diff options
Diffstat (limited to 'hw/virtio/virtio.c')
| -rw-r--r-- | hw/virtio/virtio.c | 18 |
1 files changed, 15 insertions, 3 deletions
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c index c727e92..2058b83 100644 --- a/hw/virtio/virtio.c +++ b/hw/virtio/virtio.c @@ -997,7 +997,12 @@ void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem, /* Called within rcu_read_lock(). */ static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx) { - uint16_t num_heads = vring_avail_idx(vq) - idx; + uint16_t avail_idx, num_heads; + + /* Use shadow index whenever possible. */ + avail_idx = (vq->shadow_avail_idx != idx) ? vq->shadow_avail_idx + : vring_avail_idx(vq); + num_heads = avail_idx - idx; /* Check it isn't doing very strange things with descriptor numbers. */ if (num_heads > vq->vring.num) { @@ -1005,8 +1010,15 @@ static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx) 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. */ + /* + * On success, callers read a descriptor at vq->last_avail_idx. + * Make sure descriptor read does not bypass avail index read. + * + * This is necessary even if we are using a shadow index, since + * the shadow index could have been initialized by calling + * vring_avail_idx() outside of this function, i.e., by a guest + * memory read not accompanied by a barrier. + */ if (num_heads) { smp_rmb(); } |