qemu/virtio-don-t-enable-notifications-during.patch

From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Mon, 9 Dec 2019 21:09:57 +0000
Subject: virtio: don't enable notifications during polling

Git-commit: d0435bc513e23a4961b6af20164d1c6c219eb4ea

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>
Signed-off-by: Bruce Rogers <brogers@suse.com>
---
 hw/block/virtio-blk.c      |  9 +++++++--
 hw/scsi/virtio-scsi.c      |  9 +++++++--
 hw/virtio/virtio.c         | 12 ++++++------
 include/hw/virtio/virtio.h |  1 +
 4 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 4c357d2928ff1cfe94a601c93ffa..c4e55fb3defb711dbc39b67e00a1 100644
--- 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 e8b2b64d09fb185404fa83882ba9..f080545f48e6a3e411caf641b935 100644
--- 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 04716b5f6ce1ccfb3f21a5b81b77..3211135bc8beb0856e100bcbda58 100644
--- 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;
@@ -3384,17 +3389,12 @@ 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 c32a815303730700e60c2ddd06c4..6a2044246d63ba8a3f932086f9e8 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -224,6 +224,7 @@ 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);