virtio-blk: suppress virtqueue kick during processing

The guest does not need to kick the virtqueue while we are processing
it.  This reduces the number of vmexits during periods of heavy I/O.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-8-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/block/virtio-blk.c

@@ -588,6 +588,9 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
 
     blk_io_plug(s->blk);
 
+    do {
+        virtio_queue_set_notification(vq, 0);
+
         while ((req = virtio_blk_get_request(s, vq))) {
             if (virtio_blk_handle_request(req, &mrb)) {
                 virtqueue_detach_element(req->vq, &req->elem, 0);
@@ -596,6 +599,9 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
             }
         }
 
+        virtio_queue_set_notification(vq, 1);
+    } while (!virtio_queue_empty(vq));
+
     if (mrb.num_reqs) {
         virtio_blk_submit_multireq(s->blk, &mrb);
     }