Since we're going to allow SVQ to add elements without the guest's
knowledge and without its own VirtQueueElement, it's easier to check if
an element is a valid head checking a different thing than the
VirtQueueElement.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-shadow-virtqueue.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index ffd2b2c972..e6eebd0e8d 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -414,7 +414,7 @@ static VirtQueueElement *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
         return NULL;
     }
 
-    if (unlikely(!svq->desc_state[used_elem.id].elem)) {
+    if (unlikely(!svq->desc_state[used_elem.id].ndescs)) {
         qemu_log_mask(LOG_GUEST_ERROR,
             "Device %s says index %u is used, but it was not available",
             svq->vdev->name, used_elem.id);
@@ -422,6 +422,7 @@ static VirtQueueElement *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
     }
 
     num = svq->desc_state[used_elem.id].ndescs;
+    svq->desc_state[used_elem.id].ndescs = 0;
     last_used_chain = vhost_svq_last_desc_of_chain(svq, num, used_elem.id);
     svq->desc_next[last_used_chain] = svq->free_head;
     svq->free_head = used_elem.id;
-- 
2.31.1

On Fri, Aug 5, 2022 at 2:29 AM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> Since we're going to allow SVQ to add elements without the guest's
> knowledge and without its own VirtQueueElement, it's easier to check if
> an element is a valid head checking a different thing than the
> VirtQueueElement.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---

Acked-by: Jason Wang <jasowang@redhat.com>

>  hw/virtio/vhost-shadow-virtqueue.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
> index ffd2b2c972..e6eebd0e8d 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -414,7 +414,7 @@ static VirtQueueElement *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
>          return NULL;
>      }
>
> -    if (unlikely(!svq->desc_state[used_elem.id].elem)) {
> +    if (unlikely(!svq->desc_state[used_elem.id].ndescs)) {
>          qemu_log_mask(LOG_GUEST_ERROR,
>              "Device %s says index %u is used, but it was not available",
>              svq->vdev->name, used_elem.id);
> @@ -422,6 +422,7 @@ static VirtQueueElement *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
>      }
>
>      num = svq->desc_state[used_elem.id].ndescs;
> +    svq->desc_state[used_elem.id].ndescs = 0;
>      last_used_chain = vhost_svq_last_desc_of_chain(svq, num, used_elem.id);
>      svq->desc_next[last_used_chain] = svq->free_head;
>      svq->free_head = used_elem.id;
> --
> 2.31.1
>