virtio: always use handle_aio_output if registered

Commit ad07cd69 ("virtio-scsi: always use dataplane path if ioeventfd is
active", 2016-10-30) and 9ffe337c ("virtio-blk: always use dataplane
path if ioeventfd is active", 2016-10-30) broke the virtio 1.0
indirect access registers.

The indirect access registers bypass the ioeventfd, so that virtio-blk
and virtio-scsi now repeatedly try to initialize dataplane instead of
triggering the guest->host EventNotifier.  Detect the situation by
checking vq->handle_aio_output; if it is not NULL, trigger the
EventNotifier, which is how the device expects to get notifications
and in fact the only thread-safe manner to deliver them.

Fixes: ad07cd69
Fixes: 9ffe337c
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit e49a6618)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/virtio/virtio.c

@@ -1256,7 +1256,18 @@ static void virtio_queue_notify_vq(VirtQueue *vq)
 
 void virtio_queue_notify(VirtIODevice *vdev, int n)
 {
-    virtio_queue_notify_vq(&vdev->vq[n]);
+    VirtQueue *vq = &vdev->vq[n];
+
+    if (unlikely(!vq->vring.desc || vdev->broken)) {
+        return;
+    }
+
+    trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
+    if (vq->handle_aio_output) {
+        event_notifier_set(&vq->host_notifier);
+    } else if (vq->handle_output) {
+        vq->handle_output(vdev, vq);
+    }
 }
 
 uint16_t virtio_queue_vector(VirtIODevice *vdev, int n)