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authorStefan Hajnoczi <stefanha@redhat.com>2019-12-09 21:09:57 +0000
committerMichael S. Tsirkin <mst@redhat.com>2020-01-05 07:03:03 -0500
commitd0435bc513e23a4961b6af20164d1c6c219eb4ea (patch)
tree0434e1d07119ee8ea9e80143a5aa0a8c82cff41c /hw/virtio
parent6620801f39f0cd02b30ab5ad8729c92c78ce9fd3 (diff)
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>
Diffstat (limited to 'hw/virtio')
-rw-r--r--hw/virtio/virtio.c12
1 files changed, 6 insertions, 6 deletions
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 7bc6a9455e..95d8ff8508 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;
@@ -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)