels the data through the connection used for communication with destination libvirtd (for example, qemu+tcp://192.168.122.12/system):

# virsh migrate qemu qemu+tcp://192.168.122.12/system --p2p --tunnelled

⁠A.18.14. Migration fails with Unable to allow access for disk path: No such file or directory

⁠Symptom

A guest virtual machine (or domain) cannot be migrated because libvirt cannot access the disk image(s):

# virsh migrate qemu qemu+tcp://name_of_host/system
error: Unable to allow access for disk path /var/lib/libvirt/images/qemu.img: No such file or directory

For example, if the destination hostname is "newyork", the error message will appear as:

# virsh migrate qemu qemu+tcp://newyork/system
error: Unable to allow access for disk path /var/lib/libvirt/images/qemu.img: No such file or directory

⁠Investigation

By default, migration only transfers the in-memory state of a running guest (such as memory or CPU state). Although disk images are not transferred during migration, they need to remain accessible at the same path by both hosts.

⁠Solution

Set up and mount shared storage at the same location on both hosts. The simplest way to do this is to use NFS:

⁠

Procedure A.11. Setting up shared storage

Set up an NFS server on a host serving as shared storage. The NFS server can be one of the hosts involved in the migration, as long as all hosts involved are accessing the shared storage through NFS.

# mkdir -p /exports/images
# cat >>/etc/exports <<EOF
/exports/images    192.168.122.0/24(rw,no_root_squash)
EOF

Mount the exported directory at a common location on all hosts running libvirt. For example, if the IP address of the NFS server is 192.168.122.1, mount the directory with the following commands:

# cat >>/etc/fstab <<EOF
192.168.122.1:/exports/images  /var/lib/libvirt/images  nfs  auto  0 0
EOF
# mount /var/lib/libvirt/images

Note

It is not possible to export a local directory from one host using NFS and mount it at the same path on another host — the directory used for storing disk images must be mounted from shared storage on both hosts. If this is not configured correctly, the guest virtual machine may lose access to its disk images during migration, because the source host's libvirt daemon may change the owner, permissions, and SELinux labels on the disk images after it successfully migrates the guest to its destination.

If libvirt detects that the disk images are mounted from a shared storage location, it will not make these changes.

⁠A.18.15. No guest virtual machines are present when libvirtd is started

⁠Symptom

The libvirt daemon is successfully started, but no guest virtual machines appear to be present.

# virsh list --all
 Id    Name                           State
----------------------------------------------------
#

⁠Investigation

There are various possible causes of this problem. Performing these tests will help to determine the cause of this situation:

⁠Verify KVM kernel modules

Verify that KVM kernel modules are inserted in the kernel:

# lsmod | grep kvm
kvm_intel             121346  0
kvm                   328927  1 kvm_intel

If you are using an AMD machine, verify the kvm_amd kernel modules are inserted in the kernel instead, using the similar command lsmod | grep kvm_amd in the root shell.

If the modules are not present, insert them using the modprobe <modulename> command.

Note

Although it is uncommon, KVM virtualization support may be compiled into the kernel. In this case, modules are not needed.

⁠Verify virtualization extensions

Verify that virtualization extensions are supported and enabled on the host:

# egrep "(vmx|svm)" /proc/cpuinfo
flags		: fpu vme de pse tsc ... svm ... skinit wdt npt lbrv svm_lock nrip_save
flags		: fpu vme de pse tsc ... svm ... skinit wdt npt lbrv svm_lock nrip_save

Enable virtualization extensions in your hardware's firmware configuration within the BIOS setup. Refer to your hardware documentation for further details on this.

⁠Verify client URI configuration

Verify that the URI of the client is configured as desired:

# virsh uri
vbox:///system

For example, this message shows the URI is connected to the VirtualBox hypervisor, not QEMU, and reveals a configuration error for a URI that is otherwise set to connect to a QEMU hypervisor. If the URI was correctly connecting to QEMU, the same message would appear instead as:

# virsh uri
qemu:///system

This situation occurs when there are other hypervisors present, which libvirt may speak to by default.

⁠Solution

After performing these tests, use the following command to view a list of guest virtual machines:

# virsh list --all

⁠A.18.16. Unable to connect to server at 'host:16509': Connection refused ... error: failed to connect to the hypervisor

⁠Symptom

While libvirtd should listen on TCP ports for connections, the connections fail:

# virsh -c qemu+tcp://host/system
error: unable to connect to server at 'host:16509': Connection refused
error: failed to connect to the hypervisor

The libvirt daemon is not listening on TCP ports even after changing configuration in /etc/libvirt/libvirtd.conf:

# grep listen_ /etc/libvirt/libvirtd.conf
listen_tls = 1
listen_tcp = 1
listen_addr = "0.0.0.0"

However, the TCP ports for libvirt are still not open after changing configuration:

# netstat -lntp | grep libvirtd
#

⁠Investigation

The libvirt daemon was started without the --listen option. Verify this by running this command:

# ps aux | grep libvirtd
root     27314  0.0  0.0 1000920 18304 ?       Sl   Feb16   1:19 libvirtd --daemon

The output does not contain the --listen option.

⁠Solution

Start the daemon with the --listen option.

To do this, modify the /etc/sysconfig/libvirtd file and uncomment the following line:

#LIBVIRTD_ARGS="--listen"

Then restart the libvirtd service with this command:

# /etc/init.d/libvirtd restart

⁠A.18.17. Common XML errors

The libvirt tool uses XML documents to store structured data. A variety of common errors occur with XML documents when they are passed to libvirt through the API. Several common XML errors — including erroneous XML tags, inappropriate values, and missing elements — are detailed below.

⁠A.18.17.1. Editing domain definition

Although it is not recommended, it is sometimes necessary to edit a guest virtual machine's (or a domain's) XML file manually. To access the guest's XML for editing, use the following command:

# virsh edit name_of_guest.xml

This command opens the file in a text editor with the current definition of the guest virtual machine. After finishing the edits and saving the changes, the XML is reloaded and parsed by libvirt. If the XML is correct, the following message is displayed:

# virsh edit name_of_guest.xml

Domain name_of_guest.xml XML configuration edited.

Important

When using the edit command in virsh to edit an XML document, save all changes before exiting the editor.

After saving the XML file, use the xmllint command to validate that the XML is well-formed, or the virt-xml-validate command to check for usage problems:

# xmllint --noout config.xml

# virt-xml-validate config.xml

If no errors are returned, the XML description is well-formed and matches the libvirt schema. While the schema does not catch all constraints, fixing any reported errors will further troubleshooting.

⁠XML documents stored by libvirt

These documents contain definitions of states and configurations for the guests. These documents are automatically generated and should not be edited manually. Errors in these documents contain the file name of the broken document. The file name is valid only on the host machine defined by the URI, which may refer to the machine the command was run on.

Errors in files created by libvirt are rare. However, one possible source of these errors is a downgrade of libvirt — while newer versions of libvirt can always read XML generated by older versions, older versions of libvirt may be confused by XML elements added in a newer version.

⁠A.18.17.2. XML syntax errors

Syntax errors are caught by the XML parser. The error message contains information for identifying the problem.

This example error message from the XML parser consists of three lines — the first line denotes the error message, and the two following lines contain the context and location of the XML code containing the error. The third line contains an indicator showing approximately where the error lies on the line above it:

error: (name_of_guest.xml):6: StartTag: invalid element name
<vcpu>2</vcpu><
-----------------^

⁠Information contained in this message:

⁠(name_of_guest.xml)

This is the file name of the document that contains the error. File names in parentheses are symbolic names to describe XML documents parsed from memory, and do not directly correspond to files on disk. File names that are not contained in parentheses are local files that reside on the target of the connection.

⁠6

This is the line number in the XML file that contains the error.

⁠StartTag: invalid element name

This is the error message from the libxml2 parser, which describes the specific XML error.

⁠A.18.17.2.1. Stray < in the document

⁠Symptom

The following error occurs:

error: (name_of_guest.xml):6: StartTag: invalid element name
<vcpu>2</vcpu><
-----------------^

⁠Investigation

This error message shows that the parser expects a new element name after the < symbol on line 6 of a guest's XML file.

Ensure line number display is enabled in your text editor. Open the XML file, and locate the text on line 6:

<domain type='kvm'>
   <name>name_of_guest</name>
<memory>524288</memory>
<vcpu>2</vcpu><

This snippet of a guest's XML file contains an extra < in the document:

⁠Solution

Remove the extra < or finish the new element.

⁠A.18.17.2.2. Unterminated attribute

⁠Symptom

The following error occurs:

error: (name_of_guest.xml):2: Unescaped '<' not allowed in attributes values
<name>name_of_guest</name>
--^

⁠Investigation

This snippet of a guest's XML file contains an unterminated element attribute value:

<domain type='kvm>
<name>name_of_guest</name>

In this case, 'kvm' is missing a second quotation mark. Strings of attribute values, such as quotation marks and apostrophes, must be opened and closed, similar to XML start and end tags.

⁠Solution

Correctly open and close all attribute value strings.

⁠A.18.17.2.3. Opening and ending tag mismatch

⁠Symptom

The following error occurs:

error: (name_of_guest.xml):61: Opening and ending tag mismatch: clock line 16 and domain
</domain>
---------^

⁠Investigation

The error message above contains three clues to identify the offending tag:

The message following the last colon, clock line 16 and domain, reveals that <clock> contains a mismatched tag on line 16 of the document. The last hint is the pointer in the context part of the message, which identifies the second offending tag.

Unpaired tags must be closed with />. The following snippet does not follow this rule and has produced the error message shown above:

<domain type='kvm'>
  ...
    <clock offset='utc'>

This error is caused by mismatched XML tags in the file. Every XML tag must have a matching start and end tag.

⁠Other examples of mismatched XML tags

The following examples produce similar error messages and show variations of mismatched XML tags.

This snippet contains an mismatch error for <features> because there is no end tag (</name>):

<domain type='kvm'>
 ...
 <features>
   <acpi/>
   <pae/>
 ...
 </domain>

This snippet contains an end tag (</name>) without a corresponding start tag:

<domain type='kvm'>
  </name>
  ...
</domain>

⁠Solution

Ensure all XML tags start and end correctly.

⁠A.18.17.2.4. Typographical errors in tags

⁠Symptom

The following error message appears:

error: (name_of_guest.xml):1: Specification mandate value for attribute ty
<domain ty pe='kvm'>
-----------^

⁠Investigation

XML errors are easily caused by a simple typographical error. This error message highlights the XML error — in this case, an extra white space within the word type — with a pointer.

<domain ty pe='kvm'>

These XML examples will not parse correctly because of typographical errors such as a missing special character, or an additional character:

<domain type 'kvm'>

<dom#ain type='kvm'>

⁠Solution

To identify the problematic tag, read the error message for the context of the file, and locate the error with the pointer. Correct the XML and save the changes.

⁠A.18.17.3. Logic and configuration errors

A well-formatted XML document can contain errors that are correct in syntax but libvirt cannot parse. Many of these errors exist, with two of the most common cases outlined below.

⁠A.18.17.3.1. Vanishing parts

⁠Symptom

Parts of the change you have made do not show up and have no effect after editing or defining the domain. The define or edit command works, but when dumping the XML once again, the change disappears.

⁠Investigation

This error likely results from a broken construct or syntax that libvirt does not parse. The libvirt tool will generally only look for constructs it knows but ignore everything else, resulting in some of the XML changes vanishing after libvirt parses the input.

⁠Solution

Validate the XML input before passing it to the edit or define commands. The libvirt developers maintain a set of XML schemas bundled with libvirt which define the majority of the constructs allowed in XML documents used by libvirt.

Validate libvirt XML files using the following command:

# virt-xml-validate libvirt.xml

If this command passes, libvirt will likely understand all constructs from your XML, except if the schemas cannot detect options which are valid only for a given hypervisor. Any XML generated by libvirt as a result of a virsh dump command, for example, should validate without error.

⁠A.18.17.3.2. Incorrect drive device type

⁠Symptom

The definition of the source image for the CD-ROM virtual drive is not present, despite being added:

# virsh dumpxml domain
<domain type='kvm'>
  ...
  <disk type='block' device='cdrom'>
    <driver name='qemu' type='raw'/>
    <target dev='hdc' bus='ide'/>
    <readonly/>
  </disk>
  ...
</domain>

⁠Solution

Correct the XML by adding the missing <source> parameter as follows:

<disk type='block' device='cdrom'>
  <driver name='qemu' type='raw'/>
  <source file='/path/to/image.iso'/>
  <target dev='hdc' bus='ide'/>
  <readonly/>
</disk>

A type='block' disk device expects that the source is a physical device. To use the disk with an image file, use type='file' instead.

⁠Appendix B. Virtualization restrictions

This appendix covers additional support and product restrictions of the virtualization packages in Red Hat Enterprise Linux 7.

⁠B.1. KVM restrictions

The following restrictions apply to the KVM hypervisor:

⁠Maximum vCPUs per guest

Red Hat Enterprise Linux 7.1 and above supports 240 vCPUs per guest, up from 160 in Red Hat Enterprise Linux 7.0.

⁠Nested virtualization

Nested virtualization is available as a Technology Preview in Red Hat Enterprise Linux 7.2. This feature enables KVM to launch guests that can act as hypervisors and create their own guests.

⁠Constant TSC bit

Systems without a Constant Time Stamp Counter require additional configuration. Refer to Chapter 9, KVM guest timing management for details on determining whether you have a Constant Time Stamp Counter and configuration steps for fixing any related issues.

⁠Memory overcommit

KVM supports memory overcommit and can store the memory of guest virtual machines in swap. A virtual machine will run slower if it is swapped frequently. See the following Red Hat Customer Portal article on safely and efficiently determining the size of the swap partition https://access.redhat.com/site/solutions/15244. When KSM is used for memory overcommitting, make sure that the swap size follows the recommendations described in this article.

Important

When device assignment is in use, all guest virtual machine memory must be statically pre-allocated to enable DMA with the assigned device. Memory overcommit is therefore not supported on guest virtual machines that are making use of device assignment.

⁠CPU overcommit

It is not recommended to have more than 10 virtual CPUs per physical processor core. Customers are encouraged to use a capacity planning tool in order to determine the vCPU overcommit ratio. Estimating an ideal ratio is difficult as it is highly dependent on each workload. For instance, a guest virtual machine may consume 100% CPU in one use case, and multiple guests may be completely idle on another.

Refer to Section 8.3, “Overcommitting virtualized CPUs (vCPUs)” for recommendations on overcommitting vCPUs.

⁠Emulated SCSI adapters

SCSI device emulation is only supported with the virtio-scsi para-virtualized host bus adapter (HBA). Emulated SCSI HBAs are not supported with KVM in Red Hat Enterprise Linux.

⁠Emulated IDE devices

KVM is limited to a maximum of four virtualized (emulated) IDE devices per virtual machine.

⁠Para-virtualized devices

Para-virtualized devices are also known as VirtIO devices. They are purely virtual devices designed to work optimally in a virtual machine.

Red Hat Enterprise Linux 7 supports 32 PCI device slots per virtual machine bus, and 8 PCI functions per device slot. This gives a theoretical maximum of 256 PCI functions per bus when multi-function capabilities are enabled in the virtual machine, and PCI bridges are used. Each PCI bridge adds a new bus, potentially enabling another 256 device addresses. However, some buses do not make all 256 device addresses available for the user; for example, the root bus has several built-in devices occupying slots.

Refer to Chapter 18, Guest virtual machine device configuration for more information on devices and Section 18.1.5, “Creating PCI bridges” for more information on PCI bridges.

⁠PCI device assignment

PCI device assignment (attaching PCI devices to virtual machines) requires host systems to have AMD IOMMU or Intel VT-d support to enable device assignment of PCI-e devices.

Red Hat Enterprise Linux 7 has limited PCI configuration space access by guest device drivers. This limitation could cause drivers that are dependent on device capabilities or features present in the extended PCI configuration space, to fail configuration.

There is a limit of 32 total assigned devices per Red Hat Enterprise Linux 7 virtual machine. This translates to 32 total PCI functions, regardless of the number of PCI bridges present in the virtual machine or how those functions are combined to create multi-function slots.

Platform support for interrupt remapping is required to fully isolate a guest with assigned devices from the host. Without such support, the host may be vulnerable to interrupt injection attacks from a malicious guest. In an environment where guests are trusted, the administrator may opt-in to still allow PCI device assignment using the allow_unsafe_interrupts option to the vfio_iommu_type1 module. This may either be done persistently by adding a .conf file (e.g. local.conf) to /etc/modprobe.d containing the following:

options vfio_iommu_type1 allow_unsafe_interrupts=1

or dynamically using the sysfs entry to do the same:

# echo 1 > /sys/module/vfio_iommu_type1/parameters/allow_unsafe_interrupts

⁠Migration restrictions

Device assignment refers to physical devices that have been exposed to a virtual machine, for the exclusive use of that virtual machine. Because device assignment uses hardware on the specific host where the virtual machine runs, migration and save/restore are not supported when device assignment is in use. If the guest operating system supports hot-plugging, assigned devices can be removed prior to the migration or save/restore operation to enable this feature.

Live migration is only possible between hosts with the same CPU type (that is, Intel to Intel or AMD to AMD only).

For live migration, both hosts must have the same value set for the No eXecution (NX) bit, either on or off.

For migration to work, cache=none must be specified for all block devices opened in write mode.

Warning

Failing to include the cache=none option can result in disk corruption.

⁠Storage restrictions

There are risks associated with giving guest virtual machines write access to entire disks or block devices (such as /dev/sdb). If a guest virtual machine has access to an entire block device, it can share any volume label or partition table with the host machine. If bugs exist in the host system's partition recognition code, this can create a security risk. Avoid this risk by configuring the host machine to ignore devices assigned to a guest virtual machine.

Warning

Failing to adhere to storage restrictions can result in risks to security.

⁠SR-IOV restrictions

SR-IOV is only thoroughly tested with the following devices (other SR-IOV devices may work but have not been tested at the time of release):

Intel® 82576NS Gigabit Ethernet Controller (igb driver)

Intel® 82576EB Gigabit Ethernet Controller (igb driver)

Intel® 82599ES 10 Gigabit Ethernet Controller (ixgbe driver)

Intel® 82599EB 10 Gigabit Ethernet Controller (ixgbe driver)

⁠Core dumping restrictions

Because core dumping is currently implemented on top of migration, it is not supported when device assignment is in use.

⁠Realtime kernel

KVM currently does not support the realtime kernel, and thus cannot be used on Red Hat Enterprise Linux for Real Time.

⁠IBM Power Systems

Red Hat Enterprise Linux with KVM is supported on AMD64 and Intel 64 systems, but not on IBM Power Systems. Red Hat currently provides a POWER8-based solution with Red Hat Enterprise Virtualization for IBM Power Systems.

Note

More information on version support and installation procedures can be found in the related Knowledge Base article.

⁠B.2. Application restrictions

There are aspects of virtualization which make it unsuitable for certain types of applications.

Applications with high I/O throughput requirements should use KVM's para-virtualized drivers (virtio drivers) for fully-virtualized guests. Without the virtio drivers certain applications may be unpredictable under heavy I/O loads.

The following applications should be avoided due to high I/O requirements:

kdump server

netdump server

You should carefully evaluate applications and tools that heavily utilize I/O or those that require real-time performance. Consider the virtio drivers or PCI device assignment for increased I/O performance. Refer to Chapter 6, KVM Para-virtualized (virtio) Drivers for more information on the virtio drivers for fully virtualized guests. Refer to Chapter 18, Guest virtual machine device configuration for more information on PCI device assignment.

Applications suffer a small performance loss from running in virtualized environments. The performance benefits of virtualization through consolidating to newer and faster hardware should be evaluated against the potential application performance issues associated with using virtualization.

⁠B.3. Other restrictions

For the list of all other restrictions and issues affecting virtualization read the Red Hat Enterprise Linux 7 Release Notes. The Red Hat Enterprise Linux 7 Release Notes cover the present new features, known issues and restrictions as they are updated or discovered.

⁠B.4. Storage support

The supported storage methods for virtual machines are:

files on local storage,

physical disk partitions,

locally connected physical LUNs,

LVM partitions,

NFS shared file systems,

iSCSI,

GFS2 clustered file systems,

Fibre Channel-based LUNs, and

Fibre Channel over Ethernet (FCoE).

⁠B.5. USB 3 / xHCI Support

USB 3 (xHCI) USB host adapter emulation is supported in Red Hat Enterprise Linux 7.2. All USB speeds are available, meaning any generation of USB device can be plugged into a xHCI bus. Additionally, no companion controllers (for USB 1 devices) are required. It should be noted however, that USB 3 bulk streams are not supported.

Advantages of xHCI:

Virtualization-compatible hardware design, meaning xHCI emulation requires less CPU resources than previous versions due to reduced polling overhead.

USB passthrough of USB 3 devices is available.

Limitations of xHCI:

Not supported for Red Hat Enterprise Linux 5 guests.

See Figure 18.18, “Domain XML example for USB3/xHCI devices” for a domain XML device example for xHCI devices.

⁠Appendix C. Additional resources

To learn more about virtualization and Red Hat Enterprise Linux, refer to the following resources.

⁠C.1. Online resources

http://www.libvirt.org/ is the official website for the libvirt virtualization API.

http://virt-manager.et.redhat.com/ is the project website for the Virtual Machine Manager (virt-manager), the graphical application for managing virtual machines.

Red Hat Enterprise Virtualization

http://www.redhat.com/products/cloud-computing/virtualization/

Red Hat Documentation

https://access.redhat.com/site/documentation/

Virtualization technologies overview

http://virt.kernelnewbies.org

Red Hat Emerging Technologies group

http://et.redhat.com

⁠C.2. Installed documentation

man virsh and /usr/share/doc/libvirt-<version-number> — Contains sub commands and options for the virsh virtual machine management utility as well as comprehensive information about the libvirt virtualization library API.

/usr/share/doc/gnome-applet-vm-<version-number> — Documentation for the GNOME graphical panel applet that monitors and manages locally-running virtual machines.

/usr/share/doc/libvirt-python-<version-number> — Provides details on the Python bindings for the libvirt library. The libvirt-python package allows python developers to create programs that interface with the libvirt virtualization management library.

/usr/share/doc/python-virtinst-<version-number> — Provides documentation on the virt-install command that helps in starting installations of Fedora and Red Hat Enterprise Linux related distributions inside of virtual machines.

/usr/share/doc/virt-manager-<version-number> — Provides documentation on the Virtual Machine Manager, which provides a graphical tool for administering virtual machines.

⁠Appendix D. Working with IOMMU Groups ⁠[1]

Introduced in Red Hat Enterprise Linux 7, VFIO, or Virtual Function I/O, is a set of Linux kernel modules that provide a user-space driver framework. This framework enables secure, IOMMU protected, device access for user-space drivers. VFIO enables user-space drivers such as the DPDK (Data Plane Development Kit) as well as the more common PCI device assignment. One of the features of VFIO is its use of the IOMMU (Input–output memory management unit). IOMMU groups isolate devices and prevent unintentional DMA (Direct Memory Access) between two devices running on the same host physical machine, which would impact on host and guest functionality. IOMMU groups are now available in Red Hat Enterprise Linux 7, which is a big improvement over the legacy KVM device assignment that is available in Red Hat Enterprise Linux 6. This appendix highlights the following:

An overview of IOMMU groups

The importance of device isolation

VFIO benefits

⁠D.1. IOMMU Overview

An IOMMU creates a virtual address space for the device, where each I/O Virtual Address (IOVA) may translate to different addresses in the physical system memory. When the translation is completed the devices are connected to a different address within the physical system's memory. Without an IOMMU, all devices have a shared, flat view of the physical memory because they lack memory address translation. With an IOMMU, devices receive a new address space, the IOVA space, which is useful for device assignment.

Different IOMMUs have different levels of functionality. In the past, IOMMUs were limited providing only translation, and often only for a small window of the address space. For example, the IOMMU would only reserve a small window (1GB or less) of IOVA space in low memory, which was shared by multiple devices. The AMD GART (graphics address remapping table), when used as a general purpose IOMMU, is an example of this model. These classic IOMMUs mostly provided two capabilities: bounce buffers and address coalescing.

Bounce buffers are necessary when the addressing capabilities of the device are less than that of the platform. For example, if a device's address space is limited to 4GB (32 bits) of memory and the driver was to allocate to a buffer above 4GB, the device would not be able to directly access the buffer. Such a situation necessitates using a bounce buffer; a buffer space located in lower memory, where the device can perform a DMA operation. The data in the buffer is only copied to the driver's allocated buffer on completion of the operation. In other words, the buffer is bounced from a lower memory address to a higher memory address. IOMMUs avoid bounce buffering by providing an IOVA translation within the device's address space. This allows the device to perform a DMA operation directly into the buffer, even when the buffer extends beyond the physical address space of the device. Historically, this IOMMU feature was often the exclusive use case for the IOMMU, but with the adoption of PCI-Express (PCIe), the ability to support addressing above 4GB is required for all non-legacy endpoints.

In traditional memory allocation, blocks of memory are assigned and freed based on the needs of the application. Using this method creates memory gaps scattered throughout the physical address space. It would be better if the memory gaps were coalesced so they can be used more efficiently, or in basic terms it would be better if the memory gaps were gathered together. The IOMMU coalesces these scattered memory lists through the IOVA space, sometimes referred to as scatter-gather lists. In doing so the IOMMU creates contiguous DMA operations and ultimately increases the efficiency of the I/O performance. In the simplest example, a driver may allocate two 4KB buffers that are not contiguous in the physical memory space. The IOMMU can allocate a contiguous range for these buffers allowing the I/O device to do a single 8KB DMA rather than two separate 4KB DMAs.

Although memory coalescing and bounce buffering are important for high performance I/O on the host, the IOMMU feature that is essential for a virtualization environment is the isolation capabilities of modern IOMMUs. Isolation was not possible on a wide scale prior to PCI-Express because conventional PCI does not tag transactions with an ID of the requesting device (requester ID). Even though PCI-X included some degree of a requester ID,the rules for interconnecting devices taking ownership of the transaction did not provide complete support for device isolation. With PCIe, each device’s transaction is tagged with a requester ID unique to the device (the PCI bus/device/function number, often abbreviated as BDF), which is used to reference a unique IOVA table for that device. Now that isolation is possible, the IOVA space can not only be used for translation operations such as offloading unreachable memory and coalescing memory, but it can also be used to restrict DMA access from the device. This allows devices to be isolated from each other preventing duplicate assignment of memory spaces, which is essential for proper guest virtual machine device management. Using these features on a guest virtual machine, involves populating the IOVA space for the assigned device with the guest physical to host physical memory mappings for the virtual machine. Once this is done, the device transparently performs DMA operations in the guest virtual machine’s address space.

⁠D.2. A deep-dive into IOMMU groups

An IOMMU group is defined as the smallest set of devices which can be considered isolated from the IOMMU’s perspective. The first step to achieve isolation is granularity. If the IOMMU cannot differentiate devices into separate IOVA spaces, they are not isolated. For example, if multiple devices were to alias to the same IOVA space, then the IOMMU would not be able to distinguish between them. This is the reason why a typical x86 PC will group all conventional-PCI devices together, with all of them aliased to the same requester ID, the PCIe-to-PCI bridge. Legacy KVM device assignment allows a user to assign these conventional-PCI devices separately, but the configuration fails because the IOMMU cannot distinguish between the devices. As VFIO is governed by IOMMU groups, it prevents any configuration which violates this most basic requirement of IOMMU granularity.

The next step is to determine whether the transactions from the device actually reach the IOMMU. The PCIe specification allows for transactions to be re-routed within the interconnect fabric. A PCIe downstream port can re-route a transaction from one downstream device to another. The downstream ports of a PCIe switch may be interconnected to allow re-routing from one port to another. Even within a multifunction endpoint device, a transaction from one function may be delivered directly to another function. These transactions from one device to another are called peer-to-peer transactions and can destroy the isolation of devices operating in separate IOVA spaces. Imagine for instance, if the network interface card assigned to a guest virtual machine, attempts a DMA write operation to a virtual address within its own IOVA space. However in the physical space, that same address belongs to a peer disk controller owned by the host. As the IOVA to physical translation for the device is only performed at the IOMMU, any interconnect attempting to optimize the data path of that transaction could mistakenly redirect the DMA write operation to the disk controller before it gets to the IOMMU for translation.

To solve this problem, the PCI Express specification includes support for PCIe Access Control Services, or ACS, which provides visibility and control of these redirects. This is an essential component for isolating devices from one another, which is often missing in interconnects and multifunction endpoints. Without ACS support at every level from the device to the IOMMU, it must be assumed that redirection is possible. This will therefore break the isolation of all devices below the point lacking ACS support in the PCI topology. IOMMU groups in a PCI environment take this isolation into account, grouping together devices which are capable of untranslated peer-to-peer DMA.

In summary, the IOMMU group represents the smallest set of devices for which the IOMMU has visibility and which is isolated from other groups. VFIO uses this information to enforce safe ownership of devices for userspace. With the exception of bridges, root ports, and switches (all examples of interconnect fabric), all devices within an IOMMU group must be bound to a VFIO device driver or known safe stub driver. For PCI, these drivers are vfio-pci and pci-stub. pci-stub is allowed simply because it is known that the host does not interact with devices via this driver ⁠[2] . If an error occurs indicating the group is not viable when using VFIO, it means that all of the devices in the group need to be bound to an appropriate host driver. Using virsh nodedev-dumpxml to explore the composition of an IOMMU group and virsh nodedev-detach to bind devices to VFIO compatible drivers, will help resolve such problems.

⁠D.3. How to identify and assign IOMMU Groups

This example demonstrates how to identify and assign the PCI devices that are present on the target system. For additional examples and information, refer to Section 18.6, “Assigning GPU devices”.

⁠

Procedure D.1. IOMMU groups

List the devices

Identify the devices in your system by running the virsh nodev-list [device-type] command as root in a terminal. This example demonstrates how to locate the PCI devices. The output has been amended for brevity.

# virsh nodedev-list pci

pci_0000_00_00_0
pci_0000_00_01_0
pci_0000_00_03_0
pci_0000_00_07_0
...
pci_0000_00_1c_0
pci_0000_00_1c_4
…
pci_0000_01_00_0
pci_0000_01_00_1
...
pci_0000_03_00_0
pci_0000_03_00_1
pci_0000_04_00_0
pci_0000_05_00_0
pci_0000_06_0d_0

Locate the IOMMU grouping of a device

For each device listed, further information about the device, including the IOMMU grouping, can be found using the virsh nodedev-dumpxml [name-of-device] command. For example, to find the IOMMU grouping for the PCI device named pci_0000_04_00_0 (PCI address 0000:04:00.0), run the following command as root in a terminal:

# virsh nodedev-dumpxml pci_0000_04_00_0

This command generates a XML dump similar to the one shown.

⁠

​
​
​<device>
​  <name>pci_0000_04_00_0</name>
​  <path>/sys/devices/pci0000:00/0000:00:1c.0/0000:04:00.0</path>
​  <parent>pci_0000_00_1c_0</parent>
​  <capability type='pci'>
​    <domain>0</domain>
​    <bus>4</bus>
​    <slot>0</slot>
​    <function>0</function>
​    <product id='0x10d3'>82574L Gigabit Network Connection</product>
​    <vendor id='0x8086'>Intel Corporation</vendor>
​    <iommuGroup number='8'>   <!--This is the element block you will need to use-->
​      <address domain='0x0000' bus='0x00' slot='0x1c' function='0x0'/>
​      <address domain='0x0000' bus='0x00' slot='0x1c' function='0x4'/>
​      <address domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
​      <address domain='0x0000' bus='0x05' slot='0x00' function='0x0'/>
​    </iommuGroup>
​    <pci-express>
​      <link validity='cap' port='0' speed='2.5' width='1'/>
​      <link validity='sta' speed='2.5' width='1'/>
​    </pci-express>
​  </capability>
​</device>

Figure D.1. IOMMU Group XML

View the PCI data

In the output collected above, there is one IOMMU group with 4 devices. This is an example of a multi-function PCIe root port without ACS support. The two functions in slot 0x1c are PCIe root ports, which can be identified by running the lspci command (from the pciutils package) in a terminal as root:

# lspci -s 1c

00:1c.0 PCI bridge: Intel Corporation 82801JI (ICH10 Family) PCI Express Root Port 1
00:1c.4 PCI bridge: Intel Corporation 82801JI (ICH10 Family) PCI Express Root Port 5

Repeat this step for the two PCIe devices on buses 0x04 and 0x05, which are endpoint devices.

# lspci -s 4
04:00.0 Ethernet controller: Intel Corporation 82574L Gigabit Network Connection This is used in the next step and is called 04:00.0
# lspci -s 5 This is used in the next step and is called 05:00.0
05:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5755 Gigabit Ethernet PCI Express (rev 02)

Assign the endpoints to the guest virtual machine

In order to assign either one of the endpoints to a virtual machine, the endpoint which you are not assigning at the moment, must be bound to a VFIO compatible driver so that the IOMMU group is not split between user and host drivers. If for example, using the output received above, you were to configuring a virtual machine with only 04:00.0, the virtual machine will fail to start unless 05:00.0 is detached from host drivers. To detach 05:00.0, run the virsh nodedev-detach command as root:

# virsh nodedev-detach pci_0000_05_00_0
Device pci_0000_05_00_0 detached

Assigning both endpoints to the virtual machine is another option for resolving this issue. Note that libvirt will automatically perform this operation for the attached devices when using the yes value for the managed attribute within the <hostdev> element. For example: <hostdev mode='subsystem' type='pci' managed='yes'>. Refer to the Note for more information.

Note

libvirt has two ways to handle PCI devices. They can be either managed or unmanaged. This is determined by the value given to the managed attribute within the <hostdev> element. When the device is managed, libvirt automatically detaches the device from the existing driver and then assigns it to the virtual machine by binding it to vfio-pci on boot (for the virtual machine). When the virtual machine is shutdown or deleted or the PCI device is detached from the virtual machine, libvirt unbinds the device from vfio-pci and rebinds it to the original driver. If the device is unmanaged, libvirt will not automate the process and you will have to ensure all of these management aspects as described are done before assigning the device to a virtual machine, and after the device is no longer used by the virtual machine you will have to reassign the devices as well. Failure to do these actions in an unmanaged device will cause the virtual machine to fail. Therefore, it may be easier to make sure that libvirt manages the device.

⁠D.4. IOMMU strategies and case uses

There are many ways to handle IOMMU groups that contain more devices than desired. For a plug-in card, the first option would be to determine whether installing the card into a different slot produces the desired grouping. On a typical Intel chipset, PCIe root ports are provided via both the processor and the PCH (Platform Controller Hub). The capabilities of these root ports can be very different. Red Hat Enterprise Linux 7 has support for exposing the isolation of numerous PCH root ports, even though many of them do not have native PCIe ACS support. Therefore, these root ports are good targets for creating smaller IOMMU groups. With Intel® Xeon® class processors (E5 series and above) and “High End Desktop Processors”, the processor-based PCIe root ports typically provide native support for PCIe ACS, however the lower-end client processors, such as the Core™ i3, i5, and i7 and Xeon® E3 processors do not. For these systems, the PCH root ports generally provide the most flexible isolation configurations.

Another option is to work with the hardware vendors to determine whether isolation is present and quirk the kernel to recognize this isolation. This is generally a matter of determining whether internal peer-to-peer between functions is possible, or in the case of downstream ports, also determining whether redirection is possible. The Red Hat Enterprise Linux 7 kernel includes numerous quirks for such devices and Red Hat Customer Support can help you work with hardware vendors to determine if ACS-equivalent isolation is available and how best to incorporate similar quirks into the kernel to expose this isolation. For hardware vendors, please note that multifunction endpoints that do not support peer-to-peer can expose this using a single static ACS table in configuration space, exposing no capabilities. Adding such a capability to the hardware will allow the kernel to automatically detect the functions as isolated and eliminate this issue for all users of your hardware.

In cases where the above suggestions are not available, a common reaction is that the kernel should provide an option to disable these isolation checks for certain devices or certain types of devices, specified by the user. Often the argument is made that previous technologies did not enforce isolation to this extent and everything “worked fine”. Unfortunately, bypassing these isolation features leads to an unsupportable environment. Not knowing that isolation exists, means not knowing whether the devices are actually isolated and it is best to find out before disaster strikes. Gaps in the isolation capabilities of devices may be extremely hard to trigger and even more difficult to trace back to device isolation as the cause. VFIO’s job is first and foremost to protect the host kernel from user owned devices and IOMMU groups are the mechanism used by VFIO to ensure that isolation.

In summary, by being built on top of IOMMU groups, VFIO is able to provide an increased degree of security and isolation between devices than was possible using legacy KVM device assignment. This isolation is now enforced at the Linux kernel level, allowing the kernel to protect itself and prevent dangerous configurations for the user. Additionally, hardware vendors should be encouraged to support PCIe ACS support, not only in multifunction endpoint devices, but also in chip sets and interconnect devices. For existing devices lacking this support, Red Hat may be able to work with hardware vendors to determine whether isolation is available and add Linux kernel support to expose this isolation.

[1] Content for this appendix was provided by Alex Williamson, Principal Software Engineer.

[2] The exception is legacy KVM device assignment, which often interacts with the device while bound to the pci-stub driver. Red Hat Enterprise Linux 7 does not include legacy KVM device assignment, avoiding this interaction and potential conflict. Therefore mixing the use of VFIO and legacy KVM device assignment within the same IOMMU group is not recommended.

⁠Appendix E. NetKVM Driver Parameters

Important

Red Hat Enterprise Linux 7 Windows guests are only supported under specific subscription programs such as Advanced Mission Critical (AMC). If you are unsure whether your subscription model includes support for Windows guests, please contact customer support.

After the NetKVM driver is installed, you can configure it to better suit your environment. The parameters listed in this section can be configured in the Windows Device Manager (devmgmt.msc).

Important

Modifying the driver's parameters causes Windows to re-load that driver. This interrupts existing network activity.

⁠

Procedure E.1. Configuring NetKVM Parameters

Open Device Manager

Click on the Start button. In the right-hand pane, right-click on Computer, and click Manage. If prompted, click Continue on the User Account Control window. This opens the Computer Management window.

In the left-hand pane of the Computer Management window, click Device Manager.

Locate the correct device

In the central pane of the Computer Management window, click on the + symbol beside Network adapters.

Under the list of Red Hat VirtIO Ethernet Adapter devices, double-click on NetKVM. This opens the Properties window for that device.

View device parameters

In the Properties window, click on the Advanced tab.

Modify device parameters

Click on the parameter you wish to modify to display the options for that parameter.

Modify the options as appropriate, then click on OK to save your changes.

⁠E.1. Configurable parameters for NetKVM

⁠

Logging parameters

⁠Logging.Enable

A Boolean value that determines whether logging is enabled. The default value is 1 (enabled).

⁠Logging.Level

An integer that defines the logging level. As the integer increases, so does the verbosity of the log. The default value is 0 (errors only). 1-2 adds configuration messages. 3-4 adds packet flow information. 5-6 adds interrupt and DPC level trace information.

Important

High logging levels will slow down your guest virtual machine.

⁠Logging.Statistics(sec)

An integer that defines whether log statistics are printed, and the time in seconds between each periodical statistics printout. The default value is 0 (no logging statistics).

⁠

Initial parameters

⁠Assign MAC

A string that defines the locally-administered MAC address for the para-virtualized NIC. This is not set by default.

⁠Init.ConnectionRate(Mb)

An integer that represents the connection rate in megabytes. The default value for Windows 2008 and later is 10000.

⁠Init.Do802.1PQ

A Boolean value that enables Priority/VLAN tag population and removal support. The default value is 1 (enabled).

⁠Init.UseMergedBuffers

A Boolean value that enables merge-able RX buffers. The default value is 1 (enabled).

⁠Init.UsePublishEvents

A Boolean value that enables published event use. The default value is 1 (enabled).

⁠Init.MTUSize

An integer that defines the maximum transmission unit (MTU). The default value is 1500. Any value from 500 to 65500 is acceptable.

⁠Init.IndirectTx

Controls whether indirect ring descriptors are in use. The default value is Disable, which disables use of indirect ring descriptors. Other valid values are Enable, which enables indirect ring descriptor usage; and Enable*, which enables conditional use of indirect ring descriptors.

⁠Init.MaxTxBuffers

An integer that represents the amount of TX ring descriptors that will be allocated. The default value is 1024. Valid values are: 16, 32, 64, 128, 256, 512, or 1024.

⁠Init.MaxRxBuffers

An integer that represents the amount of RX ring descriptors that will be allocated. The default value is 256. Valid values are: 16, 32, 64, 128, 256, 512, or 1024.

⁠Offload.Tx.Checksum

Specifies the TX checksum offloading mode.

In Red Hat Enterprise Linux 7, the valid values for this parameter are All (the default), which enables IP, TCP and UDP checksum offloading for both IPv4 and IPv6; TCP/UDP(v4,v6), which enables TCP and UDP checksum offloading for both IPv4 and IPv6; TCP/UDP(v4), which enables TCP and UDP checksum offloading for IPv4 only; and TCP(v4), which enables only TCP checksum offloading for IPv4 only.

⁠Offload.Tx.LSO

A Boolean value that enables TX TCP Large Segment Offload (LSO). The default value is 1 (enabled).

⁠Offload.Rx.Checksum

Specifies the RX checksum offloading mode.

In Red Hat Enterprise Linux 7, the valid values for this parameter are All (the default), which enables IP, TCP and UDP checksum offloading for both IPv4 and IPv6; TCP/UDP(v4,v6), which enables TCP and UDP checksum offloading for both IPv4 and IPv6; TCP/UDP(v4), which enables TCP and UDP checksum offloading for IPv4 only; and TCP(v4), which enables only TCP checksum offloading for IPv4 only.

⁠

Test and debug parameters

Important

Test and debug parameters should only be used for testing or debugging; they should not be used in production.

⁠TestOnly.DelayConnect(ms)

The period for which to delay connection upon startup, in milliseconds. The default value is 0.

⁠TestOnly.DPCChecking

Sets the DPC checking mode. 0 (the default) disables DPC checking. 1 enables DPC checking; each hang test verifies DPC activity and acts as if the DPC was spawned. 2 clears the device interrupt status and is otherwise identical to 1.

⁠TestOnly.Scatter-Gather

A Boolean value that determines whether scatter-gather functionality is enabled. The default value is 1 (enabled). Setting this value to 0 disables scatter-gather functionality and all dependent capabilities.

⁠TestOnly.InterruptRecovery

A Boolean value that determines whether interrupt recovery is enabled. The default value is 1 (enabled).

⁠TestOnly.PacketFilter

A Boolean value that determines whether packet filtering is enabled. The default value is 1 (enabled).

⁠TestOnly.BatchReceive

A Boolean value that determines whether packets are received in batches, or singularly. The default value is 1, which enables batched packet receipt.

⁠TestOnly.Promiscuous

A Boolean value that determines whether promiscuous mode is enabled. The default value is 0 (disabled).

⁠TestOnly.AnalyzeIPPackets

A Boolean value that determines whether the checksum fields of outgoing IP packets are tested and verified for debugging purposes. The default value is 0 (no checking).

⁠TestOnly.RXThrottle

An integer that determines the number of receive packets handled in a single DPC. The default value is 1000.

⁠TestOnly.UseSwTxChecksum

A Boolean value that determines whether hardware checksum is enabled. The default value is 0 (disabled).

⁠Appendix F. Revision History

Revision History

Revision 2-24
Thu Dec 17 2015
Laura Novich

Republished guide and fixed multiple issues (BZ#1286552)

Revision 2-23
Sun Nov 22 2015
Laura Novich

Republished guide

Revision 2-21
Thu Nov 12 2015
Laura Novich

Edited and Revised Domain XML chapter (BZ#1097365)

Removed vmhostmd Appendix(BZ#1277548)

Added IOMMU Appendix (BZ#1179072)

Added Warning message for Windows guests (BZ#1262007)

Edited Virsh chapter (BZ#1206207)

Moved KSM Chapter to Virtualization Tuning and Optimization Guide (BZ#1272286)

Backported Bonding Modes information to Networking Chapter

Revision 2-19
Thu Oct 08 2015
Jiri Herrmann

Cleaned up the Revision History

Revision 2-17
Thu Aug 27 2015
Dayle Parker

Updates for the 7.2 beta release