virtio: don't enable notifications during polling

Virtqueue notifications are not necessary during polling, so we disable
them.  This allows the guest driver to avoid MMIO vmexits.
Unfortunately the virtio-blk and virtio-scsi handler functions re-enable
notifications, defeating this optimization.

Fix virtio-blk and virtio-scsi emulation so they leave notifications
disabled.  The key thing to remember for correctness is that polling
always checks one last time after ending its loop, therefore it's safe
to lose the race when re-enabling notifications at the end of polling.

There is a measurable performance improvement of 5-10% with the null-co
block driver.  Real-life storage configurations will see a smaller
improvement because the MMIO vmexit overhead contributes less to
latency.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20191209210957.65087-1-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index d62e6377c27d0761d9e99452ef4e18ed656161be..b12157b5eb92dede6852b4c0d12dfd445f206eea 100644 (file)
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -764,13 +764,16 @@ bool virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
 {
     VirtIOBlockReq *req;
     MultiReqBuffer mrb = {};
+    bool suppress_notifications = virtio_queue_get_notification(vq);
     bool progress = false;
 
     aio_context_acquire(blk_get_aio_context(s->blk));
     blk_io_plug(s->blk);
 
     do {
-        virtio_queue_set_notification(vq, 0);
+        if (suppress_notifications) {
+            virtio_queue_set_notification(vq, 0);
+        }
 
         while ((req = virtio_blk_get_request(s, vq))) {
             progress = true;
@@ -781,7 +784,9 @@ bool virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
             }
         }
 
-        virtio_queue_set_notification(vq, 1);
+        if (suppress_notifications) {
+            virtio_queue_set_notification(vq, 1);
+        }
     } while (!virtio_queue_empty(vq));
 
     if (mrb.num_reqs) {

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index e8b2b64d09fb185404fa83882ba95742b14c6301..f080545f48e6a3e411caf641b935aa8e20337b0b 100644 (file)
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -597,12 +597,15 @@ bool virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
 {
     VirtIOSCSIReq *req, *next;
     int ret = 0;
+    bool suppress_notifications = virtio_queue_get_notification(vq);
     bool progress = false;
 
     QTAILQ_HEAD(, VirtIOSCSIReq) reqs = QTAILQ_HEAD_INITIALIZER(reqs);
 
     do {
-        virtio_queue_set_notification(vq, 0);
+        if (suppress_notifications) {
+            virtio_queue_set_notification(vq, 0);
+        }
 
         while ((req = virtio_scsi_pop_req(s, vq))) {
             progress = true;
@@ -622,7 +625,9 @@ bool virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
             }
         }
 
-        virtio_queue_set_notification(vq, 1);
+        if (suppress_notifications) {
+            virtio_queue_set_notification(vq, 1);
+        }
     } while (ret != -EINVAL && !virtio_queue_empty(vq));
 
     QTAILQ_FOREACH_SAFE(req, &reqs, next, next) {

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 7bc6a9455e7f1ba0397dda0145db512df89d66c8..95d8ff8508148bf6d29ef69d8e8afc16a77f069f 100644 (file)
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -432,6 +432,11 @@ static void virtio_queue_packed_set_notification(VirtQueue *vq, int enable)
     }
 }
 
+bool virtio_queue_get_notification(VirtQueue *vq)
+{
+    return vq->notification;
+}
+
 void virtio_queue_set_notification(VirtQueue *vq, int enable)
 {
     vq->notification = enable;
@@ -3410,17 +3415,12 @@ static bool virtio_queue_host_notifier_aio_poll(void *opaque)
 {
     EventNotifier *n = opaque;
     VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
-    bool progress;
 
     if (!vq->vring.desc || virtio_queue_empty(vq)) {
         return false;
     }
 
-    progress = virtio_queue_notify_aio_vq(vq);
-
-    /* In case the handler function re-enabled notifications */
-    virtio_queue_set_notification(vq, 0);
-    return progress;
+    return virtio_queue_notify_aio_vq(vq);
 }
 
 static void virtio_queue_host_notifier_aio_poll_end(EventNotifier *n)

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 777772475cf0b5764bc034c7517286bb16b4d352..b69d517496358d4dcba30eeb42c721e866798051 100644 (file)
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -228,6 +228,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id);
 
 void virtio_notify_config(VirtIODevice *vdev);
 
+bool virtio_queue_get_notification(VirtQueue *vq);
 void virtio_queue_set_notification(VirtQueue *vq, int enable);
 
 int virtio_queue_ready(VirtQueue *vq);