[PATCH 2/4] vhost: toggle device callbacks using used event idx

["Eugenio Pérez" <eperezma@redhat.com>]

Actually use the new field of the used ring and tell the device if SVQ
wants to be notified.

The code is not reachable at the moment.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-shadow-virtqueue.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index a518f84772..f5c0fad3fc 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -369,15 +369,27 @@ static bool vhost_svq_more_used(VhostShadowVirtqueue *svq)
  */
 static bool vhost_svq_enable_notification(VhostShadowVirtqueue *svq)
 {
-    svq->vring.avail->flags &= ~cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
-    /* Make sure the flag is written before the read of used_idx */
+    if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
+        uint16_t *used_event = (uint16_t *)&svq->vring.avail->ring[svq->vring.num];
+        *used_event = svq->shadow_used_idx;
+    } else {
+        svq->vring.avail->flags &= ~cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
+    }
+
+    /* Make sure the event is enabled before the read of used_idx */
     smp_mb();
     return !vhost_svq_more_used(svq);
 }
 
 static void vhost_svq_disable_notification(VhostShadowVirtqueue *svq)
 {
-    svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
+    /*
+     * No need to disable notification in the event idx case, since used event
+     * index is already an index too far away.
+     */
+    if (!virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
+        svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
+    }
 }
 
 static uint16_t vhost_svq_last_desc_of_chain(const VhostShadowVirtqueue *svq,
-- 
2.31.1