lguest.c

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/*P:100
 * This is the Launcher code, a simple program which lays out the "physical"
 * memory for the new Guest by mapping the kernel image and the virtual
 * devices, then opens /dev/lguest to tell the kernel about the Guest and
 * control it.
:*/
#define _LARGEFILE64_SOURCE
#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <err.h>
#include <stdint.h>
#include <stdlib.h>
#include <elf.h>
#include <sys/mman.h>
#include <sys/param.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/eventfd.h>
#include <fcntl.h>
#include <stdbool.h>
#include <errno.h>
#include <ctype.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <time.h>
#include <netinet/in.h>
#include <net/if.h>
#include <linux/sockios.h>
#include <linux/if_tun.h>
#include <sys/uio.h>
#include <termios.h>
#include <getopt.h>
#include <assert.h>
#include <sched.h>
#include <limits.h>
#include <stddef.h>
#include <signal.h>
#include <pwd.h>
#include <grp.h>

#include <linux/virtio_config.h>
#include <linux/virtio_net.h>
#include <linux/virtio_blk.h>
#include <linux/virtio_console.h>
#include <linux/virtio_rng.h>
#include <linux/virtio_ring.h>
#include <asm/bootparam.h>
#include "../../include/linux/lguest_launcher.h"
/*L:110
 * We can ignore the 43 include files we need for this program, but I do want
 * to draw attention to the use of kernel-style types.
 *
 * As Linus said, "C is a Spartan language, and so should your naming be."  I
 * like these abbreviations, so we define them here.  Note that u64 is always
 * unsigned long long, which works on all Linux systems: this means that we can
 * use %llu in printf for any u64.
 */
typedef unsigned long long u64;
typedef uint32_t u32;
typedef uint16_t u16;
typedef uint8_t u8;
/*:*/

#define BRIDGE_PFX "bridge:"
#ifndef SIOCBRADDIF
#define SIOCBRADDIF 0x89a2      /* add interface to bridge      */
#endif
/* We can have up to 256 pages for devices. */
#define DEVICE_PAGES 256
/* This will occupy 3 pages: it must be a power of 2. */
#define VIRTQUEUE_NUM 256

/*L:120
 * verbose is both a global flag and a macro.  The C preprocessor allows
 * this, and although I wouldn't recommend it, it works quite nicely here.
 */
static bool verbose;
#define verbose(args...) \
    do { if (verbose) printf(args); } while(0)
/*:*/

/* The pointer to the start of guest memory. */
static void *guest_base;
/* The maximum guest physical address allowed, and maximum possible. */
static unsigned long guest_limit, guest_max;
/* The /dev/lguest file descriptor. */
static int lguest_fd;

/* a per-cpu variable indicating whose vcpu is currently running */
static unsigned int __thread cpu_id;

/* This is our list of devices. */
struct device_list {
    /* Counter to assign interrupt numbers. */
    unsigned int next_irq;

    /* Counter to print out convenient device numbers. */
    unsigned int device_num;

    /* The descriptor page for the devices. */
    u8 *descpage;

    /* A single linked list of devices. */
    struct device *dev;
    /* And a pointer to the last device for easy append. */
    struct device *lastdev;
};

/* The list of Guest devices, based on command line arguments. */
static struct device_list devices;

/* The device structure describes a single device. */
struct device {
    /* The linked-list pointer. */
    struct device *next;

    /* The device's descriptor, as mapped into the Guest. */
    struct lguest_device_desc *desc;

    /* We can't trust desc values once Guest has booted: we use these. */
    unsigned int feature_len;
    unsigned int num_vq;

    /* The name of this device, for --verbose. */
    const char *name;

    /* Any queues attached to this device */
    struct virtqueue *vq;

    /* Is it operational */
    bool running;

    /* Device-specific data. */
    void *priv;
};

/* The virtqueue structure describes a queue attached to a device. */
struct virtqueue {
    struct virtqueue *next;

    /* Which device owns me. */
    struct device *dev;

    /* The configuration for this queue. */
    struct lguest_vqconfig config;

    /* The actual ring of buffers. */
    struct vring vring;

    /* Last available index we saw. */
    u16 last_avail_idx;

    /* How many are used since we sent last irq? */
    unsigned int pending_used;

    /* Eventfd where Guest notifications arrive. */
    int eventfd;

    /* Function for the thread which is servicing this virtqueue. */
    void (*service)(struct virtqueue *vq);
    pid_t thread;
};

/* Remember the arguments to the program so we can "reboot" */
static char **main_args;

/* The original tty settings to restore on exit. */
static struct termios orig_term;

/*
 * We have to be careful with barriers: our devices are all run in separate
 * threads and so we need to make sure that changes visible to the Guest happen
 * in precise order.
 */
#define wmb() __asm__ __volatile__("" : : : "memory")
#define mb() __asm__ __volatile__("" : : : "memory")

/*
 * Convert an iovec element to the given type.
 *
 * This is a fairly ugly trick: we need to know the size of the type and
 * alignment requirement to check the pointer is kosher.  It's also nice to
 * have the name of the type in case we report failure.
 *
 * Typing those three things all the time is cumbersome and error prone, so we
 * have a macro which sets them all up and passes to the real function.
 */
#define convert(iov, type) \
    ((type *)_convert((iov), sizeof(type), __alignof__(type), #type))

static void *_convert(struct iovec *iov, size_t size, size_t align,
              const char *name)
{
    if (iov->iov_len != size)
        errx(1, "Bad iovec size %zu for %s", iov->iov_len, name);
    if ((unsigned long)iov->iov_base % align != 0)
        errx(1, "Bad alignment %p for %s", iov->iov_base, name);
    return iov->iov_base;
}

/* Wrapper for the last available index.  Makes it easier to change. */
#define lg_last_avail(vq)   ((vq)->last_avail_idx)

/*
 * The virtio configuration space is defined to be little-endian.  x86 is
 * little-endian too, but it's nice to be explicit so we have these helpers.
 */
#define cpu_to_le16(v16) (v16)
#define cpu_to_le32(v32) (v32)
#define cpu_to_le64(v64) (v64)
#define le16_to_cpu(v16) (v16)
#define le32_to_cpu(v32) (v32)
#define le64_to_cpu(v64) (v64)

/* Is this iovec empty? */
static bool iov_empty(const struct iovec iov[], unsigned int num_iov)
{
    unsigned int i;

    for (i = 0; i < num_iov; i++)
        if (iov[i].iov_len)
            return false;
    return true;
}

/* Take len bytes from the front of this iovec. */
static void iov_consume(struct iovec iov[], unsigned num_iov, unsigned len)
{
    unsigned int i;

    for (i = 0; i < num_iov; i++) {
        unsigned int used;

        used = iov[i].iov_len < len ? iov[i].iov_len : len;
        iov[i].iov_base += used;
        iov[i].iov_len -= used;
        len -= used;
    }
    assert(len == 0);
}

/* The device virtqueue descriptors are followed by feature bitmasks. */
static u8 *get_feature_bits(struct device *dev)
{
    return (u8 *)(dev->desc + 1)
        + dev->num_vq * sizeof(struct lguest_vqconfig);
}

/*L:100
 * The Launcher code itself takes us out into userspace, that scary place where
 * pointers run wild and free!  Unfortunately, like most userspace programs,
 * it's quite boring (which is why everyone likes to hack on the kernel!).
 * Perhaps if you make up an Lguest Drinking Game at this point, it will get
 * you through this section.  Or, maybe not.
 *
 * The Launcher sets up a big chunk of memory to be the Guest's "physical"
 * memory and stores it in "guest_base".  In other words, Guest physical ==
 * Launcher virtual with an offset.
 *
 * This can be tough to get your head around, but usually it just means that we
 * use these trivial conversion functions when the Guest gives us its
 * "physical" addresses:
 */
static void *from_guest_phys(unsigned long addr)
{
    return guest_base + addr;
}

static unsigned long to_guest_phys(const void *addr)
{
    return (addr - guest_base);
}

/*L:130
 * Loading the Kernel.
 *
 * We start with couple of simple helper routines.  open_or_die() avoids
 * error-checking code cluttering the callers:
 */
static int open_or_die(const char *name, int flags)
{
    int fd = open(name, flags);
    if (fd < 0)
        err(1, "Failed to open %s", name);
    return fd;
}

/* map_zeroed_pages() takes a number of pages. */
static void *map_zeroed_pages(unsigned int num)
{
    int fd = open_or_die("/dev/zero", O_RDONLY);
    void *addr;

    /*
     * We use a private mapping (ie. if we write to the page, it will be
     * copied). We allocate an extra two pages PROT_NONE to act as guard
     * pages against read/write attempts that exceed allocated space.
     */
    addr = mmap(NULL, getpagesize() * (num+2),
            PROT_NONE, MAP_PRIVATE, fd, 0);

    if (addr == MAP_FAILED)
        err(1, "Mmapping %u pages of /dev/zero", num);

    if (mprotect(addr + getpagesize(), getpagesize() * num,
             PROT_READ|PROT_WRITE) == -1)
        err(1, "mprotect rw %u pages failed", num);

    /*
     * One neat mmap feature is that you can close the fd, and it
     * stays mapped.
     */
    close(fd);

    /* Return address after PROT_NONE page */
    return addr + getpagesize();
}

/* Get some more pages for a device. */
static void *get_pages(unsigned int num)
{
    void *addr = from_guest_phys(guest_limit);

    guest_limit += num * getpagesize();
    if (guest_limit > guest_max)
        errx(1, "Not enough memory for devices");
    return addr;
}

/*
 * This routine is used to load the kernel or initrd.  It tries mmap, but if
 * that fails (Plan 9's kernel file isn't nicely aligned on page boundaries),
 * it falls back to reading the memory in.
 */
static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
{
    ssize_t r;

    /*
     * We map writable even though for some segments are marked read-only.
     * The kernel really wants to be writable: it patches its own
     * instructions.
     *
     * MAP_PRIVATE means that the page won't be copied until a write is
     * done to it.  This allows us to share untouched memory between
     * Guests.
     */
    if (mmap(addr, len, PROT_READ|PROT_WRITE,
         MAP_FIXED|MAP_PRIVATE, fd, offset) != MAP_FAILED)
        return;

    /* pread does a seek and a read in one shot: saves a few lines. */
    r = pread(fd, addr, len, offset);
    if (r != len)
        err(1, "Reading offset %lu len %lu gave %zi", offset, len, r);
}

/*
 * This routine takes an open vmlinux image, which is in ELF, and maps it into
 * the Guest memory.  ELF = Embedded Linking Format, which is the format used
 * by all modern binaries on Linux including the kernel.
 *
 * The ELF headers give *two* addresses: a physical address, and a virtual
 * address.  We use the physical address; the Guest will map itself to the
 * virtual address.
 *
 * We return the starting address.
 */
static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr)
{
    Elf32_Phdr phdr[ehdr->e_phnum];
    unsigned int i;

    /*
     * Sanity checks on the main ELF header: an x86 executable with a
     * reasonable number of correctly-sized program headers.
     */
    if (ehdr->e_type != ET_EXEC
        || ehdr->e_machine != EM_386
        || ehdr->e_phentsize != sizeof(Elf32_Phdr)
        || ehdr->e_phnum < 1 || ehdr->e_phnum > 65536U/sizeof(Elf32_Phdr))
        errx(1, "Malformed elf header");

    /*
     * An ELF executable contains an ELF header and a number of "program"
     * headers which indicate which parts ("segments") of the program to
     * load where.
     */

    /* We read in all the program headers at once: */
    if (lseek(elf_fd, ehdr->e_phoff, SEEK_SET) < 0)
        err(1, "Seeking to program headers");
    if (read(elf_fd, phdr, sizeof(phdr)) != sizeof(phdr))
        err(1, "Reading program headers");

    /*
     * Try all the headers: there are usually only three.  A read-only one,
     * a read-write one, and a "note" section which we don't load.
     */
    for (i = 0; i < ehdr->e_phnum; i++) {
        /* If this isn't a loadable segment, we ignore it */
        if (phdr[i].p_type != PT_LOAD)
            continue;

        verbose("Section %i: size %i addr %p\n",
            i, phdr[i].p_memsz, (void *)phdr[i].p_paddr);

        /* We map this section of the file at its physical address. */
        map_at(elf_fd, from_guest_phys(phdr[i].p_paddr),
               phdr[i].p_offset, phdr[i].p_filesz);
    }

    /* The entry point is given in the ELF header. */
    return ehdr->e_entry;
}

/*L:150
 * A bzImage, unlike an ELF file, is not meant to be loaded.  You're supposed
 * to jump into it and it will unpack itself.  We used to have to perform some
 * hairy magic because the unpacking code scared me.
 *
 * Fortunately, Jeremy Fitzhardinge convinced me it wasn't that hard and wrote
 * a small patch to jump over the tricky bits in the Guest, so now we just read
 * the funky header so we know where in the file to load, and away we go!
 */
static unsigned long load_bzimage(int fd)
{
    struct boot_params boot;
    int r;
    /* Modern bzImages get loaded at 1M. */
    void *p = from_guest_phys(0x100000);

    /*
     * Go back to the start of the file and read the header.  It should be
     * a Linux boot header (see Documentation/x86/boot.txt)
     */
    lseek(fd, 0, SEEK_SET);
    read(fd, &boot, sizeof(boot));

    /* Inside the setup_hdr, we expect the magic "HdrS" */
    if (memcmp(&boot.hdr.header, "HdrS", 4) != 0)
        errx(1, "This doesn't look like a bzImage to me");

    /* Skip over the extra sectors of the header. */
    lseek(fd, (boot.hdr.setup_sects+1) * 512, SEEK_SET);

    /* Now read everything into memory. in nice big chunks. */
    while ((r = read(fd, p, 65536)) > 0)
        p += r;

    /* Finally, code32_start tells us where to enter the kernel. */
    return boot.hdr.code32_start;
}

/*L:140
 * Loading the kernel is easy when it's a "vmlinux", but most kernels
 * come wrapped up in the self-decompressing "bzImage" format.  With a little
 * work, we can load those, too.
 */
static unsigned long load_kernel(int fd)
{
    Elf32_Ehdr hdr;

    /* Read in the first few bytes. */
    if (read(fd, &hdr, sizeof(hdr)) != sizeof(hdr))
        err(1, "Reading kernel");

    /* If it's an ELF file, it starts with "\177ELF" */
    if (memcmp(hdr.e_ident, ELFMAG, SELFMAG) == 0)
        return map_elf(fd, &hdr);

    /* Otherwise we assume it's a bzImage, and try to load it. */
    return load_bzimage(fd);
}

/*
 * This is a trivial little helper to align pages.  Andi Kleen hated it because
 * it calls getpagesize() twice: "it's dumb code."
 *
 * Kernel guys get really het up about optimization, even when it's not
 * necessary.  I leave this code as a reaction against that.
 */
static inline unsigned long page_align(unsigned long addr)
{
    /* Add upwards and truncate downwards. */
    return ((addr + getpagesize()-1) & ~(getpagesize()-1));
}

/*L:180
 * An "initial ram disk" is a disk image loaded into memory along with the
 * kernel which the kernel can use to boot from without needing any drivers.
 * Most distributions now use this as standard: the initrd contains the code to
 * load the appropriate driver modules for the current machine.
 *
 * Importantly, James Morris works for RedHat, and Fedora uses initrds for its
 * kernels.  He sent me this (and tells me when I break it).
 */
static unsigned long load_initrd(const char *name, unsigned long mem)
{
    int ifd;
    struct stat st;
    unsigned long len;

    ifd = open_or_die(name, O_RDONLY);
    /* fstat() is needed to get the file size. */
    if (fstat(ifd, &st) < 0)
        err(1, "fstat() on initrd '%s'", name);

    /*
     * We map the initrd at the top of memory, but mmap wants it to be
     * page-aligned, so we round the size up for that.
     */
    len = page_align(st.st_size);
    map_at(ifd, from_guest_phys(mem - len), 0, st.st_size);
    /*
     * Once a file is mapped, you can close the file descriptor.  It's a
     * little odd, but quite useful.
     */
    close(ifd);
    verbose("mapped initrd %s size=%lu @ %p\n", name, len, (void*)mem-len);

    /* We return the initrd size. */
    return len;
}
/*:*/

/*
 * Simple routine to roll all the commandline arguments together with spaces
 * between them.
 */
static void concat(char *dst, char *args[])
{
    unsigned int i, len = 0;

    for (i = 0; args[i]; i++) {
        if (i) {
            strcat(dst+len, " ");
            len++;
        }
        strcpy(dst+len, args[i]);
        len += strlen(args[i]);
    }
    /* In case it's empty. */
    dst[len] = '\0';
}

/*L:185
 * This is where we actually tell the kernel to initialize the Guest.  We
 * saw the arguments it expects when we looked at initialize() in lguest_user.c:
 * the base of Guest "physical" memory, the top physical page to allow and the
 * entry point for the Guest.
 */
static void tell_kernel(unsigned long start)
{
    unsigned long args[] = { LHREQ_INITIALIZE,
                 (unsigned long)guest_base,
                 guest_limit / getpagesize(), start };
    verbose("Guest: %p - %p (%#lx)\n",
        guest_base, guest_base + guest_limit, guest_limit);
    lguest_fd = open_or_die("/dev/lguest", O_RDWR);
    if (write(lguest_fd, args, sizeof(args)) < 0)
        err(1, "Writing to /dev/lguest");
}
/*:*/

/*L:200
 * Device Handling.
 *
 * When the Guest gives us a buffer, it sends an array of addresses and sizes.
 * We need to make sure it's not trying to reach into the Launcher itself, so
 * we have a convenient routine which checks it and exits with an error message
 * if something funny is going on:
 */
static void *_check_pointer(unsigned long addr, unsigned int size,
                unsigned int line)
{
    /*
     * Check if the requested address and size exceeds the allocated memory,
     * or addr + size wraps around.
     */
    if ((addr + size) > guest_limit || (addr + size) < addr)
        errx(1, "%s:%i: Invalid address %#lx", __FILE__, line, addr);
    /*
     * We return a pointer for the caller's convenience, now we know it's
     * safe to use.
     */
    return from_guest_phys(addr);
}
/* A macro which transparently hands the line number to the real function. */
#define check_pointer(addr,size) _check_pointer(addr, size, __LINE__)

/*
 * Each buffer in the virtqueues is actually a chain of descriptors.  This
 * function returns the next descriptor in the chain, or vq->vring.num if we're
 * at the end.
 */
static unsigned next_desc(struct vring_desc *desc,
              unsigned int i, unsigned int max)
{
    unsigned int next;

    /* If this descriptor says it doesn't chain, we're done. */
    if (!(desc[i].flags & VRING_DESC_F_NEXT))
        return max;

    /* Check they're not leading us off end of descriptors. */
    next = desc[i].next;
    /* Make sure compiler knows to grab that: we don't want it changing! */
    wmb();

    if (next >= max)
        errx(1, "Desc next is %u", next);

    return next;
}

/*
 * This actually sends the interrupt for this virtqueue, if we've used a
 * buffer.
 */
static void trigger_irq(struct virtqueue *vq)
{
    unsigned long buf[] = { LHREQ_IRQ, vq->config.irq };

    /* Don't inform them if nothing used. */
    if (!vq->pending_used)
        return;
    vq->pending_used = 0;

    /* If they don't want an interrupt, don't send one... */
    if (vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT) {
        return;
    }

    /* Send the Guest an interrupt tell them we used something up. */
    if (write(lguest_fd, buf, sizeof(buf)) != 0)
        err(1, "Triggering irq %i", vq->config.irq);
}

/*
 * This looks in the virtqueue for the first available buffer, and converts
 * it to an iovec for convenient access.  Since descriptors consist of some
 * number of output then some number of input descriptors, it's actually two
 * iovecs, but we pack them into one and note how many of each there were.
 *
 * This function waits if necessary, and returns the descriptor number found.
 */
static unsigned wait_for_vq_desc(struct virtqueue *vq,
                 struct iovec iov[],
                 unsigned int *out_num, unsigned int *in_num)
{
    unsigned int i, head, max;
    struct vring_desc *desc;
    u16 last_avail = lg_last_avail(vq);

    /* There's nothing available? */
    while (last_avail == vq->vring.avail->idx) {
        u64 event;

        /*
         * Since we're about to sleep, now is a good time to tell the
         * Guest about what we've used up to now.
         */
        trigger_irq(vq);

        /* OK, now we need to know about added descriptors. */
        vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;

        /*
         * They could have slipped one in as we were doing that: make
         * sure it's written, then check again.
         */
        mb();
        if (last_avail != vq->vring.avail->idx) {
            vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
            break;
        }

        /* Nothing new?  Wait for eventfd to tell us they refilled. */
        if (read(vq->eventfd, &event, sizeof(event)) != sizeof(event))
            errx(1, "Event read failed?");

        /* We don't need to be notified again. */
        vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
    }

    /* Check it isn't doing very strange things with descriptor numbers. */
    if ((u16)(vq->vring.avail->idx - last_avail) > vq->vring.num)
        errx(1, "Guest moved used index from %u to %u",
             last_avail, vq->vring.avail->idx);

    /*
     * Grab the next descriptor number they're advertising, and increment
     * the index we've seen.
     */
    head = vq->vring.avail->ring[last_avail % vq->vring.num];
    lg_last_avail(vq)++;

    /* If their number is silly, that's a fatal mistake. */
    if (head >= vq->vring.num)
        errx(1, "Guest says index %u is available", head);

    /* When we start there are none of either input nor output. */
    *out_num = *in_num = 0;

    max = vq->vring.num;
    desc = vq->vring.desc;
    i = head;

    /*
     * If this is an indirect entry, then this buffer contains a descriptor
     * table which we handle as if it's any normal descriptor chain.
     */
    if (desc[i].flags & VRING_DESC_F_INDIRECT) {
        if (desc[i].len % sizeof(struct vring_desc))
            errx(1, "Invalid size for indirect buffer table");

        max = desc[i].len / sizeof(struct vring_desc);
        desc = check_pointer(desc[i].addr, desc[i].len);
        i = 0;
    }

    do {
        /* Grab the first descriptor, and check it's OK. */
        iov[*out_num + *in_num].iov_len = desc[i].len;
        iov[*out_num + *in_num].iov_base
            = check_pointer(desc[i].addr, desc[i].len);
        /* If this is an input descriptor, increment that count. */
        if (desc[i].flags & VRING_DESC_F_WRITE)
            (*in_num)++;
        else {
            /*
             * If it's an output descriptor, they're all supposed
             * to come before any input descriptors.
             */
            if (*in_num)
                errx(1, "Descriptor has out after in");
            (*out_num)++;
        }

        /* If we've got too many, that implies a descriptor loop. */
        if (*out_num + *in_num > max)
            errx(1, "Looped descriptor");
    } while ((i = next_desc(desc, i, max)) != max);

    return head;
}

/*
 * After we've used one of their buffers, we tell the Guest about it.  Sometime
 * later we'll want to send them an interrupt using trigger_irq(); note that
 * wait_for_vq_desc() does that for us if it has to wait.
 */
static void add_used(struct virtqueue *vq, unsigned int head, int len)
{
    struct vring_used_elem *used;

    /*
     * The virtqueue contains a ring of used buffers.  Get a pointer to the
     * next entry in that used ring.
     */
    used = &vq->vring.used->ring[vq->vring.used->idx % vq->vring.num];
    used->id = head;
    used->len = len;
    /* Make sure buffer is written before we update index. */
    wmb();
    vq->vring.used->idx++;
    vq->pending_used++;
}

/* And here's the combo meal deal.  Supersize me! */
static void add_used_and_trigger(struct virtqueue *vq, unsigned head, int len)
{
    add_used(vq, head, len);
    trigger_irq(vq);
}

/*
 * The Console
 *
 * We associate some data with the console for our exit hack.
 */
struct console_abort {
    /* How many times have they hit ^C? */
    int count;
    /* When did they start? */
    struct timeval start;
};

/* This is the routine which handles console input (ie. stdin). */
static void console_input(struct virtqueue *vq)
{
    int len;
    unsigned int head, in_num, out_num;
    struct console_abort *abort = vq->dev->priv;
    struct iovec iov[vq->vring.num];

    /* Make sure there's a descriptor available. */
    head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
    if (out_num)
        errx(1, "Output buffers in console in queue?");

    /* Read into it.  This is where we usually wait. */
    len = readv(STDIN_FILENO, iov, in_num);
    if (len <= 0) {
        /* Ran out of input? */
        warnx("Failed to get console input, ignoring console.");
        /*
         * For simplicity, dying threads kill the whole Launcher.  So
         * just nap here.
         */
        for (;;)
            pause();
    }

    /* Tell the Guest we used a buffer. */
    add_used_and_trigger(vq, head, len);

    /*
     * Three ^C within one second?  Exit.
     *
     * This is such a hack, but works surprisingly well.  Each ^C has to
     * be in a buffer by itself, so they can't be too fast.  But we check
     * that we get three within about a second, so they can't be too
     * slow.
     */
    if (len != 1 || ((char *)iov[0].iov_base)[0] != 3) {
        abort->count = 0;
        return;
    }

    abort->count++;
    if (abort->count == 1)
        gettimeofday(&abort->start, NULL);
    else if (abort->count == 3) {
        struct timeval now;
        gettimeofday(&now, NULL);
        /* Kill all Launcher processes with SIGINT, like normal ^C */
        if (now.tv_sec <= abort->start.tv_sec+1)
            kill(0, SIGINT);
        abort->count = 0;
    }
}

/* This is the routine which handles console output (ie. stdout). */
static void console_output(struct virtqueue *vq)
{
    unsigned int head, out, in;
    struct iovec iov[vq->vring.num];

    /* We usually wait in here, for the Guest to give us something. */
    head = wait_for_vq_desc(vq, iov, &out, &in);
    if (in)
        errx(1, "Input buffers in console output queue?");

    /* writev can return a partial write, so we loop here. */
    while (!iov_empty(iov, out)) {
        int len = writev(STDOUT_FILENO, iov, out);
        if (len <= 0) {
            warn("Write to stdout gave %i (%d)", len, errno);
            break;
        }
        iov_consume(iov, out, len);
    }

    /*
     * We're finished with that buffer: if we're going to sleep,
     * wait_for_vq_desc() will prod the Guest with an interrupt.
     */
    add_used(vq, head, 0);
}

/*
 * The Network
 *
 * Handling output for network is also simple: we get all the output buffers
 * and write them to /dev/net/tun.
 */
struct net_info {
    int tunfd;
};

static void net_output(struct virtqueue *vq)
{
    struct net_info *net_info = vq->dev->priv;
    unsigned int head, out, in;
    struct iovec iov[vq->vring.num];

    /* We usually wait in here for the Guest to give us a packet. */
    head = wait_for_vq_desc(vq, iov, &out, &in);
    if (in)
        errx(1, "Input buffers in net output queue?");
    /*
     * Send the whole thing through to /dev/net/tun.  It expects the exact
     * same format: what a coincidence!
     */
    if (writev(net_info->tunfd, iov, out) < 0)
        warnx("Write to tun failed (%d)?", errno);

    /*
     * Done with that one; wait_for_vq_desc() will send the interrupt if
     * all packets are processed.
     */
    add_used(vq, head, 0);
}

/*
 * Handling network input is a bit trickier, because I've tried to optimize it.
 *
 * First we have a helper routine which tells is if from this file descriptor
 * (ie. the /dev/net/tun device) will block:
 */
static bool will_block(int fd)
{
    fd_set fdset;
    struct timeval zero = { 0, 0 };
    FD_ZERO(&fdset);
    FD_SET(fd, &fdset);
    return select(fd+1, &fdset, NULL, NULL, &zero) != 1;
}

/*
 * This handles packets coming in from the tun device to our Guest.  Like all
 * service routines, it gets called again as soon as it returns, so you don't
 * see a while(1) loop here.
 */
static void net_input(struct virtqueue *vq)
{
    int len;
    unsigned int head, out, in;
    struct iovec iov[vq->vring.num];
    struct net_info *net_info = vq->dev->priv;

    /*
     * Get a descriptor to write an incoming packet into.  This will also
     * send an interrupt if they're out of descriptors.
     */
    head = wait_for_vq_desc(vq, iov, &out, &in);
    if (out)
        errx(1, "Output buffers in net input queue?");

    /*
     * If it looks like we'll block reading from the tun device, send them
     * an interrupt.
     */
    if (vq->pending_used && will_block(net_info->tunfd))
        trigger_irq(vq);

    /*
     * Read in the packet.  This is where we normally wait (when there's no
     * incoming network traffic).
     */
    len = readv(net_info->tunfd, iov, in);
    if (len <= 0)
        warn("Failed to read from tun (%d).", errno);

    /*
     * Mark that packet buffer as used, but don't interrupt here.  We want
     * to wait until we've done as much work as we can.
     */
    add_used(vq, head, len);
}
/*:*/

/* This is the helper to create threads: run the service routine in a loop. */
static int do_thread(void *_vq)
{
    struct virtqueue *vq = _vq;

    for (;;)
        vq->service(vq);
    return 0;
}

/*
 * When a child dies, we kill our entire process group with SIGTERM.  This
 * also has the side effect that the shell restores the console for us!
 */
static void kill_launcher(int signal)
{
    kill(0, SIGTERM);
}

static void reset_device(struct device *dev)
{
    struct virtqueue *vq;

    verbose("Resetting device %s\n", dev->name);

    /* Clear any features they've acked. */
    memset(get_feature_bits(dev) + dev->feature_len, 0, dev->feature_len);

    /* We're going to be explicitly killing threads, so ignore them. */
    signal(SIGCHLD, SIG_IGN);

    /* Zero out the virtqueues, get rid of their threads */
    for (vq = dev->vq; vq; vq = vq->next) {
        if (vq->thread != (pid_t)-1) {
            kill(vq->thread, SIGTERM);
            waitpid(vq->thread, NULL, 0);
            vq->thread = (pid_t)-1;
        }
        memset(vq->vring.desc, 0,
               vring_size(vq->config.num, LGUEST_VRING_ALIGN));
        lg_last_avail(vq) = 0;
    }
    dev->running = false;

    /* Now we care if threads die. */
    signal(SIGCHLD, (void *)kill_launcher);
}

/*L:216
 * This actually creates the thread which services the virtqueue for a device.
 */
static void create_thread(struct virtqueue *vq)
{
    /*
     * Create stack for thread.  Since the stack grows upwards, we point
     * the stack pointer to the end of this region.
     */
    char *stack = malloc(32768);
    unsigned long args[] = { LHREQ_EVENTFD,
                 vq->config.pfn*getpagesize(), 0 };

    /* Create a zero-initialized eventfd. */
    vq->eventfd = eventfd(0, 0);
    if (vq->eventfd < 0)
        err(1, "Creating eventfd");
    args[2] = vq->eventfd;

    /*
     * Attach an eventfd to this virtqueue: it will go off when the Guest
     * does an LHCALL_NOTIFY for this vq.
     */
    if (write(lguest_fd, &args, sizeof(args)) != 0)
        err(1, "Attaching eventfd");

    /*
     * CLONE_VM: because it has to access the Guest memory, and SIGCHLD so
     * we get a signal if it dies.
     */
    vq->thread = clone(do_thread, stack + 32768, CLONE_VM | SIGCHLD, vq);
    if (vq->thread == (pid_t)-1)
        err(1, "Creating clone");

    /* We close our local copy now the child has it. */
    close(vq->eventfd);
}

static void start_device(struct device *dev)
{
    unsigned int i;
    struct virtqueue *vq;

    verbose("Device %s OK: offered", dev->name);
    for (i = 0; i < dev->feature_len; i++)
        verbose(" %02x", get_feature_bits(dev)[i]);
    verbose(", accepted");
    for (i = 0; i < dev->feature_len; i++)
        verbose(" %02x", get_feature_bits(dev)
            [dev->feature_len+i]);

    for (vq = dev->vq; vq; vq = vq->next) {
        if (vq->service)
            create_thread(vq);
    }
    dev->running = true;
}

static void cleanup_devices(void)
{
    struct device *dev;

    for (dev = devices.dev; dev; dev = dev->next)
        reset_device(dev);

    /* If we saved off the original terminal settings, restore them now. */
    if (orig_term.c_lflag & (ISIG|ICANON|ECHO))
        tcsetattr(STDIN_FILENO, TCSANOW, &orig_term);
}

/* When the Guest tells us they updated the status field, we handle it. */
static void update_device_status(struct device *dev)
{
    /* A zero status is a reset, otherwise it's a set of flags. */
    if (dev->desc->status == 0)
        reset_device(dev);
    else if (dev->desc->status & VIRTIO_CONFIG_S_FAILED) {
        warnx("Device %s configuration FAILED", dev->name);
        if (dev->running)
            reset_device(dev);
    } else {
        if (dev->running)
            err(1, "Device %s features finalized twice", dev->name);
        start_device(dev);
    }
}

/*L:215
 * This is the generic routine we call when the Guest uses LHCALL_NOTIFY.  In
 * particular, it's used to notify us of device status changes during boot.
 */
static void handle_output(unsigned long addr)
{
    struct device *i;

    /* Check each device. */
    for (i = devices.dev; i; i = i->next) {
        struct virtqueue *vq;

        /*
         * Notifications to device descriptors mean they updated the
         * device status.
         */
        if (from_guest_phys(addr) == i->desc) {
            update_device_status(i);
            return;
        }

        /* Devices should not be used before features are finalized. */
        for (vq = i->vq; vq; vq = vq->next) {
            if (addr != vq->config.pfn*getpagesize())
                continue;
            errx(1, "Notification on %s before setup!", i->name);
        }
    }

    /*
     * Early console write is done using notify on a nul-terminated string
     * in Guest memory.  It's also great for hacking debugging messages
     * into a Guest.
     */
    if (addr >= guest_limit)
        errx(1, "Bad NOTIFY %#lx", addr);

    write(STDOUT_FILENO, from_guest_phys(addr),
          strnlen(from_guest_phys(addr), guest_limit - addr));
}

/*L:190
 * Device Setup
 *
 * All devices need a descriptor so the Guest knows it exists, and a "struct
 * device" so the Launcher can keep track of it.  We have common helper
 * routines to allocate and manage them.
 */

/*
 * The layout of the device page is a "struct lguest_device_desc" followed by a
 * number of virtqueue descriptors, then two sets of feature bits, then an
 * array of configuration bytes.  This routine returns the configuration
 * pointer.
 */
static u8 *device_config(const struct device *dev)
{
    return (void *)(dev->desc + 1)
        + dev->num_vq * sizeof(struct lguest_vqconfig)
        + dev->feature_len * 2;
}

/*
 * This routine allocates a new "struct lguest_device_desc" from descriptor
 * table page just above the Guest's normal memory.  It returns a pointer to
 * that descriptor.
 */
static struct lguest_device_desc *new_dev_desc(u16 type)
{
    struct lguest_device_desc d = { .type = type };
    void *p;

    /* Figure out where the next device config is, based on the last one. */
    if (devices.lastdev)
        p = device_config(devices.lastdev)
            + devices.lastdev->desc->config_len;
    else
        p = devices.descpage;

    /* We only have one page for all the descriptors. */
    if (p + sizeof(d) > (void *)devices.descpage + getpagesize())
        errx(1, "Too many devices");

    /* p might not be aligned, so we memcpy in. */
    return memcpy(p, &d, sizeof(d));
}

/*
 * Each device descriptor is followed by the description of its virtqueues.  We
 * specify how many descriptors the virtqueue is to have.
 */
static void add_virtqueue(struct device *dev, unsigned int num_descs,
              void (*service)(struct virtqueue *))
{
    unsigned int pages;
    struct virtqueue **i, *vq = malloc(sizeof(*vq));
    void *p;

    /* First we need some memory for this virtqueue. */
    pages = (vring_size(num_descs, LGUEST_VRING_ALIGN) + getpagesize() - 1)
        / getpagesize();
    p = get_pages(pages);

    /* Initialize the virtqueue */
    vq->next = NULL;
    vq->last_avail_idx = 0;
    vq->dev = dev;

    /*
     * This is the routine the service thread will run, and its Process ID
     * once it's running.
     */
    vq->service = service;
    vq->thread = (pid_t)-1;

    /* Initialize the configuration. */
    vq->config.num = num_descs;
    vq->config.irq = devices.next_irq++;
    vq->config.pfn = to_guest_phys(p) / getpagesize();

    /* Initialize the vring. */
    vring_init(&vq->vring, num_descs, p, LGUEST_VRING_ALIGN);

    /*
     * Append virtqueue to this device's descriptor.  We use
     * device_config() to get the end of the device's current virtqueues;
     * we check that we haven't added any config or feature information
     * yet, otherwise we'd be overwriting them.
     */
    assert(dev->desc->config_len == 0 && dev->desc->feature_len == 0);
    memcpy(device_config(dev), &vq->config, sizeof(vq->config));
    dev->num_vq++;
    dev->desc->num_vq++;

    verbose("Virtqueue page %#lx\n", to_guest_phys(p));

    /*
     * Add to tail of list, so dev->vq is first vq, dev->vq->next is
     * second.
     */
    for (i = &dev->vq; *i; i = &(*i)->next);
    *i = vq;
}

/*
 * The first half of the feature bitmask is for us to advertise features.  The
 * second half is for the Guest to accept features.
 */
static void add_feature(struct device *dev, unsigned bit)
{
    u8 *features = get_feature_bits(dev);

    /* We can't extend the feature bits once we've added config bytes */
    if (dev->desc->feature_len <= bit / CHAR_BIT) {
        assert(dev->desc->config_len == 0);
        dev->feature_len = dev->desc->feature_len = (bit/CHAR_BIT) + 1;
    }

    features[bit / CHAR_BIT] |= (1 << (bit % CHAR_BIT));
}

/*
 * This routine sets the configuration fields for an existing device's
 * descriptor.  It only works for the last device, but that's OK because that's
 * how we use it.
 */
static void set_config(struct device *dev, unsigned len, const void *conf)
{
    /* Check we haven't overflowed our single page. */
    if (device_config(dev) + len > devices.descpage + getpagesize())
        errx(1, "Too many devices");

    /* Copy in the config information, and store the length. */
    memcpy(device_config(dev), conf, len);
    dev->desc->config_len = len;

    /* Size must fit in config_len field (8 bits)! */
    assert(dev->desc->config_len == len);
}

/*
 * This routine does all the creation and setup of a new device, including
 * calling new_dev_desc() to allocate the descriptor and device memory.  We
 * don't actually start the service threads until later.
 *
 * See what I mean about userspace being boring?
 */
static struct device *new_device(const char *name, u16 type)
{
    struct device *dev = malloc(sizeof(*dev));

    /* Now we populate the fields one at a time. */
    dev->desc = new_dev_desc(type);
    dev->name = name;
    dev->vq = NULL;
    dev->feature_len = 0;
    dev->num_vq = 0;
    dev->running = false;
    dev->next = NULL;

    /*
     * Append to device list.  Prepending to a single-linked list is
     * easier, but the user expects the devices to be arranged on the bus
     * in command-line order.  The first network device on the command line
     * is eth0, the first block device /dev/vda, etc.
     */
    if (devices.lastdev)
        devices.lastdev->next = dev;
    else
        devices.dev = dev;
    devices.lastdev = dev;

    return dev;
}

/*
 * Our first setup routine is the console.  It's a fairly simple device, but
 * UNIX tty handling makes it uglier than it could be.
 */
static void setup_console(void)
{
    struct device *dev;

    /* If we can save the initial standard input settings... */
    if (tcgetattr(STDIN_FILENO, &orig_term) == 0) {
        struct termios term = orig_term;
        /*
         * Then we turn off echo, line buffering and ^C etc: We want a
         * raw input stream to the Guest.
         */
        term.c_lflag &= ~(ISIG|ICANON|ECHO);
        tcsetattr(STDIN_FILENO, TCSANOW, &term);
    }

    dev = new_device("console", VIRTIO_ID_CONSOLE);

    /* We store the console state in dev->priv, and initialize it. */
    dev->priv = malloc(sizeof(struct console_abort));
    ((struct console_abort *)dev->priv)->count = 0;

    /*
     * The console needs two virtqueues: the input then the output.  When
     * they put something the input queue, we make sure we're listening to
     * stdin.  When they put something in the output queue, we write it to
     * stdout.
     */
    add_virtqueue(dev, VIRTQUEUE_NUM, console_input);
    add_virtqueue(dev, VIRTQUEUE_NUM, console_output);

    verbose("device %u: console\n", ++devices.device_num);
}
/*:*/

/*M:010
 * Inter-guest networking is an interesting area.  Simplest is to have a
 * --sharenet=<name> option which opens or creates a named pipe.  This can be
 * used to send packets to another guest in a 1:1 manner.
 *
 * More sophisticated is to use one of the tools developed for project like UML
 * to do networking.
 *
 * Faster is to do virtio bonding in kernel.  Doing this 1:1 would be
 * completely generic ("here's my vring, attach to your vring") and would work
 * for any traffic.  Of course, namespace and permissions issues need to be
 * dealt with.  A more sophisticated "multi-channel" virtio_net.c could hide
 * multiple inter-guest channels behind one interface, although it would
 * require some manner of hotplugging new virtio channels.
 *
 * Finally, we could use a virtio network switch in the kernel, ie. vhost.
:*/

static u32 str2ip(const char *ipaddr)
{
    unsigned int b[4];

    if (sscanf(ipaddr, "%u.%u.%u.%u", &b[0], &b[1], &b[2], &b[3]) != 4)
        errx(1, "Failed to parse IP address '%s'", ipaddr);
    return (b[0] << 24) | (b[1] << 16) | (b[2] << 8) | b[3];
}

static void str2mac(const char *macaddr, unsigned char mac[6])
{
    unsigned int m[6];
    if (sscanf(macaddr, "%02x:%02x:%02x:%02x:%02x:%02x",
           &m[0], &m[1], &m[2], &m[3], &m[4], &m[5]) != 6)
        errx(1, "Failed to parse mac address '%s'", macaddr);
    mac[0] = m[0];
    mac[1] = m[1];
    mac[2] = m[2];
    mac[3] = m[3];
    mac[4] = m[4];
    mac[5] = m[5];
}

/*
 * This code is "adapted" from libbridge: it attaches the Host end of the
 * network device to the bridge device specified by the command line.
 *
 * This is yet another James Morris contribution (I'm an IP-level guy, so I
 * dislike bridging), and I just try not to break it.
 */
static void add_to_bridge(int fd, const char *if_name, const char *br_name)
{
    int ifidx;
    struct ifreq ifr;

    if (!*br_name)
        errx(1, "must specify bridge name");

    ifidx = if_nametoindex(if_name);
    if (!ifidx)
        errx(1, "interface %s does not exist!", if_name);

    strncpy(ifr.ifr_name, br_name, IFNAMSIZ);
    ifr.ifr_name[IFNAMSIZ-1] = '\0';
    ifr.ifr_ifindex = ifidx;
    if (ioctl(fd, SIOCBRADDIF, &ifr) < 0)
        err(1, "can't add %s to bridge %s", if_name, br_name);
}

/*
 * This sets up the Host end of the network device with an IP address, brings
 * it up so packets will flow, the copies the MAC address into the hwaddr
 * pointer.
 */
static void configure_device(int fd, const char *tapif, u32 ipaddr)
{
    struct ifreq ifr;
    struct sockaddr_in sin;

    memset(&ifr, 0, sizeof(ifr));
    strcpy(ifr.ifr_name, tapif);

    /* Don't read these incantations.  Just cut & paste them like I did! */
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = htonl(ipaddr);
    memcpy(&ifr.ifr_addr, &sin, sizeof(sin));
    if (ioctl(fd, SIOCSIFADDR, &ifr) != 0)
        err(1, "Setting %s interface address", tapif);
    ifr.ifr_flags = IFF_UP;
    if (ioctl(fd, SIOCSIFFLAGS, &ifr) != 0)
        err(1, "Bringing interface %s up", tapif);
}

static int get_tun_device(char tapif[IFNAMSIZ])
{
    struct ifreq ifr;
    int netfd;

    /* Start with this zeroed.  Messy but sure. */
    memset(&ifr, 0, sizeof(ifr));

    /*
     * We open the /dev/net/tun device and tell it we want a tap device.  A
     * tap device is like a tun device, only somehow different.  To tell
     * the truth, I completely blundered my way through this code, but it
     * works now!
     */
    netfd = open_or_die("/dev/net/tun", O_RDWR);
    ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_VNET_HDR;
    strcpy(ifr.ifr_name, "tap%d");
    if (ioctl(netfd, TUNSETIFF, &ifr) != 0)
        err(1, "configuring /dev/net/tun");

    if (ioctl(netfd, TUNSETOFFLOAD,
          TUN_F_CSUM|TUN_F_TSO4|TUN_F_TSO6|TUN_F_TSO_ECN) != 0)
        err(1, "Could not set features for tun device");

    /*
     * We don't need checksums calculated for packets coming in this
     * device: trust us!
     */
    ioctl(netfd, TUNSETNOCSUM, 1);

    memcpy(tapif, ifr.ifr_name, IFNAMSIZ);
    return netfd;
}

/*L:195
 * Our network is a Host<->Guest network.  This can either use bridging or
 * routing, but the principle is the same: it uses the "tun" device to inject
 * packets into the Host as if they came in from a normal network card.  We
 * just shunt packets between the Guest and the tun device.
 */
static void setup_tun_net(char *arg)
{
    struct device *dev;
    struct net_info *net_info = malloc(sizeof(*net_info));
    int ipfd;
    u32 ip = INADDR_ANY;
    bool bridging = false;
    char tapif[IFNAMSIZ], *p;
    struct virtio_net_config conf;

    net_info->tunfd = get_tun_device(tapif);

    /* First we create a new network device. */
    dev = new_device("net", VIRTIO_ID_NET);
    dev->priv = net_info;

    /* Network devices need a recv and a send queue, just like console. */
    add_virtqueue(dev, VIRTQUEUE_NUM, net_input);
    add_virtqueue(dev, VIRTQUEUE_NUM, net_output);

    /*
     * We need a socket to perform the magic network ioctls to bring up the
     * tap interface, connect to the bridge etc.  Any socket will do!
     */
    ipfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
    if (ipfd < 0)
        err(1, "opening IP socket");

    /* If the command line was --tunnet=bridge:<name> do bridging. */
    if (!strncmp(BRIDGE_PFX, arg, strlen(BRIDGE_PFX))) {
        arg += strlen(BRIDGE_PFX);
        bridging = true;
    }

    /* A mac address may follow the bridge name or IP address */
    p = strchr(arg, ':');
    if (p) {
        str2mac(p+1, conf.mac);
        add_feature(dev, VIRTIO_NET_F_MAC);
        *p = '\0';
    }

    /* arg is now either an IP address or a bridge name */
    if (bridging)
        add_to_bridge(ipfd, tapif, arg);
    else
        ip = str2ip(arg);

    /* Set up the tun device. */
    configure_device(ipfd, tapif, ip);

    /* Expect Guest to handle everything except UFO */
    add_feature(dev, VIRTIO_NET_F_CSUM);
    add_feature(dev, VIRTIO_NET_F_GUEST_CSUM);
    add_feature(dev, VIRTIO_NET_F_GUEST_TSO4);
    add_feature(dev, VIRTIO_NET_F_GUEST_TSO6);
    add_feature(dev, VIRTIO_NET_F_GUEST_ECN);
    add_feature(dev, VIRTIO_NET_F_HOST_TSO4);
    add_feature(dev, VIRTIO_NET_F_HOST_TSO6);
    add_feature(dev, VIRTIO_NET_F_HOST_ECN);
    /* We handle indirect ring entries */
    add_feature(dev, VIRTIO_RING_F_INDIRECT_DESC);
    set_config(dev, sizeof(conf), &conf);

    /* We don't need the socket any more; setup is done. */
    close(ipfd);

    devices.device_num++;

    if (bridging)
        verbose("device %u: tun %s attached to bridge: %s\n",
            devices.device_num, tapif, arg);
    else
        verbose("device %u: tun %s: %s\n",
            devices.device_num, tapif, arg);
}
/*:*/

/* This hangs off device->priv. */
struct vblk_info {
    /* The size of the file. */
    off64_t len;

    /* The file descriptor for the file. */
    int fd;

};

/*L:210
 * The Disk
 *
 * The disk only has one virtqueue, so it only has one thread.  It is really
 * simple: the Guest asks for a block number and we read or write that position
 * in the file.
 *
 * Before we serviced each virtqueue in a separate thread, that was unacceptably
 * slow: the Guest waits until the read is finished before running anything
 * else, even if it could have been doing useful work.
 *
 * We could have used async I/O, except it's reputed to suck so hard that
 * characters actually go missing from your code when you try to use it.
 */
static void blk_request(struct virtqueue *vq)
{
    struct vblk_info *vblk = vq->dev->priv;
    unsigned int head, out_num, in_num, wlen;
    int ret;
    u8 *in;
    struct virtio_blk_outhdr *out;
    struct iovec iov[vq->vring.num];
    off64_t off;

    /*
     * Get the next request, where we normally wait.  It triggers the
     * interrupt to acknowledge previously serviced requests (if any).
     */
    head = wait_for_vq_desc(vq, iov, &out_num, &in_num);

    /*
     * Every block request should contain at least one output buffer
     * (detailing the location on disk and the type of request) and one
     * input buffer (to hold the result).
     */
    if (out_num == 0 || in_num == 0)
        errx(1, "Bad virtblk cmd %u out=%u in=%u",
             head, out_num, in_num);

    out = convert(&iov[0], struct virtio_blk_outhdr);
    in = convert(&iov[out_num+in_num-1], u8);
    /*
     * For historical reasons, block operations are expressed in 512 byte
     * "sectors".
     */
    off = out->sector * 512;

    /*
     * In general the virtio block driver is allowed to try SCSI commands.
     * It'd be nice if we supported eject, for example, but we don't.
     */
    if (out->type & VIRTIO_BLK_T_SCSI_CMD) {
        fprintf(stderr, "Scsi commands unsupported\n");
        *in = VIRTIO_BLK_S_UNSUPP;
        wlen = sizeof(*in);
    } else if (out->type & VIRTIO_BLK_T_OUT) {
        /*
         * Write
         *
         * Move to the right location in the block file.  This can fail
         * if they try to write past end.
         */
        if (lseek64(vblk->fd, off, SEEK_SET) != off)
            err(1, "Bad seek to sector %llu", out->sector);

        ret = writev(vblk->fd, iov+1, out_num-1);
        verbose("WRITE to sector %llu: %i\n", out->sector, ret);

        /*
         * Grr... Now we know how long the descriptor they sent was, we
         * make sure they didn't try to write over the end of the block
         * file (possibly extending it).
         */
        if (ret > 0 && off + ret > vblk->len) {
            /* Trim it back to the correct length */
            ftruncate64(vblk->fd, vblk->len);
            /* Die, bad Guest, die. */
            errx(1, "Write past end %llu+%u", off, ret);
        }

        wlen = sizeof(*in);
        *in = (ret >= 0 ? VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR);
    } else if (out->type & VIRTIO_BLK_T_FLUSH) {
        /* Flush */
        ret = fdatasync(vblk->fd);
        verbose("FLUSH fdatasync: %i\n", ret);
        wlen = sizeof(*in);
        *in = (ret >= 0 ? VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR);
    } else {
        /*
         * Read
         *
         * Move to the right location in the block file.  This can fail
         * if they try to read past end.
         */
        if (lseek64(vblk->fd, off, SEEK_SET) != off)
            err(1, "Bad seek to sector %llu", out->sector);

        ret = readv(vblk->fd, iov+1, in_num-1);
        verbose("READ from sector %llu: %i\n", out->sector, ret);
        if (ret >= 0) {
            wlen = sizeof(*in) + ret;
            *in = VIRTIO_BLK_S_OK;
        } else {
            wlen = sizeof(*in);
            *in = VIRTIO_BLK_S_IOERR;
        }
    }

    /* Finished that request. */
    add_used(vq, head, wlen);
}

/*L:198 This actually sets up a virtual block device. */
static void setup_block_file(const char *filename)
{
    struct device *dev;
    struct vblk_info *vblk;
    struct virtio_blk_config conf;

    /* Creat the device. */
    dev = new_device("block", VIRTIO_ID_BLOCK);

    /* The device has one virtqueue, where the Guest places requests. */
    add_virtqueue(dev, VIRTQUEUE_NUM, blk_request);

    /* Allocate the room for our own bookkeeping */
    vblk = dev->priv = malloc(sizeof(*vblk));

    /* First we open the file and store the length. */
    vblk->fd = open_or_die(filename, O_RDWR|O_LARGEFILE);
    vblk->len = lseek64(vblk->fd, 0, SEEK_END);

    /* We support FLUSH. */
    add_feature(dev, VIRTIO_BLK_F_FLUSH);

    /* Tell Guest how many sectors this device has. */
    conf.capacity = cpu_to_le64(vblk->len / 512);

    /*
     * Tell Guest not to put in too many descriptors at once: two are used
     * for the in and out elements.
     */
    add_feature(dev, VIRTIO_BLK_F_SEG_MAX);
    conf.seg_max = cpu_to_le32(VIRTQUEUE_NUM - 2);

    /* Don't try to put whole struct: we have 8 bit limit. */
    set_config(dev, offsetof(struct virtio_blk_config, geometry), &conf);

    verbose("device %u: virtblock %llu sectors\n",
        ++devices.device_num, le64_to_cpu(conf.capacity));
}

/*L:211
 * Our random number generator device reads from /dev/random into the Guest's
 * input buffers.  The usual case is that the Guest doesn't want random numbers
 * and so has no buffers although /dev/random is still readable, whereas
 * console is the reverse.
 *
 * The same logic applies, however.
 */
struct rng_info {
    int rfd;
};

static void rng_input(struct virtqueue *vq)
{
    int len;
    unsigned int head, in_num, out_num, totlen = 0;
    struct rng_info *rng_info = vq->dev->priv;
    struct iovec iov[vq->vring.num];

    /* First we need a buffer from the Guests's virtqueue. */
    head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
    if (out_num)
        errx(1, "Output buffers in rng?");

    /*
     * Just like the console write, we loop to cover the whole iovec.
     * In this case, short reads actually happen quite a bit.
     */
    while (!iov_empty(iov, in_num)) {
        len = readv(rng_info->rfd, iov, in_num);
        if (len <= 0)
            err(1, "Read from /dev/random gave %i", len);
        iov_consume(iov, in_num, len);
        totlen += len;
    }

    /* Tell the Guest about the new input. */
    add_used(vq, head, totlen);
}

/*L:199
 * This creates a "hardware" random number device for the Guest.
 */
static void setup_rng(void)
{
    struct device *dev;
    struct rng_info *rng_info = malloc(sizeof(*rng_info));

    /* Our device's privat info simply contains the /dev/random fd. */
    rng_info->rfd = open_or_die("/dev/random", O_RDONLY);

    /* Create the new device. */
    dev = new_device("rng", VIRTIO_ID_RNG);
    dev->priv = rng_info;

    /* The device has one virtqueue, where the Guest places inbufs. */
    add_virtqueue(dev, VIRTQUEUE_NUM, rng_input);

    verbose("device %u: rng\n", devices.device_num++);
}
/* That's the end of device setup. */

/*L:230 Reboot is pretty easy: clean up and exec() the Launcher afresh. */
static void __attribute__((noreturn)) restart_guest(void)
{
    unsigned int i;

    /*
     * Since we don't track all open fds, we simply close everything beyond
     * stderr.
     */
    for (i = 3; i < FD_SETSIZE; i++)
        close(i);

    /* Reset all the devices (kills all threads). */
    cleanup_devices();

    execv(main_args[0], main_args);
    err(1, "Could not exec %s", main_args[0]);
}

/*L:220
 * Finally we reach the core of the Launcher which runs the Guest, serves
 * its input and output, and finally, lays it to rest.
 */
static void __attribute__((noreturn)) run_guest(void)
{
    for (;;) {
        unsigned long notify_addr;
        int readval;

        /* We read from the /dev/lguest device to run the Guest. */
        readval = pread(lguest_fd, &notify_addr,
                sizeof(notify_addr), cpu_id);

        /* One unsigned long means the Guest did HCALL_NOTIFY */
        if (readval == sizeof(notify_addr)) {
            verbose("Notify on address %#lx\n", notify_addr);
            handle_output(notify_addr);
        /* ENOENT means the Guest died.  Reading tells us why. */
        } else if (errno == ENOENT) {
            char reason[1024] = { 0 };
            pread(lguest_fd, reason, sizeof(reason)-1, cpu_id);
            errx(1, "%s", reason);
        /* ERESTART means that we need to reboot the guest */
        } else if (errno == ERESTART) {
            restart_guest();
        /* Anything else means a bug or incompatible change. */
        } else
            err(1, "Running guest failed");
    }
}
/*L:240
 * This is the end of the Launcher.  The good news: we are over halfway
 * through!  The bad news: the most fiendish part of the code still lies ahead
 * of us.
 *
 * Are you ready?  Take a deep breath and join me in the core of the Host, in
 * "make Host".
:*/

static struct option opts[] = {
    { "verbose", 0, NULL, 'v' },
    { "tunnet", 1, NULL, 't' },
    { "block", 1, NULL, 'b' },
    { "rng", 0, NULL, 'r' },
    { "initrd", 1, NULL, 'i' },
    { "username", 1, NULL, 'u' },
    { "chroot", 1, NULL, 'c' },
    { NULL },
};
static void usage(void)
{
    errx(1, "Usage: lguest [--verbose] "
         "[--tunnet=(<ipaddr>:<macaddr>|bridge:<bridgename>:<macaddr>)\n"
         "|--block=<filename>|--initrd=<filename>]...\n"
         "<mem-in-mb> vmlinux [args...]");
}

/*L:105 The main routine is where the real work begins: */
int main(int argc, char *argv[])
{
    /* Memory, code startpoint and size of the (optional) initrd. */
    unsigned long mem = 0, start, initrd_size = 0;
    /* Two temporaries. */
    int i, c;
    /* The boot information for the Guest. */
    struct boot_params *boot;
    /* If they specify an initrd file to load. */
    const char *initrd_name = NULL;

    /* Password structure for initgroups/setres[gu]id */
    struct passwd *user_details = NULL;

    /* Directory to chroot to */
    char *chroot_path = NULL;

    /* Save the args: we "reboot" by execing ourselves again. */
    main_args = argv;

    /*
     * First we initialize the device list.  We keep a pointer to the last
     * device, and the next interrupt number to use for devices (1:
     * remember that 0 is used by the timer).
     */
    devices.lastdev = NULL;
    devices.next_irq = 1;

    /* We're CPU 0.  In fact, that's the only CPU possible right now. */
    cpu_id = 0;

    /*
     * We need to know how much memory so we can set up the device
     * descriptor and memory pages for the devices as we parse the command
     * line.  So we quickly look through the arguments to find the amount
     * of memory now.
     */
    for (i = 1; i < argc; i++) {
        if (argv[i][0] != '-') {
            mem = atoi(argv[i]) * 1024 * 1024;
            /*
             * We start by mapping anonymous pages over all of
             * guest-physical memory range.  This fills it with 0,
             * and ensures that the Guest won't be killed when it
             * tries to access it.
             */
            guest_base = map_zeroed_pages(mem / getpagesize()
                              + DEVICE_PAGES);
            guest_limit = mem;
            guest_max = mem + DEVICE_PAGES*getpagesize();
            devices.descpage = get_pages(1);
            break;
        }
    }

    /* The options are fairly straight-forward */
    while ((c = getopt_long(argc, argv, "v", opts, NULL)) != EOF) {
        switch (c) {
        case 'v':
            verbose = true;
            break;
        case 't':
            setup_tun_net(optarg);
            break;
        case 'b':
            setup_block_file(optarg);
            break;
        case 'r':
            setup_rng();
            break;
        case 'i':
            initrd_name = optarg;
            break;
        case 'u':
            user_details = getpwnam(optarg);
            if (!user_details)
                err(1, "getpwnam failed, incorrect username?");
            break;
        case 'c':
            chroot_path = optarg;
            break;
        default:
            warnx("Unknown argument %s", argv[optind]);
            usage();
        }
    }
    /*
     * After the other arguments we expect memory and kernel image name,
     * followed by command line arguments for the kernel.
     */
    if (optind + 2 > argc)
        usage();

    verbose("Guest base is at %p\n", guest_base);

    /* We always have a console device */
    setup_console();

    /* Now we load the kernel */
    start = load_kernel(open_or_die(argv[optind+1], O_RDONLY));

    /* Boot information is stashed at physical address 0 */
    boot = from_guest_phys(0);

    /* Map the initrd image if requested (at top of physical memory) */
    if (initrd_name) {
        initrd_size = load_initrd(initrd_name, mem);
        /*
         * These are the location in the Linux boot header where the
         * start and size of the initrd are expected to be found.
         */
        boot->hdr.ramdisk_image = mem - initrd_size;
        boot->hdr.ramdisk_size = initrd_size;
        /* The bootloader type 0xFF means "unknown"; that's OK. */
        boot->hdr.type_of_loader = 0xFF;
    }

    /*
     * The Linux boot header contains an "E820" memory map: ours is a
     * simple, single region.
     */
    boot->e820_entries = 1;
    boot->e820_map[0] = ((struct e820entry) { 0, mem, E820_RAM });
    /*
     * The boot header contains a command line pointer: we put the command
     * line after the boot header.
     */
    boot->hdr.cmd_line_ptr = to_guest_phys(boot + 1);
    /* We use a simple helper to copy the arguments separated by spaces. */
    concat((char *)(boot + 1), argv+optind+2);

    /* Set kernel alignment to 16M (CONFIG_PHYSICAL_ALIGN) */
    boot->hdr.kernel_alignment = 0x1000000;

    /* Boot protocol version: 2.07 supports the fields for lguest. */
    boot->hdr.version = 0x207;

    /* The hardware_subarch value of "1" tells the Guest it's an lguest. */
    boot->hdr.hardware_subarch = 1;

    /* Tell the entry path not to try to reload segment registers. */
    boot->hdr.loadflags |= KEEP_SEGMENTS;

    /* We tell the kernel to initialize the Guest. */
    tell_kernel(start);

    /* Ensure that we terminate if a device-servicing child dies. */
    signal(SIGCHLD, kill_launcher);

    /* If we exit via err(), this kills all the threads, restores tty. */
    atexit(cleanup_devices);

    /* If requested, chroot to a directory */
    if (chroot_path) {
        if (chroot(chroot_path) != 0)
            err(1, "chroot(\"%s\") failed", chroot_path);

        if (chdir("/") != 0)
            err(1, "chdir(\"/\") failed");

        verbose("chroot done\n");
    }

    /* If requested, drop privileges */
    if (user_details) {
        uid_t u;
        gid_t g;

        u = user_details->pw_uid;
        g = user_details->pw_gid;

        if (initgroups(user_details->pw_name, g) != 0)
            err(1, "initgroups failed");

        if (setresgid(g, g, g) != 0)
            err(1, "setresgid failed");

        if (setresuid(u, u, u) != 0)
            err(1, "setresuid failed");

        verbose("Dropping privileges completed\n");
    }

    /* Finally, run the Guest.  This doesn't return. */
    run_guest();
}
/*:*/

/*M:999
 * Mastery is done: you now know everything I do.
 *
 * But surely you have seen code, features and bugs in your wanderings which
 * you now yearn to attack?  That is the real game, and I look forward to you
 * patching and forking lguest into the Your-Name-Here-visor.
 *
 * Farewell, and good coding!
 * Rusty Russell.
 */