[Qemu-devel] [PATCH v4 11/13] virtio: disable virtqueue notifications during polling

[Stefan Hajnoczi <stefanha@redhat.com>]

This is a performance optimization to eliminate vmexits during polling.
It also avoids spurious ioeventfd processing after polling ends.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/virtio/virtio.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 79b5c36..d40711a 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -2053,6 +2053,13 @@ static void virtio_queue_host_notifier_aio_read(EventNotifier *n)
     }
 }
 
+static void virtio_queue_host_notifier_aio_poll_begin(EventNotifier *n)
+{
+    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
+
+    virtio_queue_set_notification(vq, 0);
+}
+
 static bool virtio_queue_host_notifier_aio_poll(void *opaque)
 {
     EventNotifier *n = opaque;
@@ -2066,6 +2073,14 @@ static bool virtio_queue_host_notifier_aio_poll(void *opaque)
     return true;
 }
 
+static void virtio_queue_host_notifier_aio_poll_end(EventNotifier *n)
+{
+    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
+
+    /* Caller polls once more after this to catch requests that race with us */
+    virtio_queue_set_notification(vq, 1);
+}
+
 void virtio_queue_aio_set_host_notifier_handler(VirtQueue *vq, AioContext *ctx,
                                                 VirtIOHandleOutput handle_output)
 {
@@ -2074,6 +2089,9 @@ void virtio_queue_aio_set_host_notifier_handler(VirtQueue *vq, AioContext *ctx,
         aio_set_event_notifier(ctx, &vq->host_notifier, true,
                                virtio_queue_host_notifier_aio_read,
                                virtio_queue_host_notifier_aio_poll);
+        aio_set_event_notifier_poll(ctx, &vq->host_notifier,
+                                    virtio_queue_host_notifier_aio_poll_begin,
+                                    virtio_queue_host_notifier_aio_poll_end);
     } else {
         aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL);
         /* Test and clear notifier before after disabling event,
-- 
2.9.3