virtio-scsi: 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: 20161201192652.9509-9-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/scsi/virtio-scsi.c

@@ -592,10 +592,13 @@ static void virtio_scsi_handle_cmd_req_submit(VirtIOSCSI *s, VirtIOSCSIReq *req)
 void virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
 {
     VirtIOSCSIReq *req, *next;
-    int ret;
+    int ret = 0;
 
     QTAILQ_HEAD(, VirtIOSCSIReq) reqs = QTAILQ_HEAD_INITIALIZER(reqs);
 
+    do {
+        virtio_queue_set_notification(vq, 0);
+
         while ((req = virtio_scsi_pop_req(s, vq))) {
             ret = virtio_scsi_handle_cmd_req_prepare(s, req);
             if (!ret) {
@@ -613,6 +616,9 @@ void virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
             }
         }
 
+        virtio_queue_set_notification(vq, 1);
+    } while (ret != -EINVAL && !virtio_queue_empty(vq));
+
     QTAILQ_FOREACH_SAFE(req, &reqs, next, next) {
         virtio_scsi_handle_cmd_req_submit(s, req);
     }