virtio: use VRingMemoryRegionCaches for avail and used rings

[qemu.git] / hw / virtio / virtio.c

/*
 * Virtio Support
 *
 * Copyright IBM, Corp. 2007
 *
 * Authors:
 *  Anthony Liguori   <[email protected]>
 *
 * This work is licensed under the terms of the GNU GPL, version 2.  See
 * the COPYING file in the top-level directory.
 *
 */

#include "qemu/osdep.h"
#include "qapi/error.h"
#include "qemu-common.h"
#include "cpu.h"
#include "trace.h"
#include "exec/address-spaces.h"
#include "qemu/error-report.h"
#include "hw/virtio/virtio.h"
#include "qemu/atomic.h"
#include "hw/virtio/virtio-bus.h"
#include "migration/migration.h"
#include "hw/virtio/virtio-access.h"
#include "sysemu/dma.h"

/*
 * The alignment to use between consumer and producer parts of vring.
 * x86 pagesize again. This is the default, used by transports like PCI
 * which don't provide a means for the guest to tell the host the alignment.
 */
#define VIRTIO_PCI_VRING_ALIGN         4096

typedef struct VRingDesc
{
    uint64_t addr;
    uint32_t len;
    uint16_t flags;
    uint16_t next;
} VRingDesc;

typedef struct VRingAvail
{
    uint16_t flags;
    uint16_t idx;
    uint16_t ring[0];
} VRingAvail;

typedef struct VRingUsedElem
{
    uint32_t id;
    uint32_t len;
} VRingUsedElem;

typedef struct VRingUsed
{
    uint16_t flags;
    uint16_t idx;
    VRingUsedElem ring[0];
} VRingUsed;

typedef struct VRingMemoryRegionCaches {
    struct rcu_head rcu;
    MemoryRegionCache desc;
    MemoryRegionCache avail;
    MemoryRegionCache used;
} VRingMemoryRegionCaches;

typedef struct VRing
{
    unsigned int num;
    unsigned int num_default;
    unsigned int align;
    hwaddr desc;
    hwaddr avail;
    hwaddr used;
    VRingMemoryRegionCaches *caches;
} VRing;

struct VirtQueue
{
    VRing vring;

    /* Next head to pop */
    uint16_t last_avail_idx;

    /* Last avail_idx read from VQ. */
    uint16_t shadow_avail_idx;

    uint16_t used_idx;

    /* Last used index value we have signalled on */
    uint16_t signalled_used;

    /* Last used index value we have signalled on */
    bool signalled_used_valid;

    /* Notification enabled? */
    bool notification;

    uint16_t queue_index;

    unsigned int inuse;

    uint16_t vector;
    VirtIOHandleOutput handle_output;
    VirtIOHandleAIOOutput handle_aio_output;
    VirtIODevice *vdev;
    EventNotifier guest_notifier;
    EventNotifier host_notifier;
    QLIST_ENTRY(VirtQueue) node;
};

static void virtio_free_region_cache(VRingMemoryRegionCaches *caches)
{
    if (!caches) {
        return;
    }

    address_space_cache_destroy(&caches->desc);
    address_space_cache_destroy(&caches->avail);
    address_space_cache_destroy(&caches->used);
    g_free(caches);
}

static void virtio_init_region_cache(VirtIODevice *vdev, int n)
{
    VirtQueue *vq = &vdev->vq[n];
    VRingMemoryRegionCaches *old = vq->vring.caches;
    VRingMemoryRegionCaches *new;
    hwaddr addr, size;
    int event_size;

    event_size = virtio_vdev_has_feature(vq->vdev, VIRTIO_RING_F_EVENT_IDX) ? 2 : 0;

    addr = vq->vring.desc;
    if (!addr) {
        return;
    }
    new = g_new0(VRingMemoryRegionCaches, 1);
    size = virtio_queue_get_desc_size(vdev, n);
    address_space_cache_init(&new->desc, vdev->dma_as,
                             addr, size, false);

    size = virtio_queue_get_used_size(vdev, n) + event_size;
    address_space_cache_init(&new->used, vdev->dma_as,
                             vq->vring.used, size, true);

    size = virtio_queue_get_avail_size(vdev, n) + event_size;
    address_space_cache_init(&new->avail, vdev->dma_as,
                             vq->vring.avail, size, false);

    atomic_rcu_set(&vq->vring.caches, new);
    if (old) {
        call_rcu(old, virtio_free_region_cache, rcu);
    }
}

/* virt queue functions */
void virtio_queue_update_rings(VirtIODevice *vdev, int n)
{
    VRing *vring = &vdev->vq[n].vring;

    if (!vring->desc) {
        /* not yet setup -> nothing to do */
        return;
    }
    vring->avail = vring->desc + vring->num * sizeof(VRingDesc);
    vring->used = vring_align(vring->avail +
                              offsetof(VRingAvail, ring[vring->num]),
                              vring->align);
    virtio_init_region_cache(vdev, n);
}

/* Called within rcu_read_lock().  */
static void vring_desc_read(VirtIODevice *vdev, VRingDesc *desc,
                            MemoryRegionCache *cache, int i)
{
    address_space_read_cached(cache, i * sizeof(VRingDesc),
                              desc, sizeof(VRingDesc));
    virtio_tswap64s(vdev, &desc->addr);
    virtio_tswap32s(vdev, &desc->len);
    virtio_tswap16s(vdev, &desc->flags);
    virtio_tswap16s(vdev, &desc->next);
}

/* Called within rcu_read_lock().  */
static inline uint16_t vring_avail_flags(VirtQueue *vq)
{
    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
    hwaddr pa = offsetof(VRingAvail, flags);
    return virtio_lduw_phys_cached(vq->vdev, &caches->avail, pa);
}

/* Called within rcu_read_lock().  */
static inline uint16_t vring_avail_idx(VirtQueue *vq)
{
    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
    hwaddr pa = offsetof(VRingAvail, idx);
    vq->shadow_avail_idx = virtio_lduw_phys_cached(vq->vdev, &caches->avail, pa);
    return vq->shadow_avail_idx;
}

/* Called within rcu_read_lock().  */
static inline uint16_t vring_avail_ring(VirtQueue *vq, int i)
{
    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
    hwaddr pa = offsetof(VRingAvail, ring[i]);
    return virtio_lduw_phys_cached(vq->vdev, &caches->avail, pa);
}

/* Called within rcu_read_lock().  */
static inline uint16_t vring_get_used_event(VirtQueue *vq)
{
    return vring_avail_ring(vq, vq->vring.num);
}

/* Called within rcu_read_lock().  */
static inline void vring_used_write(VirtQueue *vq, VRingUsedElem *uelem,
                                    int i)
{
    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
    hwaddr pa = offsetof(VRingUsed, ring[i]);
    virtio_tswap32s(vq->vdev, &uelem->id);
    virtio_tswap32s(vq->vdev, &uelem->len);
    address_space_write_cached(&caches->used, pa, uelem, sizeof(VRingUsedElem));
    address_space_cache_invalidate(&caches->used, pa, sizeof(VRingUsedElem));
}

/* Called within rcu_read_lock().  */
static uint16_t vring_used_idx(VirtQueue *vq)
{
    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
    hwaddr pa = offsetof(VRingUsed, idx);
    return virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
}

/* Called within rcu_read_lock().  */
static inline void vring_used_idx_set(VirtQueue *vq, uint16_t val)
{
    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
    hwaddr pa = offsetof(VRingUsed, idx);
    virtio_stw_phys_cached(vq->vdev, &caches->used, pa, val);
    address_space_cache_invalidate(&caches->used, pa, sizeof(val));
    vq->used_idx = val;
}

/* Called within rcu_read_lock().  */
static inline void vring_used_flags_set_bit(VirtQueue *vq, int mask)
{
    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
    VirtIODevice *vdev = vq->vdev;
    hwaddr pa = offsetof(VRingUsed, flags);
    uint16_t flags = virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);

    virtio_stw_phys_cached(vdev, &caches->used, pa, flags | mask);
    address_space_cache_invalidate(&caches->used, pa, sizeof(flags));
}

/* Called within rcu_read_lock().  */
static inline void vring_used_flags_unset_bit(VirtQueue *vq, int mask)
{
    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
    VirtIODevice *vdev = vq->vdev;
    hwaddr pa = offsetof(VRingUsed, flags);
    uint16_t flags = virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);

    virtio_stw_phys_cached(vdev, &caches->used, pa, flags & ~mask);
    address_space_cache_invalidate(&caches->used, pa, sizeof(flags));
}

/* Called within rcu_read_lock().  */
static inline void vring_set_avail_event(VirtQueue *vq, uint16_t val)
{
    VRingMemoryRegionCaches *caches;
    hwaddr pa;
    if (!vq->notification) {
        return;
    }

    caches = atomic_rcu_read(&vq->vring.caches);
    pa = offsetof(VRingUsed, ring[vq->vring.num]);
    virtio_stw_phys_cached(vq->vdev, &caches->used, pa, val);
}

void virtio_queue_set_notification(VirtQueue *vq, int enable)
{
    vq->notification = enable;

    rcu_read_lock();
    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
        vring_set_avail_event(vq, vring_avail_idx(vq));
    } else if (enable) {
        vring_used_flags_unset_bit(vq, VRING_USED_F_NO_NOTIFY);
    } else {
        vring_used_flags_set_bit(vq, VRING_USED_F_NO_NOTIFY);
    }
    if (enable) {
        /* Expose avail event/used flags before caller checks the avail idx. */
        smp_mb();
    }
    rcu_read_unlock();
}

int virtio_queue_ready(VirtQueue *vq)
{
    return vq->vring.avail != 0;
}

/* Fetch avail_idx from VQ memory only when we really need to know if
 * guest has added some buffers.
 * Called within rcu_read_lock().  */
static int virtio_queue_empty_rcu(VirtQueue *vq)
{
    if (vq->shadow_avail_idx != vq->last_avail_idx) {
        return 0;
    }

    return vring_avail_idx(vq) == vq->last_avail_idx;
}

int virtio_queue_empty(VirtQueue *vq)
{
    bool empty;

    if (vq->shadow_avail_idx != vq->last_avail_idx) {
        return 0;
    }

    rcu_read_lock();
    empty = vring_avail_idx(vq) == vq->last_avail_idx;
    rcu_read_unlock();
    return empty;
}

static void virtqueue_unmap_sg(VirtQueue *vq, const VirtQueueElement *elem,
                               unsigned int len)
{
    AddressSpace *dma_as = vq->vdev->dma_as;
    unsigned int offset;
    int i;

    offset = 0;
    for (i = 0; i < elem->in_num; i++) {
        size_t size = MIN(len - offset, elem->in_sg[i].iov_len);

        dma_memory_unmap(dma_as, elem->in_sg[i].iov_base,
                         elem->in_sg[i].iov_len,
                         DMA_DIRECTION_FROM_DEVICE, size);

        offset += size;
    }

    for (i = 0; i < elem->out_num; i++)
        dma_memory_unmap(dma_as, elem->out_sg[i].iov_base,
                         elem->out_sg[i].iov_len,
                         DMA_DIRECTION_TO_DEVICE,
                         elem->out_sg[i].iov_len);
}

/* virtqueue_detach_element:
 * @vq: The #VirtQueue
 * @elem: The #VirtQueueElement
 * @len: number of bytes written
 *
 * Detach the element from the virtqueue.  This function is suitable for device
 * reset or other situations where a #VirtQueueElement is simply freed and will
 * not be pushed or discarded.
 */
void virtqueue_detach_element(VirtQueue *vq, const VirtQueueElement *elem,
                              unsigned int len)
{
    vq->inuse--;
    virtqueue_unmap_sg(vq, elem, len);
}

/* virtqueue_unpop:
 * @vq: The #VirtQueue
 * @elem: The #VirtQueueElement
 * @len: number of bytes written
 *
 * Pretend the most recent element wasn't popped from the virtqueue.  The next
 * call to virtqueue_pop() will refetch the element.
 */
void virtqueue_unpop(VirtQueue *vq, const VirtQueueElement *elem,
                     unsigned int len)
{
    vq->last_avail_idx--;
    virtqueue_detach_element(vq, elem, len);
}

/* virtqueue_rewind:
 * @vq: The #VirtQueue
 * @num: Number of elements to push back
 *
 * Pretend that elements weren't popped from the virtqueue.  The next
 * virtqueue_pop() will refetch the oldest element.
 *
 * Use virtqueue_unpop() instead if you have a VirtQueueElement.
 *
 * Returns: true on success, false if @num is greater than the number of in use
 * elements.
 */
bool virtqueue_rewind(VirtQueue *vq, unsigned int num)
{
    if (num > vq->inuse) {
        return false;
    }
    vq->last_avail_idx -= num;
    vq->inuse -= num;
    return true;
}

/* Called within rcu_read_lock().  */
void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
                    unsigned int len, unsigned int idx)
{
    VRingUsedElem uelem;

    trace_virtqueue_fill(vq, elem, len, idx);

    virtqueue_unmap_sg(vq, elem, len);

    if (unlikely(vq->vdev->broken)) {
        return;
    }

    idx = (idx + vq->used_idx) % vq->vring.num;

    uelem.id = elem->index;
    uelem.len = len;
    vring_used_write(vq, &uelem, idx);
}

/* Called within rcu_read_lock().  */
void virtqueue_flush(VirtQueue *vq, unsigned int count)
{
    uint16_t old, new;

    if (unlikely(vq->vdev->broken)) {
        vq->inuse -= count;
        return;
    }

    /* Make sure buffer is written before we update index. */
    smp_wmb();
    trace_virtqueue_flush(vq, count);
    old = vq->used_idx;
    new = old + count;
    vring_used_idx_set(vq, new);
    vq->inuse -= count;
    if (unlikely((int16_t)(new - vq->signalled_used) < (uint16_t)(new - old)))
        vq->signalled_used_valid = false;
}

void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
                    unsigned int len)
{
    rcu_read_lock();
    virtqueue_fill(vq, elem, len, 0);
    virtqueue_flush(vq, 1);
    rcu_read_unlock();
}

/* Called within rcu_read_lock().  */
static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
{
    uint16_t num_heads = vring_avail_idx(vq) - idx;

    /* Check it isn't doing very strange things with descriptor numbers. */
    if (num_heads > vq->vring.num) {
        virtio_error(vq->vdev, "Guest moved used index from %u to %u",
                     idx, vq->shadow_avail_idx);
        return -EINVAL;
    }
    /* On success, callers read a descriptor at vq->last_avail_idx.
     * Make sure descriptor read does not bypass avail index read. */
    if (num_heads) {
        smp_rmb();
    }

    return num_heads;
}

/* Called within rcu_read_lock().  */
static bool virtqueue_get_head(VirtQueue *vq, unsigned int idx,
                               unsigned int *head)
{
    /* Grab the next descriptor number they're advertising, and increment
     * the index we've seen. */
    *head = vring_avail_ring(vq, idx % vq->vring.num);

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

    return true;
}

enum {
    VIRTQUEUE_READ_DESC_ERROR = -1,
    VIRTQUEUE_READ_DESC_DONE = 0,   /* end of chain */
    VIRTQUEUE_READ_DESC_MORE = 1,   /* more buffers in chain */
};

static int virtqueue_read_next_desc(VirtIODevice *vdev, VRingDesc *desc,
                                    MemoryRegionCache *desc_cache, unsigned int max,
                                    unsigned int *next)
{
    /* If this descriptor says it doesn't chain, we're done. */
    if (!(desc->flags & VRING_DESC_F_NEXT)) {
        return VIRTQUEUE_READ_DESC_DONE;
    }

    /* 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! */
    smp_wmb();

    if (*next >= max) {
        virtio_error(vdev, "Desc next is %u", *next);
        return VIRTQUEUE_READ_DESC_ERROR;
    }

    vring_desc_read(vdev, desc, desc_cache, *next);
    return VIRTQUEUE_READ_DESC_MORE;
}

void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
                               unsigned int *out_bytes,
                               unsigned max_in_bytes, unsigned max_out_bytes)
{
    VirtIODevice *vdev = vq->vdev;
    unsigned int max, idx;
    unsigned int total_bufs, in_total, out_total;
    VRingMemoryRegionCaches *caches;
    MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
    int64_t len = 0;
    int rc;

    rcu_read_lock();
    idx = vq->last_avail_idx;
    total_bufs = in_total = out_total = 0;

    max = vq->vring.num;
    caches = atomic_rcu_read(&vq->vring.caches);
    if (caches->desc.len < max * sizeof(VRingDesc)) {
        virtio_error(vdev, "Cannot map descriptor ring");
        goto err;
    }

    while ((rc = virtqueue_num_heads(vq, idx)) > 0) {
        MemoryRegionCache *desc_cache = &caches->desc;
        unsigned int num_bufs;
        VRingDesc desc;
        unsigned int i;

        num_bufs = total_bufs;

        if (!virtqueue_get_head(vq, idx++, &i)) {
            goto err;
        }

        vring_desc_read(vdev, &desc, desc_cache, i);

        if (desc.flags & VRING_DESC_F_INDIRECT) {
            if (desc.len % sizeof(VRingDesc)) {
                virtio_error(vdev, "Invalid size for indirect buffer table");
                goto err;
            }

            /* If we've got too many, that implies a descriptor loop. */
            if (num_bufs >= max) {
                virtio_error(vdev, "Looped descriptor");
                goto err;
            }

            /* loop over the indirect descriptor table */
            len = address_space_cache_init(&indirect_desc_cache,
                                           vdev->dma_as,
                                           desc.addr, desc.len, false);
            desc_cache = &indirect_desc_cache;
            if (len < desc.len) {
                virtio_error(vdev, "Cannot map indirect buffer");
                goto err;
            }

            max = desc.len / sizeof(VRingDesc);
            num_bufs = i = 0;
            vring_desc_read(vdev, &desc, desc_cache, i);
        }

        do {
            /* If we've got too many, that implies a descriptor loop. */
            if (++num_bufs > max) {
                virtio_error(vdev, "Looped descriptor");
                goto err;
            }

            if (desc.flags & VRING_DESC_F_WRITE) {
                in_total += desc.len;
            } else {
                out_total += desc.len;
            }
            if (in_total >= max_in_bytes && out_total >= max_out_bytes) {
                goto done;
            }

            rc = virtqueue_read_next_desc(vdev, &desc, desc_cache, max, &i);
        } while (rc == VIRTQUEUE_READ_DESC_MORE);

        if (rc == VIRTQUEUE_READ_DESC_ERROR) {
            goto err;
        }

        if (desc_cache == &indirect_desc_cache) {
            address_space_cache_destroy(&indirect_desc_cache);
            total_bufs++;
        } else {
            total_bufs = num_bufs;
        }
    }

    if (rc < 0) {
        goto err;
    }

done:
    address_space_cache_destroy(&indirect_desc_cache);
    if (in_bytes) {
        *in_bytes = in_total;
    }
    if (out_bytes) {
        *out_bytes = out_total;
    }
    rcu_read_unlock();
    return;

err:
    in_total = out_total = 0;
    goto done;
}

int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
                          unsigned int out_bytes)
{
    unsigned int in_total, out_total;

    virtqueue_get_avail_bytes(vq, &in_total, &out_total, in_bytes, out_bytes);
    return in_bytes <= in_total && out_bytes <= out_total;
}

static bool virtqueue_map_desc(VirtIODevice *vdev, unsigned int *p_num_sg,
                               hwaddr *addr, struct iovec *iov,
                               unsigned int max_num_sg, bool is_write,
                               hwaddr pa, size_t sz)
{
    bool ok = false;
    unsigned num_sg = *p_num_sg;
    assert(num_sg <= max_num_sg);

    if (!sz) {
        virtio_error(vdev, "virtio: zero sized buffers are not allowed");
        goto out;
    }

    while (sz) {
        hwaddr len = sz;

        if (num_sg == max_num_sg) {
            virtio_error(vdev, "virtio: too many write descriptors in "
                               "indirect table");
            goto out;
        }

        iov[num_sg].iov_base = dma_memory_map(vdev->dma_as, pa, &len,
                                              is_write ?
                                              DMA_DIRECTION_FROM_DEVICE :
                                              DMA_DIRECTION_TO_DEVICE);
        if (!iov[num_sg].iov_base) {
            virtio_error(vdev, "virtio: bogus descriptor or out of resources");
            goto out;
        }

        iov[num_sg].iov_len = len;
        addr[num_sg] = pa;

        sz -= len;
        pa += len;
        num_sg++;
    }
    ok = true;

out:
    *p_num_sg = num_sg;
    return ok;
}

/* Only used by error code paths before we have a VirtQueueElement (therefore
 * virtqueue_unmap_sg() can't be used).  Assumes buffers weren't written to
 * yet.
 */
static void virtqueue_undo_map_desc(unsigned int out_num, unsigned int in_num,
                                    struct iovec *iov)
{
    unsigned int i;

    for (i = 0; i < out_num + in_num; i++) {
        int is_write = i >= out_num;

        cpu_physical_memory_unmap(iov->iov_base, iov->iov_len, is_write, 0);
        iov++;
    }
}

static void virtqueue_map_iovec(VirtIODevice *vdev, struct iovec *sg,
                                hwaddr *addr, unsigned int *num_sg,
                                int is_write)
{
    unsigned int i;
    hwaddr len;

    for (i = 0; i < *num_sg; i++) {
        len = sg[i].iov_len;
        sg[i].iov_base = dma_memory_map(vdev->dma_as,
                                        addr[i], &len, is_write ?
                                        DMA_DIRECTION_FROM_DEVICE :
                                        DMA_DIRECTION_TO_DEVICE);
        if (!sg[i].iov_base) {
            error_report("virtio: error trying to map MMIO memory");
            exit(1);
        }
        if (len != sg[i].iov_len) {
            error_report("virtio: unexpected memory split");
            exit(1);
        }
    }
}

void virtqueue_map(VirtIODevice *vdev, VirtQueueElement *elem)
{
    virtqueue_map_iovec(vdev, elem->in_sg, elem->in_addr, &elem->in_num, 1);
    virtqueue_map_iovec(vdev, elem->out_sg, elem->out_addr, &elem->out_num, 0);
}

static void *virtqueue_alloc_element(size_t sz, unsigned out_num, unsigned in_num)
{
    VirtQueueElement *elem;
    size_t in_addr_ofs = QEMU_ALIGN_UP(sz, __alignof__(elem->in_addr[0]));
    size_t out_addr_ofs = in_addr_ofs + in_num * sizeof(elem->in_addr[0]);
    size_t out_addr_end = out_addr_ofs + out_num * sizeof(elem->out_addr[0]);
    size_t in_sg_ofs = QEMU_ALIGN_UP(out_addr_end, __alignof__(elem->in_sg[0]));
    size_t out_sg_ofs = in_sg_ofs + in_num * sizeof(elem->in_sg[0]);
    size_t out_sg_end = out_sg_ofs + out_num * sizeof(elem->out_sg[0]);

    assert(sz >= sizeof(VirtQueueElement));
    elem = g_malloc(out_sg_end);
    elem->out_num = out_num;
    elem->in_num = in_num;
    elem->in_addr = (void *)elem + in_addr_ofs;
    elem->out_addr = (void *)elem + out_addr_ofs;
    elem->in_sg = (void *)elem + in_sg_ofs;
    elem->out_sg = (void *)elem + out_sg_ofs;
    return elem;
}

void *virtqueue_pop(VirtQueue *vq, size_t sz)
{
    unsigned int i, head, max;
    VRingMemoryRegionCaches *caches;
    MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
    MemoryRegionCache *desc_cache;
    int64_t len;
    VirtIODevice *vdev = vq->vdev;
    VirtQueueElement *elem = NULL;
    unsigned out_num, in_num;
    hwaddr addr[VIRTQUEUE_MAX_SIZE];
    struct iovec iov[VIRTQUEUE_MAX_SIZE];
    VRingDesc desc;
    int rc;

    if (unlikely(vdev->broken)) {
        return NULL;
    }
    rcu_read_lock();
    if (virtio_queue_empty_rcu(vq)) {
        goto done;
    }
    /* Needed after virtio_queue_empty(), see comment in
     * virtqueue_num_heads(). */
    smp_rmb();

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

    max = vq->vring.num;

    if (vq->inuse >= vq->vring.num) {
        virtio_error(vdev, "Virtqueue size exceeded");
        goto done;
    }

    if (!virtqueue_get_head(vq, vq->last_avail_idx++, &head)) {
        goto done;
    }

    if (virtio_vdev_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX)) {
        vring_set_avail_event(vq, vq->last_avail_idx);
    }

    i = head;

    caches = atomic_rcu_read(&vq->vring.caches);
    if (caches->desc.len < max * sizeof(VRingDesc)) {
        virtio_error(vdev, "Cannot map descriptor ring");
        goto done;
    }

    desc_cache = &caches->desc;
    vring_desc_read(vdev, &desc, desc_cache, i);
    if (desc.flags & VRING_DESC_F_INDIRECT) {
        if (desc.len % sizeof(VRingDesc)) {
            virtio_error(vdev, "Invalid size for indirect buffer table");
            goto done;
        }

        /* loop over the indirect descriptor table */
        len = address_space_cache_init(&indirect_desc_cache, vdev->dma_as,
                                       desc.addr, desc.len, false);
        desc_cache = &indirect_desc_cache;
        if (len < desc.len) {
            virtio_error(vdev, "Cannot map indirect buffer");
            goto done;
        }

        max = desc.len / sizeof(VRingDesc);
        i = 0;
        vring_desc_read(vdev, &desc, desc_cache, i);
    }

    /* Collect all the descriptors */
    do {
        bool map_ok;

        if (desc.flags & VRING_DESC_F_WRITE) {
            map_ok = virtqueue_map_desc(vdev, &in_num, addr + out_num,
                                        iov + out_num,
                                        VIRTQUEUE_MAX_SIZE - out_num, true,
                                        desc.addr, desc.len);
        } else {
            if (in_num) {
                virtio_error(vdev, "Incorrect order for descriptors");
                goto err_undo_map;
            }
            map_ok = virtqueue_map_desc(vdev, &out_num, addr, iov,
                                        VIRTQUEUE_MAX_SIZE, false,
                                        desc.addr, desc.len);
        }
        if (!map_ok) {
            goto err_undo_map;
        }

        /* If we've got too many, that implies a descriptor loop. */
        if ((in_num + out_num) > max) {
            virtio_error(vdev, "Looped descriptor");
            goto err_undo_map;
        }

        rc = virtqueue_read_next_desc(vdev, &desc, desc_cache, max, &i);
    } while (rc == VIRTQUEUE_READ_DESC_MORE);

    if (rc == VIRTQUEUE_READ_DESC_ERROR) {
        goto err_undo_map;
    }

    /* Now copy what we have collected and mapped */
    elem = virtqueue_alloc_element(sz, out_num, in_num);
    elem->index = head;
    for (i = 0; i < out_num; i++) {
        elem->out_addr[i] = addr[i];
        elem->out_sg[i] = iov[i];
    }
    for (i = 0; i < in_num; i++) {
        elem->in_addr[i] = addr[out_num + i];
        elem->in_sg[i] = iov[out_num + i];
    }

    vq->inuse++;

    trace_virtqueue_pop(vq, elem, elem->in_num, elem->out_num);
done:
    address_space_cache_destroy(&indirect_desc_cache);
    rcu_read_unlock();

    return elem;

err_undo_map:
    virtqueue_undo_map_desc(out_num, in_num, iov);
    goto done;
}

/* virtqueue_drop_all:
 * @vq: The #VirtQueue
 * Drops all queued buffers and indicates them to the guest
 * as if they are done. Useful when buffers can not be
 * processed but must be returned to the guest.
 */
unsigned int virtqueue_drop_all(VirtQueue *vq)
{
    unsigned int dropped = 0;
    VirtQueueElement elem = {};
    VirtIODevice *vdev = vq->vdev;
    bool fEventIdx = virtio_vdev_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);

    if (unlikely(vdev->broken)) {
        return 0;
    }

    while (!virtio_queue_empty(vq) && vq->inuse < vq->vring.num) {
        /* works similar to virtqueue_pop but does not map buffers
        * and does not allocate any memory */
        smp_rmb();
        if (!virtqueue_get_head(vq, vq->last_avail_idx, &elem.index)) {
            break;
        }
        vq->inuse++;
        vq->last_avail_idx++;
        if (fEventIdx) {
            vring_set_avail_event(vq, vq->last_avail_idx);
        }
        /* immediately push the element, nothing to unmap
         * as both in_num and out_num are set to 0 */
        virtqueue_push(vq, &elem, 0);
        dropped++;
    }

    return dropped;
}

/* Reading and writing a structure directly to QEMUFile is *awful*, but
 * it is what QEMU has always done by mistake.  We can change it sooner
 * or later by bumping the version number of the affected vm states.
 * In the meanwhile, since the in-memory layout of VirtQueueElement
 * has changed, we need to marshal to and from the layout that was
 * used before the change.
 */
typedef struct VirtQueueElementOld {
    unsigned int index;
    unsigned int out_num;
    unsigned int in_num;
    hwaddr in_addr[VIRTQUEUE_MAX_SIZE];
    hwaddr out_addr[VIRTQUEUE_MAX_SIZE];
    struct iovec in_sg[VIRTQUEUE_MAX_SIZE];
    struct iovec out_sg[VIRTQUEUE_MAX_SIZE];
} VirtQueueElementOld;

void *qemu_get_virtqueue_element(VirtIODevice *vdev, QEMUFile *f, size_t sz)
{
    VirtQueueElement *elem;
    VirtQueueElementOld data;
    int i;

    qemu_get_buffer(f, (uint8_t *)&data, sizeof(VirtQueueElementOld));

    /* TODO: teach all callers that this can fail, and return failure instead
     * of asserting here.
     * When we do, we might be able to re-enable NDEBUG below.
     */
#ifdef NDEBUG
#error building with NDEBUG is not supported
#endif
    assert(ARRAY_SIZE(data.in_addr) >= data.in_num);
    assert(ARRAY_SIZE(data.out_addr) >= data.out_num);

    elem = virtqueue_alloc_element(sz, data.out_num, data.in_num);
    elem->index = data.index;

    for (i = 0; i < elem->in_num; i++) {
        elem->in_addr[i] = data.in_addr[i];
    }

    for (i = 0; i < elem->out_num; i++) {
        elem->out_addr[i] = data.out_addr[i];
    }

    for (i = 0; i < elem->in_num; i++) {
        /* Base is overwritten by virtqueue_map.  */
        elem->in_sg[i].iov_base = 0;
        elem->in_sg[i].iov_len = data.in_sg[i].iov_len;
    }

    for (i = 0; i < elem->out_num; i++) {
        /* Base is overwritten by virtqueue_map.  */
        elem->out_sg[i].iov_base = 0;
        elem->out_sg[i].iov_len = data.out_sg[i].iov_len;
    }

    virtqueue_map(vdev, elem);
    return elem;
}

void qemu_put_virtqueue_element(QEMUFile *f, VirtQueueElement *elem)
{
    VirtQueueElementOld data;
    int i;

    memset(&data, 0, sizeof(data));
    data.index = elem->index;
    data.in_num = elem->in_num;
    data.out_num = elem->out_num;

    for (i = 0; i < elem->in_num; i++) {
        data.in_addr[i] = elem->in_addr[i];
    }

    for (i = 0; i < elem->out_num; i++) {
        data.out_addr[i] = elem->out_addr[i];
    }

    for (i = 0; i < elem->in_num; i++) {
        /* Base is overwritten by virtqueue_map when loading.  Do not
         * save it, as it would leak the QEMU address space layout.  */
        data.in_sg[i].iov_len = elem->in_sg[i].iov_len;
    }

    for (i = 0; i < elem->out_num; i++) {
        /* Do not save iov_base as above.  */
        data.out_sg[i].iov_len = elem->out_sg[i].iov_len;
    }
    qemu_put_buffer(f, (uint8_t *)&data, sizeof(VirtQueueElementOld));
}

/* virtio device */
static void virtio_notify_vector(VirtIODevice *vdev, uint16_t vector)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);

    if (unlikely(vdev->broken)) {
        return;
    }

    if (k->notify) {
        k->notify(qbus->parent, vector);
    }
}

void virtio_update_irq(VirtIODevice *vdev)
{
    virtio_notify_vector(vdev, VIRTIO_NO_VECTOR);
}

static int virtio_validate_features(VirtIODevice *vdev)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);

    if (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM) &&
        !virtio_vdev_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
        return -EFAULT;
    }

    if (k->validate_features) {
        return k->validate_features(vdev);
    } else {
        return 0;
    }
}

int virtio_set_status(VirtIODevice *vdev, uint8_t val)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    trace_virtio_set_status(vdev, val);

    if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
        if (!(vdev->status & VIRTIO_CONFIG_S_FEATURES_OK) &&
            val & VIRTIO_CONFIG_S_FEATURES_OK) {
            int ret = virtio_validate_features(vdev);

            if (ret) {
                return ret;
            }
        }
    }
    if (k->set_status) {
        k->set_status(vdev, val);
    }
    vdev->status = val;
    return 0;
}

bool target_words_bigendian(void);
static enum virtio_device_endian virtio_default_endian(void)
{
    if (target_words_bigendian()) {
        return VIRTIO_DEVICE_ENDIAN_BIG;
    } else {
        return VIRTIO_DEVICE_ENDIAN_LITTLE;
    }
}

static enum virtio_device_endian virtio_current_cpu_endian(void)
{
    CPUClass *cc = CPU_GET_CLASS(current_cpu);

    if (cc->virtio_is_big_endian(current_cpu)) {
        return VIRTIO_DEVICE_ENDIAN_BIG;
    } else {
        return VIRTIO_DEVICE_ENDIAN_LITTLE;
    }
}

void virtio_reset(void *opaque)
{
    VirtIODevice *vdev = opaque;
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    int i;

    virtio_set_status(vdev, 0);
    if (current_cpu) {
        /* Guest initiated reset */
        vdev->device_endian = virtio_current_cpu_endian();
    } else {
        /* System reset */
        vdev->device_endian = virtio_default_endian();
    }

    if (k->reset) {
        k->reset(vdev);
    }

    vdev->broken = false;
    vdev->guest_features = 0;
    vdev->queue_sel = 0;
    vdev->status = 0;
    atomic_set(&vdev->isr, 0);
    vdev->config_vector = VIRTIO_NO_VECTOR;
    virtio_notify_vector(vdev, vdev->config_vector);

    for(i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        vdev->vq[i].vring.desc = 0;
        vdev->vq[i].vring.avail = 0;
        vdev->vq[i].vring.used = 0;
        vdev->vq[i].last_avail_idx = 0;
        vdev->vq[i].shadow_avail_idx = 0;
        vdev->vq[i].used_idx = 0;
        virtio_queue_set_vector(vdev, i, VIRTIO_NO_VECTOR);
        vdev->vq[i].signalled_used = 0;
        vdev->vq[i].signalled_used_valid = false;
        vdev->vq[i].notification = true;
        vdev->vq[i].vring.num = vdev->vq[i].vring.num_default;
        vdev->vq[i].inuse = 0;
    }
}

uint32_t virtio_config_readb(VirtIODevice *vdev, uint32_t addr)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint8_t val;

    if (addr + sizeof(val) > vdev->config_len) {
        return (uint32_t)-1;
    }

    k->get_config(vdev, vdev->config);

    val = ldub_p(vdev->config + addr);
    return val;
}

uint32_t virtio_config_readw(VirtIODevice *vdev, uint32_t addr)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint16_t val;

    if (addr + sizeof(val) > vdev->config_len) {
        return (uint32_t)-1;
    }

    k->get_config(vdev, vdev->config);

    val = lduw_p(vdev->config + addr);
    return val;
}

uint32_t virtio_config_readl(VirtIODevice *vdev, uint32_t addr)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint32_t val;

    if (addr + sizeof(val) > vdev->config_len) {
        return (uint32_t)-1;
    }

    k->get_config(vdev, vdev->config);

    val = ldl_p(vdev->config + addr);
    return val;
}

void virtio_config_writeb(VirtIODevice *vdev, uint32_t addr, uint32_t data)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint8_t val = data;

    if (addr + sizeof(val) > vdev->config_len) {
        return;
    }

    stb_p(vdev->config + addr, val);

    if (k->set_config) {
        k->set_config(vdev, vdev->config);
    }
}

void virtio_config_writew(VirtIODevice *vdev, uint32_t addr, uint32_t data)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint16_t val = data;

    if (addr + sizeof(val) > vdev->config_len) {
        return;
    }

    stw_p(vdev->config + addr, val);

    if (k->set_config) {
        k->set_config(vdev, vdev->config);
    }
}

void virtio_config_writel(VirtIODevice *vdev, uint32_t addr, uint32_t data)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint32_t val = data;

    if (addr + sizeof(val) > vdev->config_len) {
        return;
    }

    stl_p(vdev->config + addr, val);

    if (k->set_config) {
        k->set_config(vdev, vdev->config);
    }
}

uint32_t virtio_config_modern_readb(VirtIODevice *vdev, uint32_t addr)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint8_t val;

    if (addr + sizeof(val) > vdev->config_len) {
        return (uint32_t)-1;
    }

    k->get_config(vdev, vdev->config);

    val = ldub_p(vdev->config + addr);
    return val;
}

uint32_t virtio_config_modern_readw(VirtIODevice *vdev, uint32_t addr)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint16_t val;

    if (addr + sizeof(val) > vdev->config_len) {
        return (uint32_t)-1;
    }

    k->get_config(vdev, vdev->config);

    val = lduw_le_p(vdev->config + addr);
    return val;
}

uint32_t virtio_config_modern_readl(VirtIODevice *vdev, uint32_t addr)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint32_t val;

    if (addr + sizeof(val) > vdev->config_len) {
        return (uint32_t)-1;
    }

    k->get_config(vdev, vdev->config);

    val = ldl_le_p(vdev->config + addr);
    return val;
}

void virtio_config_modern_writeb(VirtIODevice *vdev,
                                 uint32_t addr, uint32_t data)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint8_t val = data;

    if (addr + sizeof(val) > vdev->config_len) {
        return;
    }

    stb_p(vdev->config + addr, val);

    if (k->set_config) {
        k->set_config(vdev, vdev->config);
    }
}

void virtio_config_modern_writew(VirtIODevice *vdev,
                                 uint32_t addr, uint32_t data)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint16_t val = data;

    if (addr + sizeof(val) > vdev->config_len) {
        return;
    }

    stw_le_p(vdev->config + addr, val);

    if (k->set_config) {
        k->set_config(vdev, vdev->config);
    }
}

void virtio_config_modern_writel(VirtIODevice *vdev,
                                 uint32_t addr, uint32_t data)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint32_t val = data;

    if (addr + sizeof(val) > vdev->config_len) {
        return;
    }

    stl_le_p(vdev->config + addr, val);

    if (k->set_config) {
        k->set_config(vdev, vdev->config);
    }
}

void virtio_queue_set_addr(VirtIODevice *vdev, int n, hwaddr addr)
{
    vdev->vq[n].vring.desc = addr;
    virtio_queue_update_rings(vdev, n);
}

hwaddr virtio_queue_get_addr(VirtIODevice *vdev, int n)
{
    return vdev->vq[n].vring.desc;
}

void virtio_queue_set_rings(VirtIODevice *vdev, int n, hwaddr desc,
                            hwaddr avail, hwaddr used)
{
    vdev->vq[n].vring.desc = desc;
    vdev->vq[n].vring.avail = avail;
    vdev->vq[n].vring.used = used;
    virtio_init_region_cache(vdev, n);
}

void virtio_queue_set_num(VirtIODevice *vdev, int n, int num)
{
    /* Don't allow guest to flip queue between existent and
     * nonexistent states, or to set it to an invalid size.
     */
    if (!!num != !!vdev->vq[n].vring.num ||
        num > VIRTQUEUE_MAX_SIZE ||
        num < 0) {
        return;
    }
    vdev->vq[n].vring.num = num;
}

VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector)
{
    return QLIST_FIRST(&vdev->vector_queues[vector]);
}

VirtQueue *virtio_vector_next_queue(VirtQueue *vq)
{
    return QLIST_NEXT(vq, node);
}

int virtio_queue_get_num(VirtIODevice *vdev, int n)
{
    return vdev->vq[n].vring.num;
}

int virtio_queue_get_max_num(VirtIODevice *vdev, int n)
{
    return vdev->vq[n].vring.num_default;
}

int virtio_get_num_queues(VirtIODevice *vdev)
{
    int i;

    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        if (!virtio_queue_get_num(vdev, i)) {
            break;
        }
    }

    return i;
}

void virtio_queue_set_align(VirtIODevice *vdev, int n, int align)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);

    /* virtio-1 compliant devices cannot change the alignment */
    if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
        error_report("tried to modify queue alignment for virtio-1 device");
        return;
    }
    /* Check that the transport told us it was going to do this
     * (so a buggy transport will immediately assert rather than
     * silently failing to migrate this state)
     */
    assert(k->has_variable_vring_alignment);

    vdev->vq[n].vring.align = align;
    virtio_queue_update_rings(vdev, n);
}

static bool virtio_queue_notify_aio_vq(VirtQueue *vq)
{
    if (vq->vring.desc && vq->handle_aio_output) {
        VirtIODevice *vdev = vq->vdev;

        trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
        return vq->handle_aio_output(vdev, vq);
    }

    return false;
}

static void virtio_queue_notify_vq(VirtQueue *vq)
{
    if (vq->vring.desc && vq->handle_output) {
        VirtIODevice *vdev = vq->vdev;

        if (unlikely(vdev->broken)) {
            return;
        }

        trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
        vq->handle_output(vdev, vq);
    }
}

void virtio_queue_notify(VirtIODevice *vdev, int n)
{
    virtio_queue_notify_vq(&vdev->vq[n]);
}

uint16_t virtio_queue_vector(VirtIODevice *vdev, int n)
{
    return n < VIRTIO_QUEUE_MAX ? vdev->vq[n].vector :
        VIRTIO_NO_VECTOR;
}

void virtio_queue_set_vector(VirtIODevice *vdev, int n, uint16_t vector)
{
    VirtQueue *vq = &vdev->vq[n];

    if (n < VIRTIO_QUEUE_MAX) {
        if (vdev->vector_queues &&
            vdev->vq[n].vector != VIRTIO_NO_VECTOR) {
            QLIST_REMOVE(vq, node);
        }
        vdev->vq[n].vector = vector;
        if (vdev->vector_queues &&
            vector != VIRTIO_NO_VECTOR) {
            QLIST_INSERT_HEAD(&vdev->vector_queues[vector], vq, node);
        }
    }
}

VirtQueue *virtio_add_queue(VirtIODevice *vdev, int queue_size,
                            VirtIOHandleOutput handle_output)
{
    int i;

    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        if (vdev->vq[i].vring.num == 0)
            break;
    }

    if (i == VIRTIO_QUEUE_MAX || queue_size > VIRTQUEUE_MAX_SIZE)
        abort();

    vdev->vq[i].vring.num = queue_size;
    vdev->vq[i].vring.num_default = queue_size;
    vdev->vq[i].vring.align = VIRTIO_PCI_VRING_ALIGN;
    vdev->vq[i].handle_output = handle_output;
    vdev->vq[i].handle_aio_output = NULL;

    return &vdev->vq[i];
}

void virtio_del_queue(VirtIODevice *vdev, int n)
{
    if (n < 0 || n >= VIRTIO_QUEUE_MAX) {
        abort();
    }

    vdev->vq[n].vring.num = 0;
    vdev->vq[n].vring.num_default = 0;
}

static void virtio_set_isr(VirtIODevice *vdev, int value)
{
    uint8_t old = atomic_read(&vdev->isr);

    /* Do not write ISR if it does not change, so that its cacheline remains
     * shared in the common case where the guest does not read it.
     */
    if ((old & value) != value) {
        atomic_or(&vdev->isr, value);
    }
}

/* Called within rcu_read_lock().  */
static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
{
    uint16_t old, new;
    bool v;
    /* We need to expose used array entries before checking used event. */
    smp_mb();
    /* Always notify when queue is empty (when feature acknowledge) */
    if (virtio_vdev_has_feature(vdev, VIRTIO_F_NOTIFY_ON_EMPTY) &&
        !vq->inuse && virtio_queue_empty(vq)) {
        return true;
    }

    if (!virtio_vdev_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX)) {
        return !(vring_avail_flags(vq) & VRING_AVAIL_F_NO_INTERRUPT);
    }

    v = vq->signalled_used_valid;
    vq->signalled_used_valid = true;
    old = vq->signalled_used;
    new = vq->signalled_used = vq->used_idx;
    return !v || vring_need_event(vring_get_used_event(vq), new, old);
}

void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
{
    bool should_notify;
    rcu_read_lock();
    should_notify = virtio_should_notify(vdev, vq);
    rcu_read_unlock();

    if (!should_notify) {
        return;
    }

    trace_virtio_notify_irqfd(vdev, vq);

    /*
     * virtio spec 1.0 says ISR bit 0 should be ignored with MSI, but
     * windows drivers included in virtio-win 1.8.0 (circa 2015) are
     * incorrectly polling this bit during crashdump and hibernation
     * in MSI mode, causing a hang if this bit is never updated.
     * Recent releases of Windows do not really shut down, but rather
     * log out and hibernate to make the next startup faster.  Hence,
     * this manifested as a more serious hang during shutdown with
     *
     * Next driver release from 2016 fixed this problem, so working around it
     * is not a must, but it's easy to do so let's do it here.
     *
     * Note: it's safe to update ISR from any thread as it was switched
     * to an atomic operation.
     */
    virtio_set_isr(vq->vdev, 0x1);
    event_notifier_set(&vq->guest_notifier);
}

void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
{
    bool should_notify;
    rcu_read_lock();
    should_notify = virtio_should_notify(vdev, vq);
    rcu_read_unlock();

    if (!should_notify) {
        return;
    }

    trace_virtio_notify(vdev, vq);
    virtio_set_isr(vq->vdev, 0x1);
    virtio_notify_vector(vdev, vq->vector);
}

void virtio_notify_config(VirtIODevice *vdev)
{
    if (!(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK))
        return;

    virtio_set_isr(vdev, 0x3);
    vdev->generation++;
    virtio_notify_vector(vdev, vdev->config_vector);
}

static bool virtio_device_endian_needed(void *opaque)
{
    VirtIODevice *vdev = opaque;

    assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
    if (!virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
        return vdev->device_endian != virtio_default_endian();
    }
    /* Devices conforming to VIRTIO 1.0 or later are always LE. */
    return vdev->device_endian != VIRTIO_DEVICE_ENDIAN_LITTLE;
}

static bool virtio_64bit_features_needed(void *opaque)
{
    VirtIODevice *vdev = opaque;

    return (vdev->host_features >> 32) != 0;
}

static bool virtio_virtqueue_needed(void *opaque)
{
    VirtIODevice *vdev = opaque;

    return virtio_host_has_feature(vdev, VIRTIO_F_VERSION_1);
}

static bool virtio_ringsize_needed(void *opaque)
{
    VirtIODevice *vdev = opaque;
    int i;

    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        if (vdev->vq[i].vring.num != vdev->vq[i].vring.num_default) {
            return true;
        }
    }
    return false;
}

static bool virtio_extra_state_needed(void *opaque)
{
    VirtIODevice *vdev = opaque;
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);

    return k->has_extra_state &&
        k->has_extra_state(qbus->parent);
}

static bool virtio_broken_needed(void *opaque)
{
    VirtIODevice *vdev = opaque;

    return vdev->broken;
}

static const VMStateDescription vmstate_virtqueue = {
    .name = "virtqueue_state",
    .version_id = 1,
    .minimum_version_id = 1,
    .fields = (VMStateField[]) {
        VMSTATE_UINT64(vring.avail, struct VirtQueue),
        VMSTATE_UINT64(vring.used, struct VirtQueue),
        VMSTATE_END_OF_LIST()
    }
};

static const VMStateDescription vmstate_virtio_virtqueues = {
    .name = "virtio/virtqueues",
    .version_id = 1,
    .minimum_version_id = 1,
    .needed = &virtio_virtqueue_needed,
    .fields = (VMStateField[]) {
        VMSTATE_STRUCT_VARRAY_POINTER_KNOWN(vq, struct VirtIODevice,
                      VIRTIO_QUEUE_MAX, 0, vmstate_virtqueue, VirtQueue),
        VMSTATE_END_OF_LIST()
    }
};

static const VMStateDescription vmstate_ringsize = {
    .name = "ringsize_state",
    .version_id = 1,
    .minimum_version_id = 1,
    .fields = (VMStateField[]) {
        VMSTATE_UINT32(vring.num_default, struct VirtQueue),
        VMSTATE_END_OF_LIST()
    }
};

static const VMStateDescription vmstate_virtio_ringsize = {
    .name = "virtio/ringsize",
    .version_id = 1,
    .minimum_version_id = 1,
    .needed = &virtio_ringsize_needed,
    .fields = (VMStateField[]) {
        VMSTATE_STRUCT_VARRAY_POINTER_KNOWN(vq, struct VirtIODevice,
                      VIRTIO_QUEUE_MAX, 0, vmstate_ringsize, VirtQueue),
        VMSTATE_END_OF_LIST()
    }
};

static int get_extra_state(QEMUFile *f, void *pv, size_t size,
                           VMStateField *field)
{
    VirtIODevice *vdev = pv;
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);

    if (!k->load_extra_state) {
        return -1;
    } else {
        return k->load_extra_state(qbus->parent, f);
    }
}

static int put_extra_state(QEMUFile *f, void *pv, size_t size,
                           VMStateField *field, QJSON *vmdesc)
{
    VirtIODevice *vdev = pv;
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);

    k->save_extra_state(qbus->parent, f);
    return 0;
}

static const VMStateInfo vmstate_info_extra_state = {
    .name = "virtqueue_extra_state",
    .get = get_extra_state,
    .put = put_extra_state,
};

static const VMStateDescription vmstate_virtio_extra_state = {
    .name = "virtio/extra_state",
    .version_id = 1,
    .minimum_version_id = 1,
    .needed = &virtio_extra_state_needed,
    .fields = (VMStateField[]) {
        {
            .name         = "extra_state",
            .version_id   = 0,
            .field_exists = NULL,
            .size         = 0,
            .info         = &vmstate_info_extra_state,
            .flags        = VMS_SINGLE,
            .offset       = 0,
        },
        VMSTATE_END_OF_LIST()
    }
};

static const VMStateDescription vmstate_virtio_device_endian = {
    .name = "virtio/device_endian",
    .version_id = 1,
    .minimum_version_id = 1,
    .needed = &virtio_device_endian_needed,
    .fields = (VMStateField[]) {
        VMSTATE_UINT8(device_endian, VirtIODevice),
        VMSTATE_END_OF_LIST()
    }
};

static const VMStateDescription vmstate_virtio_64bit_features = {
    .name = "virtio/64bit_features",
    .version_id = 1,
    .minimum_version_id = 1,
    .needed = &virtio_64bit_features_needed,
    .fields = (VMStateField[]) {
        VMSTATE_UINT64(guest_features, VirtIODevice),
        VMSTATE_END_OF_LIST()
    }
};

static const VMStateDescription vmstate_virtio_broken = {
    .name = "virtio/broken",
    .version_id = 1,
    .minimum_version_id = 1,
    .needed = &virtio_broken_needed,
    .fields = (VMStateField[]) {
        VMSTATE_BOOL(broken, VirtIODevice),
        VMSTATE_END_OF_LIST()
    }
};

static const VMStateDescription vmstate_virtio = {
    .name = "virtio",
    .version_id = 1,
    .minimum_version_id = 1,
    .minimum_version_id_old = 1,
    .fields = (VMStateField[]) {
        VMSTATE_END_OF_LIST()
    },
    .subsections = (const VMStateDescription*[]) {
        &vmstate_virtio_device_endian,
        &vmstate_virtio_64bit_features,
        &vmstate_virtio_virtqueues,
        &vmstate_virtio_ringsize,
        &vmstate_virtio_broken,
        &vmstate_virtio_extra_state,
        NULL
    }
};

void virtio_save(VirtIODevice *vdev, QEMUFile *f)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
    uint32_t guest_features_lo = (vdev->guest_features & 0xffffffff);
    int i;

    if (k->save_config) {
        k->save_config(qbus->parent, f);
    }

    qemu_put_8s(f, &vdev->status);
    qemu_put_8s(f, &vdev->isr);
    qemu_put_be16s(f, &vdev->queue_sel);
    qemu_put_be32s(f, &guest_features_lo);
    qemu_put_be32(f, vdev->config_len);
    qemu_put_buffer(f, vdev->config, vdev->config_len);

    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        if (vdev->vq[i].vring.num == 0)
            break;
    }

    qemu_put_be32(f, i);

    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        if (vdev->vq[i].vring.num == 0)
            break;

        qemu_put_be32(f, vdev->vq[i].vring.num);
        if (k->has_variable_vring_alignment) {
            qemu_put_be32(f, vdev->vq[i].vring.align);
        }
        /* XXX virtio-1 devices */
        qemu_put_be64(f, vdev->vq[i].vring.desc);
        qemu_put_be16s(f, &vdev->vq[i].last_avail_idx);
        if (k->save_queue) {
            k->save_queue(qbus->parent, i, f);
        }
    }

    if (vdc->save != NULL) {
        vdc->save(vdev, f);
    }

    if (vdc->vmsd) {
        vmstate_save_state(f, vdc->vmsd, vdev, NULL);
    }

    /* Subsections */
    vmstate_save_state(f, &vmstate_virtio, vdev, NULL);
}

/* A wrapper for use as a VMState .put function */
static int virtio_device_put(QEMUFile *f, void *opaque, size_t size,
                              VMStateField *field, QJSON *vmdesc)
{
    virtio_save(VIRTIO_DEVICE(opaque), f);

    return 0;
}

/* A wrapper for use as a VMState .get function */
static int virtio_device_get(QEMUFile *f, void *opaque, size_t size,
                             VMStateField *field)
{
    VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
    DeviceClass *dc = DEVICE_CLASS(VIRTIO_DEVICE_GET_CLASS(vdev));

    return virtio_load(vdev, f, dc->vmsd->version_id);
}

const VMStateInfo  virtio_vmstate_info = {
    .name = "virtio",
    .get = virtio_device_get,
    .put = virtio_device_put,
};

static int virtio_set_features_nocheck(VirtIODevice *vdev, uint64_t val)
{
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
    bool bad = (val & ~(vdev->host_features)) != 0;

    val &= vdev->host_features;
    if (k->set_features) {
        k->set_features(vdev, val);
    }
    vdev->guest_features = val;
    return bad ? -1 : 0;
}

int virtio_set_features(VirtIODevice *vdev, uint64_t val)
{
   /*
     * The driver must not attempt to set features after feature negotiation
     * has finished.
     */
    if (vdev->status & VIRTIO_CONFIG_S_FEATURES_OK) {
        return -EINVAL;
    }
    return virtio_set_features_nocheck(vdev, val);
}

int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
{
    int i, ret;
    int32_t config_len;
    uint32_t num;
    uint32_t features;
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);

    /*
     * We poison the endianness to ensure it does not get used before
     * subsections have been loaded.
     */
    vdev->device_endian = VIRTIO_DEVICE_ENDIAN_UNKNOWN;

    if (k->load_config) {
        ret = k->load_config(qbus->parent, f);
        if (ret)
            return ret;
    }

    qemu_get_8s(f, &vdev->status);
    qemu_get_8s(f, &vdev->isr);
    qemu_get_be16s(f, &vdev->queue_sel);
    if (vdev->queue_sel >= VIRTIO_QUEUE_MAX) {
        return -1;
    }
    qemu_get_be32s(f, &features);

    /*
     * Temporarily set guest_features low bits - needed by
     * virtio net load code testing for VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
     * VIRTIO_NET_F_GUEST_ANNOUNCE and VIRTIO_NET_F_CTRL_VQ.
     *
     * Note: devices should always test host features in future - don't create
     * new dependencies like this.
     */
    vdev->guest_features = features;

    config_len = qemu_get_be32(f);

    /*
     * There are cases where the incoming config can be bigger or smaller
     * than what we have; so load what we have space for, and skip
     * any excess that's in the stream.
     */
    qemu_get_buffer(f, vdev->config, MIN(config_len, vdev->config_len));

    while (config_len > vdev->config_len) {
        qemu_get_byte(f);
        config_len--;
    }

    num = qemu_get_be32(f);

    if (num > VIRTIO_QUEUE_MAX) {
        error_report("Invalid number of virtqueues: 0x%x", num);
        return -1;
    }

    for (i = 0; i < num; i++) {
        vdev->vq[i].vring.num = qemu_get_be32(f);
        if (k->has_variable_vring_alignment) {
            vdev->vq[i].vring.align = qemu_get_be32(f);
        }
        vdev->vq[i].vring.desc = qemu_get_be64(f);
        qemu_get_be16s(f, &vdev->vq[i].last_avail_idx);
        vdev->vq[i].signalled_used_valid = false;
        vdev->vq[i].notification = true;

        if (vdev->vq[i].vring.desc) {
            /* XXX virtio-1 devices */
            virtio_queue_update_rings(vdev, i);
        } else if (vdev->vq[i].last_avail_idx) {
            error_report("VQ %d address 0x0 "
                         "inconsistent with Host index 0x%x",
                         i, vdev->vq[i].last_avail_idx);
                return -1;
        }
        if (k->load_queue) {
            ret = k->load_queue(qbus->parent, i, f);
            if (ret)
                return ret;
        }
    }

    virtio_notify_vector(vdev, VIRTIO_NO_VECTOR);

    if (vdc->load != NULL) {
        ret = vdc->load(vdev, f, version_id);
        if (ret) {
            return ret;
        }
    }

    if (vdc->vmsd) {
        ret = vmstate_load_state(f, vdc->vmsd, vdev, version_id);
        if (ret) {
            return ret;
        }
    }

    /* Subsections */
    ret = vmstate_load_state(f, &vmstate_virtio, vdev, 1);
    if (ret) {
        return ret;
    }

    if (vdev->device_endian == VIRTIO_DEVICE_ENDIAN_UNKNOWN) {
        vdev->device_endian = virtio_default_endian();
    }

    if (virtio_64bit_features_needed(vdev)) {
        /*
         * Subsection load filled vdev->guest_features.  Run them
         * through virtio_set_features to sanity-check them against
         * host_features.
         */
        uint64_t features64 = vdev->guest_features;
        if (virtio_set_features_nocheck(vdev, features64) < 0) {
            error_report("Features 0x%" PRIx64 " unsupported. "
                         "Allowed features: 0x%" PRIx64,
                         features64, vdev->host_features);
            return -1;
        }
    } else {
        if (virtio_set_features_nocheck(vdev, features) < 0) {
            error_report("Features 0x%x unsupported. "
                         "Allowed features: 0x%" PRIx64,
                         features, vdev->host_features);
            return -1;
        }
    }

    rcu_read_lock();
    for (i = 0; i < num; i++) {
        if (vdev->vq[i].vring.desc) {
            uint16_t nheads;
            nheads = vring_avail_idx(&vdev->vq[i]) - vdev->vq[i].last_avail_idx;
            /* Check it isn't doing strange things with descriptor numbers. */
            if (nheads > vdev->vq[i].vring.num) {
                error_report("VQ %d size 0x%x Guest index 0x%x "
                             "inconsistent with Host index 0x%x: delta 0x%x",
                             i, vdev->vq[i].vring.num,
                             vring_avail_idx(&vdev->vq[i]),
                             vdev->vq[i].last_avail_idx, nheads);
                return -1;
            }
            vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]);
            vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i]);

            /*
             * Some devices migrate VirtQueueElements that have been popped
             * from the avail ring but not yet returned to the used ring.
             * Since max ring size < UINT16_MAX it's safe to use modulo
             * UINT16_MAX + 1 subtraction.
             */
            vdev->vq[i].inuse = (uint16_t)(vdev->vq[i].last_avail_idx -
                                vdev->vq[i].used_idx);
            if (vdev->vq[i].inuse > vdev->vq[i].vring.num) {
                error_report("VQ %d size 0x%x < last_avail_idx 0x%x - "
                             "used_idx 0x%x",
                             i, vdev->vq[i].vring.num,
                             vdev->vq[i].last_avail_idx,
                             vdev->vq[i].used_idx);
                return -1;
            }
        }
    }
    rcu_read_unlock();

    return 0;
}

void virtio_cleanup(VirtIODevice *vdev)
{
    qemu_del_vm_change_state_handler(vdev->vmstate);
}

static void virtio_vmstate_change(void *opaque, int running, RunState state)
{
    VirtIODevice *vdev = opaque;
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
    bool backend_run = running && (vdev->status & VIRTIO_CONFIG_S_DRIVER_OK);
    vdev->vm_running = running;

    if (backend_run) {
        virtio_set_status(vdev, vdev->status);
    }

    if (k->vmstate_change) {
        k->vmstate_change(qbus->parent, backend_run);
    }

    if (!backend_run) {
        virtio_set_status(vdev, vdev->status);
    }
}

void virtio_instance_init_common(Object *proxy_obj, void *data,
                                 size_t vdev_size, const char *vdev_name)
{
    DeviceState *vdev = data;

    object_initialize(vdev, vdev_size, vdev_name);
    object_property_add_child(proxy_obj, "virtio-backend", OBJECT(vdev), NULL);
    object_unref(OBJECT(vdev));
    qdev_alias_all_properties(vdev, proxy_obj);
}

void virtio_init(VirtIODevice *vdev, const char *name,
                 uint16_t device_id, size_t config_size)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
    int i;
    int nvectors = k->query_nvectors ? k->query_nvectors(qbus->parent) : 0;

    if (nvectors) {
        vdev->vector_queues =
            g_malloc0(sizeof(*vdev->vector_queues) * nvectors);
    }

    vdev->device_id = device_id;
    vdev->status = 0;
    atomic_set(&vdev->isr, 0);
    vdev->queue_sel = 0;
    vdev->config_vector = VIRTIO_NO_VECTOR;
    vdev->vq = g_malloc0(sizeof(VirtQueue) * VIRTIO_QUEUE_MAX);
    vdev->vm_running = runstate_is_running();
    vdev->broken = false;
    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        vdev->vq[i].vector = VIRTIO_NO_VECTOR;
        vdev->vq[i].vdev = vdev;
        vdev->vq[i].queue_index = i;
    }

    vdev->name = name;
    vdev->config_len = config_size;
    if (vdev->config_len) {
        vdev->config = g_malloc0(config_size);
    } else {
        vdev->config = NULL;
    }
    vdev->vmstate = qemu_add_vm_change_state_handler(virtio_vmstate_change,
                                                     vdev);
    vdev->device_endian = virtio_default_endian();
    vdev->use_guest_notifier_mask = true;
}

hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n)
{
    return vdev->vq[n].vring.desc;
}

hwaddr virtio_queue_get_avail_addr(VirtIODevice *vdev, int n)
{
    return vdev->vq[n].vring.avail;
}

hwaddr virtio_queue_get_used_addr(VirtIODevice *vdev, int n)
{
    return vdev->vq[n].vring.used;
}

hwaddr virtio_queue_get_desc_size(VirtIODevice *vdev, int n)
{
    return sizeof(VRingDesc) * vdev->vq[n].vring.num;
}

hwaddr virtio_queue_get_avail_size(VirtIODevice *vdev, int n)
{
    return offsetof(VRingAvail, ring) +
        sizeof(uint16_t) * vdev->vq[n].vring.num;
}

hwaddr virtio_queue_get_used_size(VirtIODevice *vdev, int n)
{
    return offsetof(VRingUsed, ring) +
        sizeof(VRingUsedElem) * vdev->vq[n].vring.num;
}

uint16_t virtio_queue_get_last_avail_idx(VirtIODevice *vdev, int n)
{
    return vdev->vq[n].last_avail_idx;
}

void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx)
{
    vdev->vq[n].last_avail_idx = idx;
    vdev->vq[n].shadow_avail_idx = idx;
}

void virtio_queue_update_used_idx(VirtIODevice *vdev, int n)
{
    rcu_read_lock();
    if (vdev->vq[n].vring.desc) {
        vdev->vq[n].used_idx = vring_used_idx(&vdev->vq[n]);
    }
    rcu_read_unlock();
}

void virtio_queue_invalidate_signalled_used(VirtIODevice *vdev, int n)
{
    vdev->vq[n].signalled_used_valid = false;
}

VirtQueue *virtio_get_queue(VirtIODevice *vdev, int n)
{
    return vdev->vq + n;
}

uint16_t virtio_get_queue_index(VirtQueue *vq)
{
    return vq->queue_index;
}

static void virtio_queue_guest_notifier_read(EventNotifier *n)
{
    VirtQueue *vq = container_of(n, VirtQueue, guest_notifier);
    if (event_notifier_test_and_clear(n)) {
        virtio_notify_vector(vq->vdev, vq->vector);
    }
}

void virtio_queue_set_guest_notifier_fd_handler(VirtQueue *vq, bool assign,
                                                bool with_irqfd)
{
    if (assign && !with_irqfd) {
        event_notifier_set_handler(&vq->guest_notifier,
                                   virtio_queue_guest_notifier_read);
    } else {
        event_notifier_set_handler(&vq->guest_notifier, NULL);
    }
    if (!assign) {
        /* Test and clear notifier before closing it,
         * in case poll callback didn't have time to run. */
        virtio_queue_guest_notifier_read(&vq->guest_notifier);
    }
}

EventNotifier *virtio_queue_get_guest_notifier(VirtQueue *vq)
{
    return &vq->guest_notifier;
}

static void virtio_queue_host_notifier_aio_read(EventNotifier *n)
{
    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
    if (event_notifier_test_and_clear(n)) {
        virtio_queue_notify_aio_vq(vq);
    }
}

static void virtio_queue_host_notifier_aio_poll_begin(EventNotifier *n)
{
    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);

    virtio_queue_set_notification(vq, 0);
}

static bool virtio_queue_host_notifier_aio_poll(void *opaque)
{
    EventNotifier *n = opaque;
    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
    bool progress;

    if (virtio_queue_empty(vq)) {
        return false;
    }

    progress = virtio_queue_notify_aio_vq(vq);

    /* In case the handler function re-enabled notifications */
    virtio_queue_set_notification(vq, 0);
    return progress;
}

static void virtio_queue_host_notifier_aio_poll_end(EventNotifier *n)
{
    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);

    /* Caller polls once more after this to catch requests that race with us */
    virtio_queue_set_notification(vq, 1);
}

void virtio_queue_aio_set_host_notifier_handler(VirtQueue *vq, AioContext *ctx,
                                                VirtIOHandleAIOOutput handle_output)
{
    if (handle_output) {
        vq->handle_aio_output = handle_output;
        aio_set_event_notifier(ctx, &vq->host_notifier, true,
                               virtio_queue_host_notifier_aio_read,
                               virtio_queue_host_notifier_aio_poll);
        aio_set_event_notifier_poll(ctx, &vq->host_notifier,
                                    virtio_queue_host_notifier_aio_poll_begin,
                                    virtio_queue_host_notifier_aio_poll_end);
    } else {
        aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL);
        /* Test and clear notifier before after disabling event,
         * in case poll callback didn't have time to run. */
        virtio_queue_host_notifier_aio_read(&vq->host_notifier);
        vq->handle_aio_output = NULL;
    }
}

void virtio_queue_host_notifier_read(EventNotifier *n)
{
    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
    if (event_notifier_test_and_clear(n)) {
        virtio_queue_notify_vq(vq);
    }
}

EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq)
{
    return &vq->host_notifier;
}

void virtio_device_set_child_bus_name(VirtIODevice *vdev, char *bus_name)
{
    g_free(vdev->bus_name);
    vdev->bus_name = g_strdup(bus_name);
}

void GCC_FMT_ATTR(2, 3) virtio_error(VirtIODevice *vdev, const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    error_vreport(fmt, ap);
    va_end(ap);

    vdev->broken = true;

    if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
        virtio_set_status(vdev, vdev->status | VIRTIO_CONFIG_S_NEEDS_RESET);
        virtio_notify_config(vdev);
    }
}

static void virtio_memory_listener_commit(MemoryListener *listener)
{
    VirtIODevice *vdev = container_of(listener, VirtIODevice, listener);
    int i;

    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        if (vdev->vq[i].vring.num == 0) {
            break;
        }
        virtio_init_region_cache(vdev, i);
    }
}

static void virtio_device_realize(DeviceState *dev, Error **errp)
{
    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(dev);
    Error *err = NULL;

    /* Devices should either use vmsd or the load/save methods */
    assert(!vdc->vmsd || !vdc->load);

    if (vdc->realize != NULL) {
        vdc->realize(dev, &err);
        if (err != NULL) {
            error_propagate(errp, err);
            return;
        }
    }

    virtio_bus_device_plugged(vdev, &err);
    if (err != NULL) {
        error_propagate(errp, err);
        return;
    }

    vdev->listener.commit = virtio_memory_listener_commit;
    memory_listener_register(&vdev->listener, vdev->dma_as);
}

static void virtio_device_unrealize(DeviceState *dev, Error **errp)
{
    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(dev);
    Error *err = NULL;

    virtio_bus_device_unplugged(vdev);

    if (vdc->unrealize != NULL) {
        vdc->unrealize(dev, &err);
        if (err != NULL) {
            error_propagate(errp, err);
            return;
        }
    }

    g_free(vdev->bus_name);
    vdev->bus_name = NULL;
}

static void virtio_device_free_virtqueues(VirtIODevice *vdev)
{
    int i;
    if (!vdev->vq) {
        return;
    }

    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
        VRingMemoryRegionCaches *caches;
        if (vdev->vq[i].vring.num == 0) {
            break;
        }
        caches = atomic_read(&vdev->vq[i].vring.caches);
        atomic_set(&vdev->vq[i].vring.caches, NULL);
        virtio_free_region_cache(caches);
    }
    g_free(vdev->vq);
}

static void virtio_device_instance_finalize(Object *obj)
{
    VirtIODevice *vdev = VIRTIO_DEVICE(obj);

    memory_listener_unregister(&vdev->listener);
    virtio_device_free_virtqueues(vdev);

    g_free(vdev->config);
    g_free(vdev->vector_queues);
}

static Property virtio_properties[] = {
    DEFINE_VIRTIO_COMMON_FEATURES(VirtIODevice, host_features),
    DEFINE_PROP_END_OF_LIST(),
};

static int virtio_device_start_ioeventfd_impl(VirtIODevice *vdev)
{
    VirtioBusState *qbus = VIRTIO_BUS(qdev_get_parent_bus(DEVICE(vdev)));
    int n, r, err;

    for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
        VirtQueue *vq = &vdev->vq[n];
        if (!virtio_queue_get_num(vdev, n)) {
            continue;
        }
        r = virtio_bus_set_host_notifier(qbus, n, true);
        if (r < 0) {
            err = r;
            goto assign_error;
        }
        event_notifier_set_handler(&vq->host_notifier,
                                   virtio_queue_host_notifier_read);
    }

    for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
        /* Kick right away to begin processing requests already in vring */
        VirtQueue *vq = &vdev->vq[n];
        if (!vq->vring.num) {
            continue;
        }
        event_notifier_set(&vq->host_notifier);
    }
    return 0;

assign_error:
    while (--n >= 0) {
        VirtQueue *vq = &vdev->vq[n];
        if (!virtio_queue_get_num(vdev, n)) {
            continue;
        }

        event_notifier_set_handler(&vq->host_notifier, NULL);
        r = virtio_bus_set_host_notifier(qbus, n, false);
        assert(r >= 0);
    }
    return err;
}

int virtio_device_start_ioeventfd(VirtIODevice *vdev)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusState *vbus = VIRTIO_BUS(qbus);

    return virtio_bus_start_ioeventfd(vbus);
}

static void virtio_device_stop_ioeventfd_impl(VirtIODevice *vdev)
{
    VirtioBusState *qbus = VIRTIO_BUS(qdev_get_parent_bus(DEVICE(vdev)));
    int n, r;

    for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
        VirtQueue *vq = &vdev->vq[n];

        if (!virtio_queue_get_num(vdev, n)) {
            continue;
        }
        event_notifier_set_handler(&vq->host_notifier, NULL);
        r = virtio_bus_set_host_notifier(qbus, n, false);
        assert(r >= 0);
    }
}

void virtio_device_stop_ioeventfd(VirtIODevice *vdev)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusState *vbus = VIRTIO_BUS(qbus);

    virtio_bus_stop_ioeventfd(vbus);
}

int virtio_device_grab_ioeventfd(VirtIODevice *vdev)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusState *vbus = VIRTIO_BUS(qbus);

    return virtio_bus_grab_ioeventfd(vbus);
}

void virtio_device_release_ioeventfd(VirtIODevice *vdev)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusState *vbus = VIRTIO_BUS(qbus);

    virtio_bus_release_ioeventfd(vbus);
}

static void virtio_device_class_init(ObjectClass *klass, void *data)
{
    /* Set the default value here. */
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_CLASS(klass);
    DeviceClass *dc = DEVICE_CLASS(klass);

    dc->realize = virtio_device_realize;
    dc->unrealize = virtio_device_unrealize;
    dc->bus_type = TYPE_VIRTIO_BUS;
    dc->props = virtio_properties;
    vdc->start_ioeventfd = virtio_device_start_ioeventfd_impl;
    vdc->stop_ioeventfd = virtio_device_stop_ioeventfd_impl;

    vdc->legacy_features |= VIRTIO_LEGACY_FEATURES;
}

bool virtio_device_ioeventfd_enabled(VirtIODevice *vdev)
{
    BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
    VirtioBusState *vbus = VIRTIO_BUS(qbus);

    return virtio_bus_ioeventfd_enabled(vbus);
}

static const TypeInfo virtio_device_info = {
    .name = TYPE_VIRTIO_DEVICE,
    .parent = TYPE_DEVICE,
    .instance_size = sizeof(VirtIODevice),
    .class_init = virtio_device_class_init,
    .instance_finalize = virtio_device_instance_finalize,
    .abstract = true,
    .class_size = sizeof(VirtioDeviceClass),
};

static void virtio_register_types(void)
{
    type_register_static(&virtio_device_info);
}

type_init(virtio_register_types)