vhost: introduce vhost_vq_avail_empty()

[mirror_ubuntu-bionic-kernel.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/virtual/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/uio.h>
#include <linux/mm.h>
#include <linux/mmu_context.h>
#include <linux/miscdevice.h>
#include <linux/mutex.h>
#include <linux/poll.h>
#include <linux/file.h>
#include <linux/highmem.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/kthread.h>
#include <linux/cgroup.h>
#include <linux/module.h>
#include <linux/sort.h>

#include "vhost.h"

static ushort max_mem_regions = 64;
module_param(max_mem_regions, ushort, 0444);
MODULE_PARM_DESC(max_mem_regions,
        "Maximum number of memory regions in memory map. (default: 64)");

enum {
        VHOST_MEMORY_F_LOG = 0x1,
};

#define vhost_used_event(vq) ((__virtio16 __user *)&vq->avail->ring[vq->num])
#define vhost_avail_event(vq) ((__virtio16 __user *)&vq->used->ring[vq->num])

#ifdef CONFIG_VHOST_CROSS_ENDIAN_LEGACY
static void vhost_disable_cross_endian(struct vhost_virtqueue *vq)
{
        vq->user_be = !virtio_legacy_is_little_endian();
}

static void vhost_enable_cross_endian_big(struct vhost_virtqueue *vq)
{
        vq->user_be = true;
}

static void vhost_enable_cross_endian_little(struct vhost_virtqueue *vq)
{
        vq->user_be = false;
}

static long vhost_set_vring_endian(struct vhost_virtqueue *vq, int __user *argp)
{
        struct vhost_vring_state s;

        if (vq->private_data)
                return -EBUSY;

        if (copy_from_user(&s, argp, sizeof(s)))
                return -EFAULT;

        if (s.num != VHOST_VRING_LITTLE_ENDIAN &&
            s.num != VHOST_VRING_BIG_ENDIAN)
                return -EINVAL;

        if (s.num == VHOST_VRING_BIG_ENDIAN)
                vhost_enable_cross_endian_big(vq);
        else
                vhost_enable_cross_endian_little(vq);

        return 0;
}

static long vhost_get_vring_endian(struct vhost_virtqueue *vq, u32 idx,
                                   int __user *argp)
{
        struct vhost_vring_state s = {
                .index = idx,
                .num = vq->user_be
        };

        if (copy_to_user(argp, &s, sizeof(s)))
                return -EFAULT;

        return 0;
}

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
        /* Note for legacy virtio: user_be is initialized at reset time
         * according to the host endianness. If userspace does not set an
         * explicit endianness, the default behavior is native endian, as
         * expected by legacy virtio.
         */
        vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be;
}
#else
static void vhost_disable_cross_endian(struct vhost_virtqueue *vq)
{
}

static long vhost_set_vring_endian(struct vhost_virtqueue *vq, int __user *argp)
{
        return -ENOIOCTLCMD;
}

static long vhost_get_vring_endian(struct vhost_virtqueue *vq, u32 idx,
                                   int __user *argp)
{
        return -ENOIOCTLCMD;
}

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
        if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
                vq->is_le = true;
}
#endif /* CONFIG_VHOST_CROSS_ENDIAN_LEGACY */

static void vhost_reset_is_le(struct vhost_virtqueue *vq)
{
        vq->is_le = virtio_legacy_is_little_endian();
}

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 = container_of(wait, struct vhost_poll, wait);

        if (!((unsigned long)key & poll->mask))
                return 0;

        vhost_poll_queue(poll);
        return 0;
}

void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
{
        INIT_LIST_HEAD(&work->node);
        work->fn = fn;
        init_waitqueue_head(&work->done);
        work->flushing = 0;
        work->queue_seq = work->done_seq = 0;
}
EXPORT_SYMBOL_GPL(vhost_work_init);

/* Init poll structure */
void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
                     unsigned long mask, struct vhost_dev *dev)
{
        init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
        init_poll_funcptr(&poll->table, vhost_poll_func);
        poll->mask = mask;
        poll->dev = dev;
        poll->wqh = NULL;

        vhost_work_init(&poll->work, fn);
}
EXPORT_SYMBOL_GPL(vhost_poll_init);

/* 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. */
int vhost_poll_start(struct vhost_poll *poll, struct file *file)
{
        unsigned long mask;
        int ret = 0;

        if (poll->wqh)
                return 0;

        mask = file->f_op->poll(file, &poll->table);
        if (mask)
                vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
        if (mask & POLLERR) {
                if (poll->wqh)
                        remove_wait_queue(poll->wqh, &poll->wait);
                ret = -EINVAL;
        }

        return ret;
}
EXPORT_SYMBOL_GPL(vhost_poll_start);

/* 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)
{
        if (poll->wqh) {
                remove_wait_queue(poll->wqh, &poll->wait);
                poll->wqh = NULL;
        }
}
EXPORT_SYMBOL_GPL(vhost_poll_stop);

static bool vhost_work_seq_done(struct vhost_dev *dev, struct vhost_work *work,
                                unsigned seq)
{
        int left;

        spin_lock_irq(&dev->work_lock);
        left = seq - work->done_seq;
        spin_unlock_irq(&dev->work_lock);
        return left <= 0;
}

void vhost_work_flush(struct vhost_dev *dev, struct vhost_work *work)
{
        unsigned seq;
        int flushing;

        spin_lock_irq(&dev->work_lock);
        seq = work->queue_seq;
        work->flushing++;
        spin_unlock_irq(&dev->work_lock);
        wait_event(work->done, vhost_work_seq_done(dev, work, seq));
        spin_lock_irq(&dev->work_lock);
        flushing = --work->flushing;
        spin_unlock_irq(&dev->work_lock);
        BUG_ON(flushing < 0);
}
EXPORT_SYMBOL_GPL(vhost_work_flush);

/* 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)
{
        vhost_work_flush(poll->dev, &poll->work);
}
EXPORT_SYMBOL_GPL(vhost_poll_flush);

void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
{
        unsigned long flags;

        spin_lock_irqsave(&dev->work_lock, flags);
        if (list_empty(&work->node)) {
                list_add_tail(&work->node, &dev->work_list);
                work->queue_seq++;
                spin_unlock_irqrestore(&dev->work_lock, flags);
                wake_up_process(dev->worker);
        } else {
                spin_unlock_irqrestore(&dev->work_lock, flags);
        }
}
EXPORT_SYMBOL_GPL(vhost_work_queue);

/* A lockless hint for busy polling code to exit the loop */
bool vhost_has_work(struct vhost_dev *dev)
{
        return !list_empty(&dev->work_list);
}
EXPORT_SYMBOL_GPL(vhost_has_work);

void vhost_poll_queue(struct vhost_poll *poll)
{
        vhost_work_queue(poll->dev, &poll->work);
}
EXPORT_SYMBOL_GPL(vhost_poll_queue);

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->signalled_used = 0;
        vq->signalled_used_valid = false;
        vq->used_flags = 0;
        vq->log_used = false;
        vq->log_addr = -1ull;
        vq->private_data = NULL;
        vq->acked_features = 0;
        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;
        vq->memory = NULL;
        vhost_reset_is_le(vq);
        vhost_disable_cross_endian(vq);
}

static int vhost_worker(void *data)
{
        struct vhost_dev *dev = data;
        struct vhost_work *work = NULL;
        unsigned uninitialized_var(seq);
        mm_segment_t oldfs = get_fs();

        set_fs(USER_DS);
        use_mm(dev->mm);

        for (;;) {
                /* mb paired w/ kthread_stop */
                set_current_state(TASK_INTERRUPTIBLE);

                spin_lock_irq(&dev->work_lock);
                if (work) {
                        work->done_seq = seq;
                        if (work->flushing)
                                wake_up_all(&work->done);
                }

                if (kthread_should_stop()) {
                        spin_unlock_irq(&dev->work_lock);
                        __set_current_state(TASK_RUNNING);
                        break;
                }
                if (!list_empty(&dev->work_list)) {
                        work = list_first_entry(&dev->work_list,
                                                struct vhost_work, node);
                        list_del_init(&work->node);
                        seq = work->queue_seq;
                } else
                        work = NULL;
                spin_unlock_irq(&dev->work_lock);

                if (work) {
                        __set_current_state(TASK_RUNNING);
                        work->fn(work);
                        if (need_resched())
                                schedule();
                } else
                        schedule();

        }
        unuse_mm(dev->mm);
        set_fs(oldfs);
        return 0;
}

static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
{
        kfree(vq->indirect);
        vq->indirect = NULL;
        kfree(vq->log);
        vq->log = NULL;
        kfree(vq->heads);
        vq->heads = NULL;
}

/* Helper to allocate iovec buffers for all vqs. */
static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
{
        struct vhost_virtqueue *vq;
        int i;

        for (i = 0; i < dev->nvqs; ++i) {
                vq = dev->vqs[i];
                vq->indirect = kmalloc(sizeof *vq->indirect * UIO_MAXIOV,
                                       GFP_KERNEL);
                vq->log = kmalloc(sizeof *vq->log * UIO_MAXIOV, GFP_KERNEL);
                vq->heads = kmalloc(sizeof *vq->heads * UIO_MAXIOV, GFP_KERNEL);
                if (!vq->indirect || !vq->log || !vq->heads)
                        goto err_nomem;
        }
        return 0;

err_nomem:
        for (; i >= 0; --i)
                vhost_vq_free_iovecs(dev->vqs[i]);
        return -ENOMEM;
}

static void vhost_dev_free_iovecs(struct vhost_dev *dev)
{
        int i;

        for (i = 0; i < dev->nvqs; ++i)
                vhost_vq_free_iovecs(dev->vqs[i]);
}

void vhost_dev_init(struct vhost_dev *dev,
                    struct vhost_virtqueue **vqs, int nvqs)
{
        struct vhost_virtqueue *vq;
        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;
        spin_lock_init(&dev->work_lock);
        INIT_LIST_HEAD(&dev->work_list);
        dev->worker = NULL;

        for (i = 0; i < dev->nvqs; ++i) {
                vq = dev->vqs[i];
                vq->log = NULL;
                vq->indirect = NULL;
                vq->heads = NULL;
                vq->dev = dev;
                mutex_init(&vq->mutex);
                vhost_vq_reset(dev, vq);
                if (vq->handle_kick)
                        vhost_poll_init(&vq->poll, vq->handle_kick,
                                        POLLIN, dev);
        }
}
EXPORT_SYMBOL_GPL(vhost_dev_init);

/* 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;
}
EXPORT_SYMBOL_GPL(vhost_dev_check_owner);

struct vhost_attach_cgroups_struct {
        struct vhost_work work;
        struct task_struct *owner;
        int ret;
};

static void vhost_attach_cgroups_work(struct vhost_work *work)
{
        struct vhost_attach_cgroups_struct *s;

        s = container_of(work, struct vhost_attach_cgroups_struct, work);
        s->ret = cgroup_attach_task_all(s->owner, current);
}

static int vhost_attach_cgroups(struct vhost_dev *dev)
{
        struct vhost_attach_cgroups_struct attach;

        attach.owner = current;
        vhost_work_init(&attach.work, vhost_attach_cgroups_work);
        vhost_work_queue(dev, &attach.work);
        vhost_work_flush(dev, &attach.work);
        return attach.ret;
}

/* Caller should have device mutex */
bool vhost_dev_has_owner(struct vhost_dev *dev)
{
        return dev->mm;
}
EXPORT_SYMBOL_GPL(vhost_dev_has_owner);

/* Caller should have device mutex */
long vhost_dev_set_owner(struct vhost_dev *dev)
{
        struct task_struct *worker;
        int err;

        /* Is there an owner already? */
        if (vhost_dev_has_owner(dev)) {
                err = -EBUSY;
                goto err_mm;
        }

        /* No owner, become one */
        dev->mm = get_task_mm(current);
        worker = kthread_create(vhost_worker, dev, "vhost-%d", current->pid);
        if (IS_ERR(worker)) {
                err = PTR_ERR(worker);
                goto err_worker;
        }

        dev->worker = worker;
        wake_up_process(worker);        /* avoid contributing to loadavg */

        err = vhost_attach_cgroups(dev);
        if (err)
                goto err_cgroup;

        err = vhost_dev_alloc_iovecs(dev);
        if (err)
                goto err_cgroup;

        return 0;
err_cgroup:
        kthread_stop(worker);
        dev->worker = NULL;
err_worker:
        if (dev->mm)
                mmput(dev->mm);
        dev->mm = NULL;
err_mm:
        return err;
}
EXPORT_SYMBOL_GPL(vhost_dev_set_owner);

struct vhost_memory *vhost_dev_reset_owner_prepare(void)
{
        return kmalloc(offsetof(struct vhost_memory, regions), GFP_KERNEL);
}
EXPORT_SYMBOL_GPL(vhost_dev_reset_owner_prepare);

/* Caller should have device mutex */
void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_memory *memory)
{
        int i;

        vhost_dev_cleanup(dev, true);

        /* Restore memory to default empty mapping. */
        memory->nregions = 0;
        dev->memory = memory;
        /* We don't need VQ locks below since vhost_dev_cleanup makes sure
         * VQs aren't running.
         */
        for (i = 0; i < dev->nvqs; ++i)
                dev->vqs[i]->memory = memory;
}
EXPORT_SYMBOL_GPL(vhost_dev_reset_owner);

void vhost_dev_stop(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);
                }
        }
}
EXPORT_SYMBOL_GPL(vhost_dev_stop);

/* Caller should have device mutex if and only if locked is set */
void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
{
        int i;

        for (i = 0; i < dev->nvqs; ++i) {
                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]);
        }
        vhost_dev_free_iovecs(dev);
        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 */
        kvfree(dev->memory);
        dev->memory = NULL;
        WARN_ON(!list_empty(&dev->work_list));
        if (dev->worker) {
                kthread_stop(dev->worker);
                dev->worker = NULL;
        }
        if (dev->mm)
                mmput(dev->mm);
        dev->mm = NULL;
}
EXPORT_SYMBOL_GPL(vhost_dev_cleanup);

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 0;

        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;

        if (!mem)
                return 0;

        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;
                bool log;

                mutex_lock(&d->vqs[i]->mutex);
                log = log_all || vhost_has_feature(d->vqs[i], VHOST_F_LOG_ALL);
                /* 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);
                else
                        ok = 1;
                mutex_unlock(&d->vqs[i]->mutex);
                if (!ok)
                        return 0;
        }
        return 1;
}

static int vq_access_ok(struct vhost_virtqueue *vq, unsigned int num,
                        struct vring_desc __user *desc,
                        struct vring_avail __user *avail,
                        struct vring_used __user *used)
{
        size_t s = vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX) ? 2 : 0;
        return access_ok(VERIFY_READ, desc, num * sizeof *desc) &&
               access_ok(VERIFY_READ, avail,
                         sizeof *avail + num * sizeof *avail->ring + s) &&
               access_ok(VERIFY_WRITE, used,
                        sizeof *used + num * sizeof *used->ring + s);
}

/* 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);
}
EXPORT_SYMBOL_GPL(vhost_log_access_ok);

/* 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)
{
        size_t s = vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX) ? 2 : 0;

        return vq_memory_access_ok(log_base, vq->memory,
                                   vhost_has_feature(vq, VHOST_F_LOG_ALL)) &&
                (!vq->log_used || log_access_ok(log_base, vq->log_addr,
                                        sizeof *vq->used +
                                        vq->num * sizeof *vq->used->ring + s));
}

/* 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, vq->num, vq->desc, vq->avail, vq->used) &&
                vq_log_access_ok(vq, vq->log_base);
}
EXPORT_SYMBOL_GPL(vhost_vq_access_ok);

static int vhost_memory_reg_sort_cmp(const void *p1, const void *p2)
{
        const struct vhost_memory_region *r1 = p1, *r2 = p2;
        if (r1->guest_phys_addr < r2->guest_phys_addr)
                return 1;
        if (r1->guest_phys_addr > r2->guest_phys_addr)
                return -1;
        return 0;
}

static void *vhost_kvzalloc(unsigned long size)
{
        void *n = kzalloc(size, GFP_KERNEL | __GFP_NOWARN | __GFP_REPEAT);

        if (!n)
                n = vzalloc(size);
        return n;
}

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);
        int i;

        if (copy_from_user(&mem, m, size))
                return -EFAULT;
        if (mem.padding)
                return -EOPNOTSUPP;
        if (mem.nregions > max_mem_regions)
                return -E2BIG;
        newmem = vhost_kvzalloc(size + mem.nregions * sizeof(*m->regions));
        if (!newmem)
                return -ENOMEM;

        memcpy(newmem, &mem, size);
        if (copy_from_user(newmem->regions, m->regions,
                           mem.nregions * sizeof *m->regions)) {
                kvfree(newmem);
                return -EFAULT;
        }
        sort(newmem->regions, newmem->nregions, sizeof(*newmem->regions),
                vhost_memory_reg_sort_cmp, NULL);

        if (!memory_access_ok(d, newmem, 0)) {
                kvfree(newmem);
                return -EFAULT;
        }
        oldmem = d->memory;
        d->memory = newmem;

        /* All memory accesses are done under some VQ mutex. */
        for (i = 0; i < d->nvqs; ++i) {
                mutex_lock(&d->vqs[i]->mutex);
                d->vqs[i]->memory = newmem;
                mutex_unlock(&d->vqs[i]->mutex);
        }
        kvfree(oldmem);
        return 0;
}

long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
{
        struct file *eventfp, *filep = NULL;
        bool pollstart = false, pollstop = false;
        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;
                }
                if (copy_from_user(&s, argp, sizeof s)) {
                        r = -EFAULT;
                        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;
                }
                if (copy_from_user(&s, argp, sizeof s)) {
                        r = -EFAULT;
                        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;
                if (copy_to_user(argp, &s, sizeof s))
                        r = -EFAULT;
                break;
        case VHOST_SET_VRING_ADDR:
                if (copy_from_user(&a, argp, sizeof a)) {
                        r = -EFAULT;
                        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;
                }

                /* Make sure it's safe to cast pointers to vring types. */
                BUILD_BUG_ON(__alignof__ *vq->avail > VRING_AVAIL_ALIGN_SIZE);
                BUILD_BUG_ON(__alignof__ *vq->used > VRING_USED_ALIGN_SIZE);
                if ((a.avail_user_addr & (VRING_AVAIL_ALIGN_SIZE - 1)) ||
                    (a.used_user_addr & (VRING_USED_ALIGN_SIZE - 1)) ||
                    (a.log_guest_addr & (VRING_USED_ALIGN_SIZE - 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, 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;
                        }
                }

                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:
                if (copy_from_user(&f, argp, sizeof f)) {
                        r = -EFAULT;
                        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) != NULL;
                        pollstart = (vq->kick = eventfp) != NULL;
                } else
                        filep = eventfp;
                break;
        case VHOST_SET_VRING_CALL:
                if (copy_from_user(&f, argp, sizeof f)) {
                        r = -EFAULT;
                        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:
                if (copy_from_user(&f, argp, sizeof f)) {
                        r = -EFAULT;
                        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;
        case VHOST_SET_VRING_ENDIAN:
                r = vhost_set_vring_endian(vq, argp);
                break;
        case VHOST_GET_VRING_ENDIAN:
                r = vhost_get_vring_endian(vq, idx, argp);
                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)
                r = vhost_poll_start(&vq->poll, vq->kick);

        mutex_unlock(&vq->mutex);

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

/* Caller must have device mutex */
long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
{
        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:
                if (copy_from_user(&p, argp, sizeof p)) {
                        r = -EFAULT;
                        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;
                        d->log_file = eventfp;
                        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 = -ENOIOCTLCMD;
                break;
        }
done:
        return r;
}
EXPORT_SYMBOL_GPL(vhost_dev_ioctl);

static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
                                                     __u64 addr, __u32 len)
{
        const struct vhost_memory_region *reg;
        int start = 0, end = mem->nregions;

        while (start < end) {
                int slot = start + (end - start) / 2;
                reg = mem->regions + slot;
                if (addr >= reg->guest_phys_addr)
                        end = slot;
                else
                        start = slot + 1;
        }

        reg = mem->regions + start;
        if (addr >= reg->guest_phys_addr &&
                reg->guest_phys_addr + reg->memory_size > 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);
        set_bit(bit, base);
        kunmap_atomic(base);
        set_page_dirty_lock(page);
        put_page(page);
        return 0;
}

static int log_write(void __user *log_base,
                     u64 write_address, u64 write_length)
{
        u64 write_page = write_address / VHOST_PAGE_SIZE;
        int r;

        if (!write_length)
                return 0;
        write_length += write_address % VHOST_PAGE_SIZE;
        for (;;) {
                u64 base = (u64)(unsigned long)log_base;
                u64 log = base + write_page / 8;
                int bit = write_page % 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_page += 1;
        }
        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) {
                        if (vq->log_ctx)
                                eventfd_signal(vq->log_ctx, 1);
                        return 0;
                }
        }
        /* Length written exceeds what we have stored. This is a bug. */
        BUG();
        return 0;
}
EXPORT_SYMBOL_GPL(vhost_log_write);

static int vhost_update_used_flags(struct vhost_virtqueue *vq)
{
        void __user *used;
        if (__put_user(cpu_to_vhost16(vq, vq->used_flags), &vq->used->flags) < 0)
                return -EFAULT;
        if (unlikely(vq->log_used)) {
                /* Make sure the flag is seen before log. */
                smp_wmb();
                /* Log used flag write. */
                used = &vq->used->flags;
                log_write(vq->log_base, vq->log_addr +
                          (used - (void __user *)vq->used),
                          sizeof vq->used->flags);
                if (vq->log_ctx)
                        eventfd_signal(vq->log_ctx, 1);
        }
        return 0;
}

static int vhost_update_avail_event(struct vhost_virtqueue *vq, u16 avail_event)
{
        if (__put_user(cpu_to_vhost16(vq, vq->avail_idx), vhost_avail_event(vq)))
                return -EFAULT;
        if (unlikely(vq->log_used)) {
                void __user *used;
                /* Make sure the event is seen before log. */
                smp_wmb();
                /* Log avail event write */
                used = vhost_avail_event(vq);
                log_write(vq->log_base, vq->log_addr +
                          (used - (void __user *)vq->used),
                          sizeof *vhost_avail_event(vq));
                if (vq->log_ctx)
                        eventfd_signal(vq->log_ctx, 1);
        }
        return 0;
}

int vhost_vq_init_access(struct vhost_virtqueue *vq)
{
        __virtio16 last_used_idx;
        int r;
        bool is_le = vq->is_le;

        if (!vq->private_data) {
                vhost_reset_is_le(vq);
                return 0;
        }

        vhost_init_is_le(vq);

        r = vhost_update_used_flags(vq);
        if (r)
                goto err;
        vq->signalled_used_valid = false;
        if (!access_ok(VERIFY_READ, &vq->used->idx, sizeof vq->used->idx)) {
                r = -EFAULT;
                goto err;
        }
        r = __get_user(last_used_idx, &vq->used->idx);
        if (r)
                goto err;
        vq->last_used_idx = vhost16_to_cpu(vq, last_used_idx);
        return 0;
err:
        vq->is_le = is_le;
        return r;
}
EXPORT_SYMBOL_GPL(vhost_vq_init_access);

static int translate_desc(struct vhost_virtqueue *vq, 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;

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

        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 vhost_virtqueue *vq, struct vring_desc *desc)
{
        unsigned int next;

        /* If this descriptor says it doesn't chain, we're done. */
        if (!(desc->flags & cpu_to_vhost16(vq, VRING_DESC_F_NEXT)))
                return -1U;

        /* Check they're not leading us off end of descriptors. */
        next = vhost16_to_cpu(vq, 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 int get_indirect(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;
        u32 len = vhost32_to_cpu(vq, indirect->len);
        struct iov_iter from;
        int ret;

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

        ret = translate_desc(vq, vhost64_to_cpu(vq, indirect->addr), len, vq->indirect,
                             UIO_MAXIOV);
        if (unlikely(ret < 0)) {
                vq_err(vq, "Translation failure %d in indirect.\n", ret);
                return ret;
        }
        iov_iter_init(&from, READ, vq->indirect, ret, len);

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

        count = len / sizeof desc;
        /* Buffers are chained via a 16 bit next field, so
         * we can have at most 2^16 of these. */
        if (unlikely(count > USHRT_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 (unlikely(++found > count)) {
                        vq_err(vq, "Loop detected: last one at %u "
                               "indirect size %u\n",
                               i, count);
                        return -EINVAL;
                }
                if (unlikely(copy_from_iter(&desc, sizeof(desc), &from) !=
                             sizeof(desc))) {
                        vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
                               i, (size_t)vhost64_to_cpu(vq, indirect->addr) + i * sizeof desc);
                        return -EINVAL;
                }
                if (unlikely(desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT))) {
                        vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
                               i, (size_t)vhost64_to_cpu(vq, indirect->addr) + i * sizeof desc);
                        return -EINVAL;
                }

                ret = translate_desc(vq, vhost64_to_cpu(vq, desc.addr),
                                     vhost32_to_cpu(vq, desc.len), iov + iov_count,
                                     iov_size - iov_count);
                if (unlikely(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 & cpu_to_vhost16(vq, VRING_DESC_F_WRITE)) {
                        *in_num += ret;
                        if (unlikely(log)) {
                                log[*log_num].addr = vhost64_to_cpu(vq, desc.addr);
                                log[*log_num].len = vhost32_to_cpu(vq, desc.len);
                                ++*log_num;
                        }
                } else {
                        /* If it's an output descriptor, they're all supposed
                         * to come before any input descriptors. */
                        if (unlikely(*in_num)) {
                                vq_err(vq, "Indirect descriptor "
                                       "has out after in: idx %d\n", i);
                                return -EINVAL;
                        }
                        *out_num += ret;
                }
        } while ((i = next_desc(vq, &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.  A negative code is
 * returned on error. */
int vhost_get_vq_desc(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;
        __virtio16 avail_idx;
        __virtio16 ring_head;
        int ret;

        /* Check it isn't doing very strange things with descriptor numbers. */
        last_avail_idx = vq->last_avail_idx;
        if (unlikely(__get_user(avail_idx, &vq->avail->idx))) {
                vq_err(vq, "Failed to access avail idx at %p\n",
                       &vq->avail->idx);
                return -EFAULT;
        }
        vq->avail_idx = vhost16_to_cpu(vq, avail_idx);

        if (unlikely((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 -EFAULT;
        }

        /* 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 (unlikely(__get_user(ring_head,
                                &vq->avail->ring[last_avail_idx & (vq->num - 1)]))) {
                vq_err(vq, "Failed to read head: idx %d address %p\n",
                       last_avail_idx,
                       &vq->avail->ring[last_avail_idx % vq->num]);
                return -EFAULT;
        }

        head = vhost16_to_cpu(vq, ring_head);

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

        /* 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 (unlikely(i >= vq->num)) {
                        vq_err(vq, "Desc index is %u > %u, head = %u",
                               i, vq->num, head);
                        return -EINVAL;
                }
                if (unlikely(++found > vq->num)) {
                        vq_err(vq, "Loop detected: last one at %u "
                               "vq size %u head %u\n",
                               i, vq->num, head);
                        return -EINVAL;
                }
                ret = __copy_from_user(&desc, vq->desc + i, sizeof desc);
                if (unlikely(ret)) {
                        vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
                               i, vq->desc + i);
                        return -EFAULT;
                }
                if (desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT)) {
                        ret = get_indirect(vq, iov, iov_size,
                                           out_num, in_num,
                                           log, log_num, &desc);
                        if (unlikely(ret < 0)) {
                                vq_err(vq, "Failure detected "
                                       "in indirect descriptor at idx %d\n", i);
                                return ret;
                        }
                        continue;
                }

                ret = translate_desc(vq, vhost64_to_cpu(vq, desc.addr),
                                     vhost32_to_cpu(vq, desc.len), iov + iov_count,
                                     iov_size - iov_count);
                if (unlikely(ret < 0)) {
                        vq_err(vq, "Translation failure %d descriptor idx %d\n",
                               ret, i);
                        return ret;
                }
                if (desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_WRITE)) {
                        /* If this is an input descriptor,
                         * increment that count. */
                        *in_num += ret;
                        if (unlikely(log)) {
                                log[*log_num].addr = vhost64_to_cpu(vq, desc.addr);
                                log[*log_num].len = vhost32_to_cpu(vq, desc.len);
                                ++*log_num;
                        }
                } else {
                        /* If it's an output descriptor, they're all supposed
                         * to come before any input descriptors. */
                        if (unlikely(*in_num)) {
                                vq_err(vq, "Descriptor has out after in: "
                                       "idx %d\n", i);
                                return -EINVAL;
                        }
                        *out_num += ret;
                }
        } while ((i = next_desc(vq, &desc)) != -1);

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

        /* Assume notifications from guest are disabled at this point,
         * if they aren't we would need to update avail_event index. */
        BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
        return head;
}
EXPORT_SYMBOL_GPL(vhost_get_vq_desc);

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

/* 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 heads = {
                cpu_to_vhost32(vq, head),
                cpu_to_vhost32(vq, len)
        };

        return vhost_add_used_n(vq, &heads, 1);
}
EXPORT_SYMBOL_GPL(vhost_add_used);

static int __vhost_add_used_n(struct vhost_virtqueue *vq,
                            struct vring_used_elem *heads,
                            unsigned count)
{
        struct vring_used_elem __user *used;
        u16 old, new;
        int start;

        start = vq->last_used_idx & (vq->num - 1);
        used = vq->used->ring + start;
        if (count == 1) {
                if (__put_user(heads[0].id, &used->id)) {
                        vq_err(vq, "Failed to write used id");
                        return -EFAULT;
                }
                if (__put_user(heads[0].len, &used->len)) {
                        vq_err(vq, "Failed to write used len");
                        return -EFAULT;
                }
        } else if (__copy_to_user(used, heads, count * sizeof *used)) {
                vq_err(vq, "Failed to write used");
                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 __user *)used - (void __user *)vq->used),
                          count * sizeof *used);
        }
        old = vq->last_used_idx;
        new = (vq->last_used_idx += count);
        /* If the driver never bothers to signal in a very long while,
         * used index might wrap around. If that happens, invalidate
         * signalled_used index we stored. TODO: make sure driver
         * signals at least once in 2^16 and remove this. */
        if (unlikely((u16)(new - vq->signalled_used) < (u16)(new - old)))
                vq->signalled_used_valid = false;
        return 0;
}

/* 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_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
                     unsigned count)
{
        int start, n, r;

        start = vq->last_used_idx & (vq->num - 1);
        n = vq->num - start;
        if (n < count) {
                r = __vhost_add_used_n(vq, heads, n);
                if (r < 0)
                        return r;
                heads += n;
                count -= n;
        }
        r = __vhost_add_used_n(vq, heads, count);

        /* Make sure buffer is written before we update index. */
        smp_wmb();
        if (__put_user(cpu_to_vhost16(vq, vq->last_used_idx), &vq->used->idx)) {
                vq_err(vq, "Failed to increment used idx");
                return -EFAULT;
        }
        if (unlikely(vq->log_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);
        }
        return r;
}
EXPORT_SYMBOL_GPL(vhost_add_used_n);

static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
        __u16 old, new;
        __virtio16 event;
        bool v;
        /* Flush out used index updates. This is paired
         * with the barrier that the Guest executes when enabling
         * interrupts. */
        smp_mb();

        if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
            unlikely(vq->avail_idx == vq->last_avail_idx))
                return true;

        if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
                __virtio16 flags;
                if (__get_user(flags, &vq->avail->flags)) {
                        vq_err(vq, "Failed to get flags");
                        return true;
                }
                return !(flags & cpu_to_vhost16(vq, VRING_AVAIL_F_NO_INTERRUPT));
        }
        old = vq->signalled_used;
        v = vq->signalled_used_valid;
        new = vq->signalled_used = vq->last_used_idx;
        vq->signalled_used_valid = true;

        if (unlikely(!v))
                return true;

        if (__get_user(event, vhost_used_event(vq))) {
                vq_err(vq, "Failed to get used event idx");
                return true;
        }
        return vring_need_event(vhost16_to_cpu(vq, event), new, old);
}

/* This actually signals the guest, using eventfd. */
void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
        /* Signal the Guest tell them we used something up. */
        if (vq->call_ctx && vhost_notify(dev, vq))
                eventfd_signal(vq->call_ctx, 1);
}
EXPORT_SYMBOL_GPL(vhost_signal);

/* 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);
}
EXPORT_SYMBOL_GPL(vhost_add_used_and_signal);

/* multi-buffer version of vhost_add_used_and_signal */
void vhost_add_used_and_signal_n(struct vhost_dev *dev,
                                 struct vhost_virtqueue *vq,
                                 struct vring_used_elem *heads, unsigned count)
{
        vhost_add_used_n(vq, heads, count);
        vhost_signal(dev, vq);
}
EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);

/* return true if we're sure that avaiable ring is empty */
bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
        __virtio16 avail_idx;
        int r;

        r = __get_user(avail_idx, &vq->avail->idx);
        if (r)
                return false;

        return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
}
EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);

/* OK, now we need to know about added descriptors. */
bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
        __virtio16 avail_idx;
        int r;

        if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
                return false;
        vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
        if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
                r = vhost_update_used_flags(vq);
                if (r) {
                        vq_err(vq, "Failed to enable notification at %p: %d\n",
                               &vq->used->flags, r);
                        return false;
                }
        } else {
                r = vhost_update_avail_event(vq, vq->avail_idx);
                if (r) {
                        vq_err(vq, "Failed to update avail event index at %p: %d\n",
                               vhost_avail_event(vq), 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 vhost16_to_cpu(vq, avail_idx) != vq->avail_idx;
}
EXPORT_SYMBOL_GPL(vhost_enable_notify);

/* We don't need to be notified again. */
void vhost_disable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
        int r;

        if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
                return;
        vq->used_flags |= VRING_USED_F_NO_NOTIFY;
        if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
                r = vhost_update_used_flags(vq);
                if (r)
                        vq_err(vq, "Failed to enable notification at %p: %d\n",
                               &vq->used->flags, r);
        }
}
EXPORT_SYMBOL_GPL(vhost_disable_notify);

static int __init vhost_init(void)
{
        return 0;
}

static void __exit vhost_exit(void)
{
}

module_init(vhost_init);
module_exit(vhost_exit);

MODULE_VERSION("0.0.1");
MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("Michael S. Tsirkin");
MODULE_DESCRIPTION("Host kernel accelerator for virtio");