Re: [Qemu-devel] [PATCH v1 09/16] virtio: fill/flush/pop for packed ring

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Wei,

On 11/22/18 3:06 PM, wexu@redhat.com wrote:
> +void virtqueue_flush(VirtQueue *vq, unsigned int count)
> +{
> +    if (unlikely(vq->vdev->broken)) {
> +        vq->inuse -= count;
> +        return;
> +    }
> +
> +    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> +        virtqueue_packed_flush(vq, count);
> +    } else {
> +        virtqueue_split_flush(vq, count);
> +    }
> +}
> +
>   void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
>                       unsigned int len)
>   {
> @@ -1074,7 +1180,7 @@ static void *virtqueue_alloc_element(size_t sz, unsigned out_num, unsigned in_nu
>       return elem;
>   }
>   
> -void *virtqueue_pop(VirtQueue *vq, size_t sz)
> +static void *virtqueue_split_pop(VirtQueue *vq, size_t sz)
>   {
>       unsigned int i, head, max;
>       VRingMemoryRegionCaches *caches;
> @@ -1089,9 +1195,6 @@ void *virtqueue_pop(VirtQueue *vq, size_t sz)
>       VRingDesc desc;
>       int rc;
>   
> -    if (unlikely(vdev->broken)) {
> -        return NULL;
> -    }
>       rcu_read_lock();
>       if (virtio_queue_empty_rcu(vq)) {
>           goto done;
> @@ -1209,6 +1312,159 @@ err_undo_map:
>       goto done;
>   }
>   
> +static void *virtqueue_packed_pop(VirtQueue *vq, size_t sz)
> +{
> +    unsigned int i, head, max;
> +    VRingMemoryRegionCaches *caches;
> +    MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
> +    MemoryRegionCache *cache;
> +    int64_t len;
> +    VirtIODevice *vdev = vq->vdev;
> +    VirtQueueElement *elem = NULL;
> +    unsigned out_num, in_num, elem_entries;
> +    hwaddr addr[VIRTQUEUE_MAX_SIZE];
> +    struct iovec iov[VIRTQUEUE_MAX_SIZE];
> +    VRingPackedDesc desc;
> +    uint16_t id;
> +
> +    rcu_read_lock();
> +    if (virtio_queue_packed_empty_rcu(vq)) {
> +        goto done;
> +    }
> +
> +    /* When we start there are none of either input nor output. */
> +    out_num = in_num = elem_entries = 0;
> +
> +    max = vq->vring.num;
> +
> +    if (vq->inuse >= vq->vring.num) {
> +        virtio_error(vdev, "Virtqueue size exceeded");
> +        goto done;
> +    }
> +
> +    head = vq->last_avail_idx;
> +    i = head;
> +
> +    caches = vring_get_region_caches(vq);
> +    cache = &caches->desc;
> +
> +    /* Empty check has been done at the beginning, so it is an available
> +     * entry already, make sure all fields has been exposed by guest */
> +    smp_rmb();
> +    vring_packed_desc_read(vdev, &desc, cache, i);
> +
> +    id = desc.id;
> +    if (desc.flags & VRING_DESC_F_INDIRECT) {
> +
> +        if (desc.len % sizeof(VRingPackedDesc)) {
> +            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);
> +        cache = &indirect_desc_cache;
> +        if (len < desc.len) {
> +            virtio_error(vdev, "Cannot map indirect buffer");
> +            goto done;
> +        }
> +
> +        max = desc.len / sizeof(VRingPackedDesc);
> +        i = 0;
> +        vring_packed_desc_read(vdev, &desc, cache, i);
> +        /* Make sure we see all the fields*/
> +        smp_rmb();
> +    }
> +
> +    /* Collect all the descriptors */
> +    while (1) {
> +        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 (++elem_entries > max) {
> +            virtio_error(vdev, "Looped descriptor");
> +            goto err_undo_map;
> +        }
> +
> +        if (++i >= vq->vring.num) {
> +            i -= vq->vring.num;
> +        }
> +
> +        if (desc.flags & VRING_DESC_F_NEXT) {
> +            vring_packed_desc_read(vq->vdev, &desc, cache, i);
> +        } else {
> +            break;
> +        }
> +    }
> +
> +    /* Now copy what we have collected and mapped */
> +    elem = virtqueue_alloc_element(sz, out_num, in_num);
> +    elem->index = id;
> +    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[head + out_num + i];
> +        elem->in_sg[i] = iov[out_num + i];
> +    }
> +
> +    vq->last_avail_idx += (cache == &indirect_desc_cache) ?
> +                          1 : out_num + in_num;

I think you cannot rely on out_num and in_num to deduce the number of 
descriptors.

Indeed, in virtqueue_map_desc(), in_num and out_num can be incremented
by more than one for a single descriptor, when the desc buffer is not
contiguous in the QEMU address space.