Index: sys/dev/virtio/random/virtio_random.c
===================================================================
--- sys/dev/virtio/random/virtio_random.c
+++ sys/dev/virtio/random/virtio_random.c
@@ -229,6 +229,7 @@
 	struct sglist_seg segs[1];
 	struct sglist sg;
 	struct virtqueue *vq;
+	void *cookie;
 	uint32_t value[HARVESTSIZE] __aligned(sizeof(uint32_t) * HARVESTSIZE);
 	uint32_t rdlen;
 	int error;
@@ -252,7 +253,9 @@
 	 * done when we return from the notify.
 	 */
 	virtqueue_notify(vq);
-	virtqueue_poll(vq, &rdlen);
+	cookie = virtqueue_poll(vq, &rdlen);
+	/* Almost trivially true, but could indicate something bad. */
+	KASSERT(cookie == buf, ("inconsistent virtqueue state"));
 
 	if (rdlen > *sz)
 		panic("%s: random device wrote %zu bytes beyond end of provided"