vdpa: rewind at get_base, not set_base

At this moment it is only possible to migrate to a vdpa device running
with x-svq=on. As a protective measure, the rewind of the inflight
descriptors was done at the destination. That way if the source sent a
virtqueue with inuse descriptors they are always discarded.

Since this series allows to migrate also to passthrough devices with no
SVQ, the right thing to do is to rewind at the source so the base of
vrings are correct.

Support for inflight descriptors may be added in the future.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230303172445.1089785-5-eperezma@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index 515ccf870d..8361e70d1b 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -694,13 +694,17 @@ void vhost_svq_stop(VhostShadowVirtqueue *svq)
         g_autofree VirtQueueElement *elem = NULL;
         elem = g_steal_pointer(&svq->desc_state[i].elem);
         if (elem) {
-            virtqueue_detach_element(svq->vq, elem, 0);
+            /*
+             * TODO: This is ok for networking, but other kinds of devices
+             * might have problems with just unpop these.
+             */
+            virtqueue_unpop(svq->vq, elem, 0);
         }
     }
 
     next_avail_elem = g_steal_pointer(&svq->next_guest_avail_elem);
     if (next_avail_elem) {
-        virtqueue_detach_element(svq->vq, next_avail_elem, 0);
+        virtqueue_unpop(svq->vq, next_avail_elem, 0);
     }
     svq->vq = NULL;
     g_free(svq->desc_next);