virtio: add virtqueue_rewind()

virtqueue_discard() requires a VirtQueueElement but virtio-balloon does
not migrate its in-use element.  Introduce a new function that is
similar to virtqueue_discard() but doesn't require a VirtQueueElement.

This will allow virtio-balloon to access element again after migration
with the usual proviso that the guest may have modified the vring since
last time.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Roman Kagan <rkagan@virtuozzo.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NLadi Prosek <lprosek@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -272,6 +272,28 @@ void virtqueue_discard(VirtQueue *vq, const VirtQueueElement *elem,
     virtqueue_unmap_sg(vq, elem, len);
 }
 
+/* virtqueue_rewind:
+ * @vq: The #VirtQueue
+ * @num: Number of elements to push back
+ *
+ * Pretend that elements weren't popped from the virtqueue.  The next
+ * virtqueue_pop() will refetch the oldest element.
+ *
+ * Use virtqueue_discard() instead if you have a VirtQueueElement.
+ *
+ * Returns: true on success, false if @num is greater than the number of in use
+ * elements.
+ */
+bool virtqueue_rewind(VirtQueue *vq, unsigned int num)
+{
+    if (num > vq->inuse) {
+        return false;
+    }
+    vq->last_avail_idx -= num;
+    vq->inuse -= num;
+    return true;
+}
+
 void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
                     unsigned int len, unsigned int idx)
 {

include/hw/virtio/virtio.h

@@ -154,6 +154,7 @@ void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
 void virtqueue_flush(VirtQueue *vq, unsigned int count);
 void virtqueue_discard(VirtQueue *vq, const VirtQueueElement *elem,
                        unsigned int len);
+bool virtqueue_rewind(VirtQueue *vq, unsigned int num);
 void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
                     unsigned int len, unsigned int idx);