vhost: use avail event idx on vhost_svq_kick

So SVQ code knows if an event is needed.

The code is not reachable at the moment.

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/virtio/vhost-shadow-virtqueue.c

@@ -218,12 +218,22 @@ static bool vhost_svq_add_split(VhostShadowVirtqueue *svq,
 
 static void vhost_svq_kick(VhostShadowVirtqueue *svq)
 {
+    bool needs_kick;
+
     /*
      * We need to expose the available array entries before checking the used
      * flags
      */
     smp_mb();
-    if (svq->vring.used->flags & VRING_USED_F_NO_NOTIFY) {
+
+    if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
+        uint16_t avail_event = *(uint16_t *)(&svq->vring.used->ring[svq->vring.num]);
+        needs_kick = vring_need_event(avail_event, svq->shadow_avail_idx, svq->shadow_avail_idx - 1);
+    } else {
+        needs_kick = !(svq->vring.used->flags & VRING_USED_F_NO_NOTIFY);
+    }
+
+    if (!needs_kick) {
         return;
     }