From d0435bc513e23a4961b6af20164d1c6c219eb4ea Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Mon, 9 Dec 2019 21:09:57 +0000 Subject: 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 Message-Id: <20191209210957.65087-1-stefanha@redhat.com> Reviewed-by: Michael S. Tsirkin Signed-off-by: Michael S. Tsirkin --- hw/block/virtio-blk.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) (limited to 'hw/block') diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c index d62e637..b12157b 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) { -- cgit v1.1