virtio-crypto: fix virtio_queue_set_notification() race

We must check for new virtqueue buffers after re-enabling notifications.
This prevents the race condition where the guest added buffers just
after we stopped popping the virtqueue but before we re-enabled
notifications.

I think the virtio-crypto code was based on virtio-net but this crucial
detail was missed.  virtio-net does not have the race condition because
it processes the virtqueue one more time after re-enabling
notifications.

Cc: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>

hw/virtio/virtio-crypto.c

@@ -692,8 +692,17 @@ static void virtio_crypto_dataq_bh(void *opaque)
         return;
     }
 
-    virtio_crypto_handle_dataq(vdev, q->dataq);
-    virtio_queue_set_notification(q->dataq, 1);
+    for (;;) {
+        virtio_crypto_handle_dataq(vdev, q->dataq);
+        virtio_queue_set_notification(q->dataq, 1);
+
+        /* Are we done or did the guest add more buffers? */
+        if (virtio_queue_empty(q->dataq)) {
+            break;
+        }
+
+        virtio_queue_set_notification(q->dataq, 0);
+    }
 }
 
 static void