xenbus_dev_backend.c

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#include <linux/slab.h>
#include <linux/types.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/miscdevice.h>
#include <linux/module.h>
#include <linux/capability.h>

#include <xen/xen.h>
#include <xen/page.h>
#include <xen/xenbus.h>
#include <xen/xenbus_dev.h>
#include <xen/grant_table.h>
#include <xen/events.h>
#include <asm/xen/hypervisor.h>

#include "xenbus_comms.h"

MODULE_LICENSE("GPL");

static int xenbus_backend_open(struct inode *inode, struct file *filp)
{
    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    return nonseekable_open(inode, filp);
}

static long xenbus_alloc(domid_t domid)
{
    struct evtchn_alloc_unbound arg;
    int err = -EEXIST;

    xs_suspend();

    /* If xenstored_ready is nonzero, that means we have already talked to
     * xenstore and set up watches. These watches will be restored by
     * xs_resume, but that requires communication over the port established
     * below that is not visible to anyone until the ioctl returns.
     *
     * This can be resolved by splitting the ioctl into two parts
     * (postponing the resume until xenstored is active) but this is
     * unnecessarily complex for the intended use where xenstored is only
     * started once - so return -EEXIST if it's already running.
     */
    if (xenstored_ready)
        goto out_err;

    gnttab_grant_foreign_access_ref(GNTTAB_RESERVED_XENSTORE, domid,
            virt_to_mfn(xen_store_interface), 0 /* writable */);

    arg.dom = DOMID_SELF;
    arg.remote_dom = domid;

    err = HYPERVISOR_event_channel_op(EVTCHNOP_alloc_unbound, &arg);
    if (err)
        goto out_err;

    if (xen_store_evtchn > 0)
        xb_deinit_comms();

    xen_store_evtchn = arg.port;

    xs_resume();

    return arg.port;

 out_err:
    xs_suspend_cancel();
    return err;
}

static long xenbus_backend_ioctl(struct file *file, unsigned int cmd, unsigned long data)
{
    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    switch (cmd) {
        case IOCTL_XENBUS_BACKEND_EVTCHN:
            if (xen_store_evtchn > 0)
                return xen_store_evtchn;
            return -ENODEV;

        case IOCTL_XENBUS_BACKEND_SETUP:
            return xenbus_alloc(data);

        default:
            return -ENOTTY;
    }
}

static int xenbus_backend_mmap(struct file *file, struct vm_area_struct *vma)
{
    size_t size = vma->vm_end - vma->vm_start;

    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    if ((size > PAGE_SIZE) || (vma->vm_pgoff != 0))
        return -EINVAL;

    if (remap_pfn_range(vma, vma->vm_start,
                virt_to_pfn(xen_store_interface),
                size, vma->vm_page_prot))
        return -EAGAIN;

    return 0;
}

static const struct file_operations xenbus_backend_fops = {
    .open = xenbus_backend_open,
    .mmap = xenbus_backend_mmap,
    .unlocked_ioctl = xenbus_backend_ioctl,
};

static struct miscdevice xenbus_backend_dev = {
    .minor = MISC_DYNAMIC_MINOR,
    .name = "xen/xenbus_backend",
    .fops = &xenbus_backend_fops,
};

static int __init xenbus_backend_init(void)
{
    int err;

    if (!xen_initial_domain())
        return -ENODEV;

    err = misc_register(&xenbus_backend_dev);
    if (err)
        printk(KERN_ERR "Could not register xenbus backend device\n");
    return err;
}

static void __exit xenbus_backend_exit(void)
{
    misc_deregister(&xenbus_backend_dev);
}

module_init(xenbus_backend_init);
module_exit(xenbus_backend_exit);