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

@@ -592,26 +592,32 @@ 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);
 
-    while ((req = virtio_scsi_pop_req(s, vq))) {
-        ret = virtio_scsi_handle_cmd_req_prepare(s, req);
-        if (!ret) {
-            QTAILQ_INSERT_TAIL(&reqs, req, next);
-        } else if (ret == -EINVAL) {
-            /* The device is broken and shouldn't process any request */
-            while (!QTAILQ_EMPTY(&reqs)) {
-                req = QTAILQ_FIRST(&reqs);
-                QTAILQ_REMOVE(&reqs, req, next);
-                blk_io_unplug(req->sreq->dev->conf.blk);
-                scsi_req_unref(req->sreq);
-                virtqueue_detach_element(req->vq, &req->elem, 0);
-                virtio_scsi_free_req(req);
+    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) {
+                QTAILQ_INSERT_TAIL(&reqs, req, next);
+            } else if (ret == -EINVAL) {
+                /* The device is broken and shouldn't process any request */
+                while (!QTAILQ_EMPTY(&reqs)) {
+                    req = QTAILQ_FIRST(&reqs);
+                    QTAILQ_REMOVE(&reqs, req, next);
+                    blk_io_unplug(req->sreq->dev->conf.blk);
+                    scsi_req_unref(req->sreq);
+                    virtqueue_detach_element(req->vq, &req->elem, 0);
+                    virtio_scsi_free_req(req);
+                }
             }
         }
-    }
+
+        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);