[PATCH v6 02/12] vhost: use SVQ element ndescs instead of opaque data for desc validation

["Eugenio Pérez" <eperezma@redhat.com>]

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