Linux 2.6.34-rc3

[pohmelfs.git] / drivers / vhost / vhost.c

/* Copyright (C) 2009 Red Hat, Inc.
 * Copyright (C) 2006 Rusty Russell IBM Corporation
 *
 * Author: Michael S. Tsirkin <mst@redhat.com>
 *
 * Inspiration, some code, and most witty comments come from
 * Documentation/lguest/lguest.c, by Rusty Russell
 *
 * This work is licensed under the terms of the GNU GPL, version 2.
 *
 * Generic code for virtio server in host kernel.
 */

#include <linux/eventfd.h>
#include <linux/vhost.h>
#include <linux/virtio_net.h>
#include <linux/mm.h>
#include <linux/miscdevice.h>
#include <linux/mutex.h>
#include <linux/workqueue.h>
#include <linux/rcupdate.h>
#include <linux/poll.h>
#include <linux/file.h>
#include <linux/highmem.h>

#include <linux/net.h>
#include <linux/if_packet.h>
#include <linux/if_arp.h>

#include <net/sock.h>

#include "vhost.h"

enum {
        VHOST_MEMORY_MAX_NREGIONS = 64,
        VHOST_MEMORY_F_LOG = 0x1,
};

static struct workqueue_struct *vhost_workqueue;

static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
                            poll_table *pt)
{
        struct vhost_poll *poll;
        poll = container_of(pt, struct vhost_poll, table);

        poll->wqh = wqh;
        add_wait_queue(wqh, &poll->wait);
}

static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,
                             void *key)
{
        struct vhost_poll *poll;
        poll = container_of(wait, struct vhost_poll, wait);
        if (!((unsigned long)key & poll->mask))
                return 0;

        queue_work(vhost_workqueue, &poll->work);
        return 0;
}

/* Init poll structure */
void vhost_poll_init(struct vhost_poll *poll, work_func_t func,
                     unsigned long mask)
{
        INIT_WORK(&poll->work, func);
        init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
        init_poll_funcptr(&poll->table, vhost_poll_func);
        poll->mask = mask;
}

/* Start polling a file. We add ourselves to file's wait queue. The caller must
 * keep a reference to a file until after vhost_poll_stop is called. */
void vhost_poll_start(struct vhost_poll *poll, struct file *file)
{
        unsigned long mask;
        mask = file->f_op->poll(file, &poll->table);
        if (mask)
                vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
}

/* Stop polling a file. After this function returns, it becomes safe to drop the
 * file reference. You must also flush afterwards. */
void vhost_poll_stop(struct vhost_poll *poll)
{
        remove_wait_queue(poll->wqh, &poll->wait);
}

/* Flush any work that has been scheduled. When calling this, don't hold any
 * locks that are also used by the callback. */
void vhost_poll_flush(struct vhost_poll *poll)
{
        flush_work(&poll->work);
}

void vhost_poll_queue(struct vhost_poll *poll)
{
        queue_work(vhost_workqueue, &poll->work);
}

static void vhost_vq_reset(struct vhost_dev *dev,
                           struct vhost_virtqueue *vq)
{
        vq->num = 1;
        vq->desc = NULL;
        vq->avail = NULL;
        vq->used = NULL;
        vq->last_avail_idx = 0;
        vq->avail_idx = 0;
        vq->last_used_idx = 0;
        vq->used_flags = 0;
        vq->used_flags = 0;
        vq->log_used = false;
        vq->log_addr = -1ull;
        vq->hdr_size = 0;
        vq->private_data = NULL;
        vq->log_base = NULL;
        vq->error_ctx = NULL;
        vq->error = NULL;
        vq->kick = NULL;
        vq->call_ctx = NULL;
        vq->call = NULL;
        vq->log_ctx = NULL;
}

long vhost_dev_init(struct vhost_dev *dev,
                    struct vhost_virtqueue *vqs, int nvqs)
{
        int i;
        dev->vqs = vqs;
        dev->nvqs = nvqs;
        mutex_init(&dev->mutex);
        dev->log_ctx = NULL;
        dev->log_file = NULL;
        dev->memory = NULL;
        dev->mm = NULL;

        for (i = 0; i < dev->nvqs; ++i) {
                dev->vqs[i].dev = dev;
                mutex_init(&dev->vqs[i].mutex);
                vhost_vq_reset(dev, dev->vqs + i);
                if (dev->vqs[i].handle_kick)
                        vhost_poll_init(&dev->vqs[i].poll,
                                        dev->vqs[i].handle_kick,
                                        POLLIN);
        }
        return 0;
}

/* Caller should have device mutex */
long vhost_dev_check_owner(struct vhost_dev *dev)
{
        /* Are you the owner? If not, I don't think you mean to do that */
        return dev->mm == current->mm ? 0 : -EPERM;
}

/* Caller should have device mutex */
static long vhost_dev_set_owner(struct vhost_dev *dev)
{
        /* Is there an owner already? */
        if (dev->mm)
                return -EBUSY;
        /* No owner, become one */
        dev->mm = get_task_mm(current);
        return 0;
}

/* Caller should have device mutex */
long vhost_dev_reset_owner(struct vhost_dev *dev)
{
        struct vhost_memory *memory;

        /* Restore memory to default empty mapping. */
        memory = kmalloc(offsetof(struct vhost_memory, regions), GFP_KERNEL);
        if (!memory)
                return -ENOMEM;

        vhost_dev_cleanup(dev);

        memory->nregions = 0;
        dev->memory = memory;
        return 0;
}

/* Caller should have device mutex */
void vhost_dev_cleanup(struct vhost_dev *dev)
{
        int i;
        for (i = 0; i < dev->nvqs; ++i) {
                if (dev->vqs[i].kick && dev->vqs[i].handle_kick) {
                        vhost_poll_stop(&dev->vqs[i].poll);
                        vhost_poll_flush(&dev->vqs[i].poll);
                }
                if (dev->vqs[i].error_ctx)
                        eventfd_ctx_put(dev->vqs[i].error_ctx);
                if (dev->vqs[i].error)
                        fput(dev->vqs[i].error);
                if (dev->vqs[i].kick)
                        fput(dev->vqs[i].kick);
                if (dev->vqs[i].call_ctx)
                        eventfd_ctx_put(dev->vqs[i].call_ctx);
                if (dev->vqs[i].call)
                        fput(dev->vqs[i].call);
                vhost_vq_reset(dev, dev->vqs + i);
        }
        if (dev->log_ctx)
                eventfd_ctx_put(dev->log_ctx);
        dev->log_ctx = NULL;
        if (dev->log_file)
                fput(dev->log_file);
        dev->log_file = NULL;
        /* No one will access memory at this point */
        kfree(dev->memory);
        dev->memory = NULL;
        if (dev->mm)
                mmput(dev->mm);
        dev->mm = NULL;
}

static int log_access_ok(void __user *log_base, u64 addr, unsigned long sz)
{
        u64 a = addr / VHOST_PAGE_SIZE / 8;
        /* Make sure 64 bit math will not overflow. */
        if (a > ULONG_MAX - (unsigned long)log_base ||
            a + (unsigned long)log_base > ULONG_MAX)
                return -EFAULT;

        return access_ok(VERIFY_WRITE, log_base + a,
                         (sz + VHOST_PAGE_SIZE * 8 - 1) / VHOST_PAGE_SIZE / 8);
}

/* Caller should have vq mutex and device mutex. */
static int vq_memory_access_ok(void __user *log_base, struct vhost_memory *mem,
                               int log_all)
{
        int i;
        for (i = 0; i < mem->nregions; ++i) {
                struct vhost_memory_region *m = mem->regions + i;
                unsigned long a = m->userspace_addr;
                if (m->memory_size > ULONG_MAX)
                        return 0;
                else if (!access_ok(VERIFY_WRITE, (void __user *)a,
                                    m->memory_size))
                        return 0;
                else if (log_all && !log_access_ok(log_base,
                                                   m->guest_phys_addr,
                                                   m->memory_size))
                        return 0;
        }
        return 1;
}

/* Can we switch to this memory table? */
/* Caller should have device mutex but not vq mutex */
static int memory_access_ok(struct vhost_dev *d, struct vhost_memory *mem,
                            int log_all)
{
        int i;
        for (i = 0; i < d->nvqs; ++i) {
                int ok;
                mutex_lock(&d->vqs[i].mutex);
                /* If ring is inactive, will check when it's enabled. */
                if (d->vqs[i].private_data)
                        ok = vq_memory_access_ok(d->vqs[i].log_base, mem,
                                                 log_all);
                else
                        ok = 1;
                mutex_unlock(&d->vqs[i].mutex);
                if (!ok)
                        return 0;
        }
        return 1;
}

static int vq_access_ok(unsigned int num,
                        struct vring_desc __user *desc,
                        struct vring_avail __user *avail,
                        struct vring_used __user *used)
{
        return access_ok(VERIFY_READ, desc, num * sizeof *desc) &&
               access_ok(VERIFY_READ, avail,
                         sizeof *avail + num * sizeof *avail->ring) &&
               access_ok(VERIFY_WRITE, used,
                        sizeof *used + num * sizeof *used->ring);
}

/* Can we log writes? */
/* Caller should have device mutex but not vq mutex */
int vhost_log_access_ok(struct vhost_dev *dev)
{
        return memory_access_ok(dev, dev->memory, 1);
}

/* Verify access for write logging. */
/* Caller should have vq mutex and device mutex */
static int vq_log_access_ok(struct vhost_virtqueue *vq, void __user *log_base)
{
        return vq_memory_access_ok(log_base, vq->dev->memory,
                            vhost_has_feature(vq->dev, VHOST_F_LOG_ALL)) &&
                (!vq->log_used || log_access_ok(log_base, vq->log_addr,
                                        sizeof *vq->used +
                                        vq->num * sizeof *vq->used->ring));
}

/* Can we start vq? */
/* Caller should have vq mutex and device mutex */
int vhost_vq_access_ok(struct vhost_virtqueue *vq)
{
        return vq_access_ok(vq->num, vq->desc, vq->avail, vq->used) &&
                vq_log_access_ok(vq, vq->log_base);
}

static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
{
        struct vhost_memory mem, *newmem, *oldmem;
        unsigned long size = offsetof(struct vhost_memory, regions);
        long r;
        r = copy_from_user(&mem, m, size);
        if (r)
                return r;
        if (mem.padding)
                return -EOPNOTSUPP;
        if (mem.nregions > VHOST_MEMORY_MAX_NREGIONS)
                return -E2BIG;
        newmem = kmalloc(size + mem.nregions * sizeof *m->regions, GFP_KERNEL);
        if (!newmem)
                return -ENOMEM;

        memcpy(newmem, &mem, size);
        r = copy_from_user(newmem->regions, m->regions,
                           mem.nregions * sizeof *m->regions);
        if (r) {
                kfree(newmem);
                return r;
        }

        if (!memory_access_ok(d, newmem, vhost_has_feature(d, VHOST_F_LOG_ALL)))
                return -EFAULT;
        oldmem = d->memory;
        rcu_assign_pointer(d->memory, newmem);
        synchronize_rcu();
        kfree(oldmem);
        return 0;
}

static int init_used(struct vhost_virtqueue *vq,
                     struct vring_used __user *used)
{
        int r = put_user(vq->used_flags, &used->flags);
        if (r)
                return r;
        return get_user(vq->last_used_idx, &used->idx);
}

static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
{
        struct file *eventfp, *filep = NULL,
                    *pollstart = NULL, *pollstop = NULL;
        struct eventfd_ctx *ctx = NULL;
        u32 __user *idxp = argp;
        struct vhost_virtqueue *vq;
        struct vhost_vring_state s;
        struct vhost_vring_file f;
        struct vhost_vring_addr a;
        u32 idx;
        long r;

        r = get_user(idx, idxp);
        if (r < 0)
                return r;
        if (idx > d->nvqs)
                return -ENOBUFS;

        vq = d->vqs + idx;

        mutex_lock(&vq->mutex);

        switch (ioctl) {
        case VHOST_SET_VRING_NUM:
                /* Resizing ring with an active backend?
                 * You don't want to do that. */
                if (vq->private_data) {
                        r = -EBUSY;
                        break;
                }
                r = copy_from_user(&s, argp, sizeof s);
                if (r < 0)
                        break;
                if (!s.num || s.num > 0xffff || (s.num & (s.num - 1))) {
                        r = -EINVAL;
                        break;
                }
                vq->num = s.num;
                break;
        case VHOST_SET_VRING_BASE:
                /* Moving base with an active backend?
                 * You don't want to do that. */
                if (vq->private_data) {
                        r = -EBUSY;
                        break;
                }
                r = copy_from_user(&s, argp, sizeof s);
                if (r < 0)
                        break;
                if (s.num > 0xffff) {
                        r = -EINVAL;
                        break;
                }
                vq->last_avail_idx = s.num;
                /* Forget the cached index value. */
                vq->avail_idx = vq->last_avail_idx;
                break;
        case VHOST_GET_VRING_BASE:
                s.index = idx;
                s.num = vq->last_avail_idx;
                r = copy_to_user(argp, &s, sizeof s);
                break;
        case VHOST_SET_VRING_ADDR:
                r = copy_from_user(&a, argp, sizeof a);
                if (r < 0)
                        break;
                if (a.flags & ~(0x1 << VHOST_VRING_F_LOG)) {
                        r = -EOPNOTSUPP;
                        break;
                }
                /* For 32bit, verify that the top 32bits of the user
                   data are set to zero. */
                if ((u64)(unsigned long)a.desc_user_addr != a.desc_user_addr ||
                    (u64)(unsigned long)a.used_user_addr != a.used_user_addr ||
                    (u64)(unsigned long)a.avail_user_addr != a.avail_user_addr) {
                        r = -EFAULT;
                        break;
                }
                if ((a.avail_user_addr & (sizeof *vq->avail->ring - 1)) ||
                    (a.used_user_addr & (sizeof *vq->used->ring - 1)) ||
                    (a.log_guest_addr & (sizeof *vq->used->ring - 1))) {
                        r = -EINVAL;
                        break;
                }

                /* We only verify access here if backend is configured.
                 * If it is not, we don't as size might not have been setup.
                 * We will verify when backend is configured. */
                if (vq->private_data) {
                        if (!vq_access_ok(vq->num,
                                (void __user *)(unsigned long)a.desc_user_addr,
                                (void __user *)(unsigned long)a.avail_user_addr,
                                (void __user *)(unsigned long)a.used_user_addr)) {
                                r = -EINVAL;
                                break;
                        }

                        /* Also validate log access for used ring if enabled. */
                        if ((a.flags & (0x1 << VHOST_VRING_F_LOG)) &&
                            !log_access_ok(vq->log_base, a.log_guest_addr,
                                           sizeof *vq->used +
                                           vq->num * sizeof *vq->used->ring)) {
                                r = -EINVAL;
                                break;
                        }
                }

                r = init_used(vq, (struct vring_used __user *)(unsigned long)
                              a.used_user_addr);
                if (r)
                        break;
                vq->log_used = !!(a.flags & (0x1 << VHOST_VRING_F_LOG));
                vq->desc = (void __user *)(unsigned long)a.desc_user_addr;
                vq->avail = (void __user *)(unsigned long)a.avail_user_addr;
                vq->log_addr = a.log_guest_addr;
                vq->used = (void __user *)(unsigned long)a.used_user_addr;
                break;
        case VHOST_SET_VRING_KICK:
                r = copy_from_user(&f, argp, sizeof f);
                if (r < 0)
                        break;
                eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
                if (IS_ERR(eventfp)) {
                        r = PTR_ERR(eventfp);
                        break;
                }
                if (eventfp != vq->kick) {
                        pollstop = filep = vq->kick;
                        pollstart = vq->kick = eventfp;
                } else
                        filep = eventfp;
                break;
        case VHOST_SET_VRING_CALL:
                r = copy_from_user(&f, argp, sizeof f);
                if (r < 0)
                        break;
                eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
                if (IS_ERR(eventfp)) {
                        r = PTR_ERR(eventfp);
                        break;
                }
                if (eventfp != vq->call) {
                        filep = vq->call;
                        ctx = vq->call_ctx;
                        vq->call = eventfp;
                        vq->call_ctx = eventfp ?
                                eventfd_ctx_fileget(eventfp) : NULL;
                } else
                        filep = eventfp;
                break;
        case VHOST_SET_VRING_ERR:
                r = copy_from_user(&f, argp, sizeof f);
                if (r < 0)
                        break;
                eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
                if (IS_ERR(eventfp)) {
                        r = PTR_ERR(eventfp);
                        break;
                }
                if (eventfp != vq->error) {
                        filep = vq->error;
                        vq->error = eventfp;
                        ctx = vq->error_ctx;
                        vq->error_ctx = eventfp ?
                                eventfd_ctx_fileget(eventfp) : NULL;
                } else
                        filep = eventfp;
                break;
        default:
                r = -ENOIOCTLCMD;
        }

        if (pollstop && vq->handle_kick)
                vhost_poll_stop(&vq->poll);

        if (ctx)
                eventfd_ctx_put(ctx);
        if (filep)
                fput(filep);

        if (pollstart && vq->handle_kick)
                vhost_poll_start(&vq->poll, vq->kick);

        mutex_unlock(&vq->mutex);

        if (pollstop && vq->handle_kick)
                vhost_poll_flush(&vq->poll);
        return r;
}

/* Caller must have device mutex */
long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, unsigned long arg)
{
        void __user *argp = (void __user *)arg;
        struct file *eventfp, *filep = NULL;
        struct eventfd_ctx *ctx = NULL;
        u64 p;
        long r;
        int i, fd;

        /* If you are not the owner, you can become one */
        if (ioctl == VHOST_SET_OWNER) {
                r = vhost_dev_set_owner(d);
                goto done;
        }

        /* You must be the owner to do anything else */
        r = vhost_dev_check_owner(d);
        if (r)
                goto done;

        switch (ioctl) {
        case VHOST_SET_MEM_TABLE:
                r = vhost_set_memory(d, argp);
                break;
        case VHOST_SET_LOG_BASE:
                r = copy_from_user(&p, argp, sizeof p);
                if (r < 0)
                        break;
                if ((u64)(unsigned long)p != p) {
                        r = -EFAULT;
                        break;
                }
                for (i = 0; i < d->nvqs; ++i) {
                        struct vhost_virtqueue *vq;
                        void __user *base = (void __user *)(unsigned long)p;
                        vq = d->vqs + i;
                        mutex_lock(&vq->mutex);
                        /* If ring is inactive, will check when it's enabled. */
                        if (vq->private_data && !vq_log_access_ok(vq, base))
                                r = -EFAULT;
                        else
                                vq->log_base = base;
                        mutex_unlock(&vq->mutex);
                }
                break;
        case VHOST_SET_LOG_FD:
                r = get_user(fd, (int __user *)argp);
                if (r < 0)
                        break;
                eventfp = fd == -1 ? NULL : eventfd_fget(fd);
                if (IS_ERR(eventfp)) {
                        r = PTR_ERR(eventfp);
                        break;
                }
                if (eventfp != d->log_file) {
                        filep = d->log_file;
                        ctx = d->log_ctx;
                        d->log_ctx = eventfp ?
                                eventfd_ctx_fileget(eventfp) : NULL;
                } else
                        filep = eventfp;
                for (i = 0; i < d->nvqs; ++i) {
                        mutex_lock(&d->vqs[i].mutex);
                        d->vqs[i].log_ctx = d->log_ctx;
                        mutex_unlock(&d->vqs[i].mutex);
                }
                if (ctx)
                        eventfd_ctx_put(ctx);
                if (filep)
                        fput(filep);
                break;
        default:
                r = vhost_set_vring(d, ioctl, argp);
                break;
        }
done:
        return r;
}

static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
                                                     __u64 addr, __u32 len)
{
        struct vhost_memory_region *reg;
        int i;
        /* linear search is not brilliant, but we really have on the order of 6
         * regions in practice */
        for (i = 0; i < mem->nregions; ++i) {
                reg = mem->regions + i;
                if (reg->guest_phys_addr <= addr &&
                    reg->guest_phys_addr + reg->memory_size - 1 >= addr)
                        return reg;
        }
        return NULL;
}

/* TODO: This is really inefficient.  We need something like get_user()
 * (instruction directly accesses the data, with an exception table entry
 * returning -EFAULT). See Documentation/x86/exception-tables.txt.
 */
static int set_bit_to_user(int nr, void __user *addr)
{
        unsigned long log = (unsigned long)addr;
        struct page *page;
        void *base;
        int bit = nr + (log % PAGE_SIZE) * 8;
        int r;
        r = get_user_pages_fast(log, 1, 1, &page);
        if (r < 0)
                return r;
        BUG_ON(r != 1);
        base = kmap_atomic(page, KM_USER0);
        set_bit(bit, base);
        kunmap_atomic(base, KM_USER0);
        set_page_dirty_lock(page);
        put_page(page);
        return 0;
}

static int log_write(void __user *log_base,
                     u64 write_address, u64 write_length)
{
        int r;
        if (!write_length)
                return 0;
        write_address /= VHOST_PAGE_SIZE;
        for (;;) {
                u64 base = (u64)(unsigned long)log_base;
                u64 log = base + write_address / 8;
                int bit = write_address % 8;
                if ((u64)(unsigned long)log != log)
                        return -EFAULT;
                r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
                if (r < 0)
                        return r;
                if (write_length <= VHOST_PAGE_SIZE)
                        break;
                write_length -= VHOST_PAGE_SIZE;
                write_address += VHOST_PAGE_SIZE;
        }
        return r;
}

int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
                    unsigned int log_num, u64 len)
{
        int i, r;

        /* Make sure data written is seen before log. */
        smp_wmb();
        for (i = 0; i < log_num; ++i) {
                u64 l = min(log[i].len, len);
                r = log_write(vq->log_base, log[i].addr, l);
                if (r < 0)
                        return r;
                len -= l;
                if (!len)
                        return 0;
        }
        if (vq->log_ctx)
                eventfd_signal(vq->log_ctx, 1);
        /* Length written exceeds what we have stored. This is a bug. */
        BUG();
        return 0;
}

int translate_desc(struct vhost_dev *dev, u64 addr, u32 len,
                   struct iovec iov[], int iov_size)
{
        const struct vhost_memory_region *reg;
        struct vhost_memory *mem;
        struct iovec *_iov;
        u64 s = 0;
        int ret = 0;

        rcu_read_lock();

        mem = rcu_dereference(dev->memory);
        while ((u64)len > s) {
                u64 size;
                if (ret >= iov_size) {
                        ret = -ENOBUFS;
                        break;
                }
                reg = find_region(mem, addr, len);
                if (!reg) {
                        ret = -EFAULT;
                        break;
                }
                _iov = iov + ret;
                size = reg->memory_size - addr + reg->guest_phys_addr;
                _iov->iov_len = min((u64)len, size);
                _iov->iov_base = (void *)(unsigned long)
                        (reg->userspace_addr + addr - reg->guest_phys_addr);
                s += size;
                addr += size;
                ++ret;
        }

        rcu_read_unlock();
        return ret;
}

/* Each buffer in the virtqueues is actually a chain of descriptors.  This
 * function returns the next descriptor in the chain,
 * or -1U if we're at the end. */
static unsigned next_desc(struct vring_desc *desc)
{
        unsigned int next;

        /* If this descriptor says it doesn't chain, we're done. */
        if (!(desc->flags & VRING_DESC_F_NEXT))
                return -1U;

        /* Check they're not leading us off end of descriptors. */
        next = desc->next;
        /* Make sure compiler knows to grab that: we don't want it changing! */
        /* We will use the result as an index in an array, so most
         * architectures only need a compiler barrier here. */
        read_barrier_depends();

        return next;
}

static unsigned get_indirect(struct vhost_dev *dev, struct vhost_virtqueue *vq,
                             struct iovec iov[], unsigned int iov_size,
                             unsigned int *out_num, unsigned int *in_num,
                             struct vhost_log *log, unsigned int *log_num,
                             struct vring_desc *indirect)
{
        struct vring_desc desc;
        unsigned int i = 0, count, found = 0;
        int ret;

        /* Sanity check */
        if (indirect->len % sizeof desc) {
                vq_err(vq, "Invalid length in indirect descriptor: "
                       "len 0x%llx not multiple of 0x%zx\n",
                       (unsigned long long)indirect->len,
                       sizeof desc);
                return -EINVAL;
        }

        ret = translate_desc(dev, indirect->addr, indirect->len, vq->indirect,
                             ARRAY_SIZE(vq->indirect));
        if (ret < 0) {
                vq_err(vq, "Translation failure %d in indirect.\n", ret);
                return ret;
        }

        /* We will use the result as an address to read from, so most
         * architectures only need a compiler barrier here. */
        read_barrier_depends();

        count = indirect->len / sizeof desc;
        /* Buffers are chained via a 16 bit next field, so
         * we can have at most 2^16 of these. */
        if (count > USHORT_MAX + 1) {
                vq_err(vq, "Indirect buffer length too big: %d\n",
                       indirect->len);
                return -E2BIG;
        }

        do {
                unsigned iov_count = *in_num + *out_num;
                if (++found > count) {
                        vq_err(vq, "Loop detected: last one at %u "
                               "indirect size %u\n",
                               i, count);
                        return -EINVAL;
                }
                if (memcpy_fromiovec((unsigned char *)&desc, vq->indirect,
                                     sizeof desc)) {
                        vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
                               i, (size_t)indirect->addr + i * sizeof desc);
                        return -EINVAL;
                }
                if (desc.flags & VRING_DESC_F_INDIRECT) {
                        vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
                               i, (size_t)indirect->addr + i * sizeof desc);
                        return -EINVAL;
                }

                ret = translate_desc(dev, desc.addr, desc.len, iov + iov_count,
                                     iov_size - iov_count);
                if (ret < 0) {
                        vq_err(vq, "Translation failure %d indirect idx %d\n",
                               ret, i);
                        return ret;
                }
                /* If this is an input descriptor, increment that count. */
                if (desc.flags & VRING_DESC_F_WRITE) {
                        *in_num += ret;
                        if (unlikely(log)) {
                                log[*log_num].addr = desc.addr;
                                log[*log_num].len = desc.len;
                                ++*log_num;
                        }
                } else {
                        /* If it's an output descriptor, they're all supposed
                         * to come before any input descriptors. */
                        if (*in_num) {
                                vq_err(vq, "Indirect descriptor "
                                       "has out after in: idx %d\n", i);
                                return -EINVAL;
                        }
                        *out_num += ret;
                }
        } while ((i = next_desc(&desc)) != -1);
        return 0;
}

/* This looks in the virtqueue and 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 returns the descriptor number found, or vq->num (which
 * is never a valid descriptor number) if none was found. */
unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
                           struct iovec iov[], unsigned int iov_size,
                           unsigned int *out_num, unsigned int *in_num,
                           struct vhost_log *log, unsigned int *log_num)
{
        struct vring_desc desc;
        unsigned int i, head, found = 0;
        u16 last_avail_idx;
        int ret;

        /* Check it isn't doing very strange things with descriptor numbers. */
        last_avail_idx = vq->last_avail_idx;
        if (get_user(vq->avail_idx, &vq->avail->idx)) {
                vq_err(vq, "Failed to access avail idx at %p\n",
                       &vq->avail->idx);
                return vq->num;
        }

        if ((u16)(vq->avail_idx - last_avail_idx) > vq->num) {
                vq_err(vq, "Guest moved used index from %u to %u",
                       last_avail_idx, vq->avail_idx);
                return vq->num;
        }

        /* If there's nothing new since last we looked, return invalid. */
        if (vq->avail_idx == last_avail_idx)
                return vq->num;

        /* Only get avail ring entries after they have been exposed by guest. */
        smp_rmb();

        /* Grab the next descriptor number they're advertising, and increment
         * the index we've seen. */
        if (get_user(head, &vq->avail->ring[last_avail_idx % vq->num])) {
                vq_err(vq, "Failed to read head: idx %d address %p\n",
                       last_avail_idx,
                       &vq->avail->ring[last_avail_idx % vq->num]);
                return vq->num;
        }

        /* If their number is silly, that's an error. */
        if (head >= vq->num) {
                vq_err(vq, "Guest says index %u > %u is available",
                       head, vq->num);
                return vq->num;
        }

        /* When we start there are none of either input nor output. */
        *out_num = *in_num = 0;
        if (unlikely(log))
                *log_num = 0;

        i = head;
        do {
                unsigned iov_count = *in_num + *out_num;
                if (i >= vq->num) {
                        vq_err(vq, "Desc index is %u > %u, head = %u",
                               i, vq->num, head);
                        return vq->num;
                }
                if (++found > vq->num) {
                        vq_err(vq, "Loop detected: last one at %u "
                               "vq size %u head %u\n",
                               i, vq->num, head);
                        return vq->num;
                }
                ret = copy_from_user(&desc, vq->desc + i, sizeof desc);
                if (ret) {
                        vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
                               i, vq->desc + i);
                        return vq->num;
                }
                if (desc.flags & VRING_DESC_F_INDIRECT) {
                        ret = get_indirect(dev, vq, iov, iov_size,
                                           out_num, in_num,
                                           log, log_num, &desc);
                        if (ret < 0) {
                                vq_err(vq, "Failure detected "
                                       "in indirect descriptor at idx %d\n", i);
                                return vq->num;
                        }
                        continue;
                }

                ret = translate_desc(dev, desc.addr, desc.len, iov + iov_count,
                                     iov_size - iov_count);
                if (ret < 0) {
                        vq_err(vq, "Translation failure %d descriptor idx %d\n",
                               ret, i);
                        return vq->num;
                }
                if (desc.flags & VRING_DESC_F_WRITE) {
                        /* If this is an input descriptor,
                         * increment that count. */
                        *in_num += ret;
                        if (unlikely(log)) {
                                log[*log_num].addr = desc.addr;
                                log[*log_num].len = desc.len;
                                ++*log_num;
                        }
                } else {
                        /* If it's an output descriptor, they're all supposed
                         * to come before any input descriptors. */
                        if (*in_num) {
                                vq_err(vq, "Descriptor has out after in: "
                                       "idx %d\n", i);
                                return vq->num;
                        }
                        *out_num += ret;
                }
        } while ((i = next_desc(&desc)) != -1);

        /* On success, increment avail index. */
        vq->last_avail_idx++;
        return head;
}

/* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
void vhost_discard_vq_desc(struct vhost_virtqueue *vq)
{
        vq->last_avail_idx--;
}

/* After we've used one of their buffers, we tell them about it.  We'll then
 * want to notify the guest, using eventfd. */
int vhost_add_used(struct vhost_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->used->ring[vq->last_used_idx % vq->num];
        if (put_user(head, &used->id)) {
                vq_err(vq, "Failed to write used id");
                return -EFAULT;
        }
        if (put_user(len, &used->len)) {
                vq_err(vq, "Failed to write used len");
                return -EFAULT;
        }
        /* Make sure buffer is written before we update index. */
        smp_wmb();
        if (put_user(vq->last_used_idx + 1, &vq->used->idx)) {
                vq_err(vq, "Failed to increment used idx");
                return -EFAULT;
        }
        if (unlikely(vq->log_used)) {
                /* Make sure data is seen before log. */
                smp_wmb();
                /* Log used ring entry write. */
                log_write(vq->log_base,
                          vq->log_addr + ((void *)used - (void *)vq->used),
                          sizeof *used);
                /* Log used index update. */
                log_write(vq->log_base,
                          vq->log_addr + offsetof(struct vring_used, idx),
                          sizeof vq->used->idx);
                if (vq->log_ctx)
                        eventfd_signal(vq->log_ctx, 1);
        }
        vq->last_used_idx++;
        return 0;
}

/* This actually signals the guest, using eventfd. */
void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
        __u16 flags = 0;
        if (get_user(flags, &vq->avail->flags)) {
                vq_err(vq, "Failed to get flags");
                return;
        }

        /* If they don't want an interrupt, don't signal, unless empty. */
        if ((flags & VRING_AVAIL_F_NO_INTERRUPT) &&
            (vq->avail_idx != vq->last_avail_idx ||
             !vhost_has_feature(dev, VIRTIO_F_NOTIFY_ON_EMPTY)))
                return;

        /* Signal the Guest tell them we used something up. */
        if (vq->call_ctx)
                eventfd_signal(vq->call_ctx, 1);
}

/* And here's the combo meal deal.  Supersize me! */
void vhost_add_used_and_signal(struct vhost_dev *dev,
                               struct vhost_virtqueue *vq,
                               unsigned int head, int len)
{
        vhost_add_used(vq, head, len);
        vhost_signal(dev, vq);
}

/* OK, now we need to know about added descriptors. */
bool vhost_enable_notify(struct vhost_virtqueue *vq)
{
        u16 avail_idx;
        int r;
        if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
                return false;
        vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
        r = put_user(vq->used_flags, &vq->used->flags);
        if (r) {
                vq_err(vq, "Failed to enable notification at %p: %d\n",
                       &vq->used->flags, r);
                return false;
        }
        /* They could have slipped one in as we were doing that: make
         * sure it's written, then check again. */
        smp_mb();
        r = get_user(avail_idx, &vq->avail->idx);
        if (r) {
                vq_err(vq, "Failed to check avail idx at %p: %d\n",
                       &vq->avail->idx, r);
                return false;
        }

        return avail_idx != vq->last_avail_idx;
}

/* We don't need to be notified again. */
void vhost_disable_notify(struct vhost_virtqueue *vq)
{
        int r;
        if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
                return;
        vq->used_flags |= VRING_USED_F_NO_NOTIFY;
        r = put_user(vq->used_flags, &vq->used->flags);
        if (r)
                vq_err(vq, "Failed to enable notification at %p: %d\n",
                       &vq->used->flags, r);
}

int vhost_init(void)
{
        vhost_workqueue = create_singlethread_workqueue("vhost");
        if (!vhost_workqueue)
                return -ENOMEM;
        return 0;
}

void vhost_cleanup(void)
{
        destroy_workqueue(vhost_workqueue);
}