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>

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);

hw/virtio/vhost-vdpa.c

@@ -1170,18 +1170,7 @@ static int vhost_vdpa_set_vring_base(struct vhost_dev *dev,
                                        struct vhost_vring_state *ring)
 {
     struct vhost_vdpa *v = dev->opaque;
-    VirtQueue *vq = virtio_get_queue(dev->vdev, ring->index);
 
-    /*
-     * vhost-vdpa devices does not support in-flight requests. Set all of them
-     * as available.
-     *
-     * TODO: This is ok for networking, but other kinds of devices might
-     * have problems with these retransmissions.
-     */
-    while (virtqueue_rewind(vq, 1)) {
-        continue;
-    }
     if (v->shadow_vqs_enabled) {
         /*
          * Device vring base was set at device start. SVQ base is handled by