diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 874377f37a..a5214bca61 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -634,6 +634,7 @@ static int virtio_queue_split_empty(VirtQueue *vq)
     return empty;
 }
 
+/* Called within rcu_read_lock().  */
 static int virtio_queue_packed_empty_rcu(VirtQueue *vq)
 {
     struct VRingPackedDesc desc;