[Qemu-devel] [PATCH v3 03/10] virtio-scsi: suppress virtqueue kick during processing

[Stefan Hajnoczi <stefanha@redhat.com>]

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: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/scsi/virtio-scsi.c | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 10fd687..24a6272 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -578,26 +578,32 @@ 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);
-- 
2.7.4