From d0435bc513e23a4961b6af20164d1c6c219eb4ea Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Mon, 9 Dec 2019 21:09:57 +0000 Subject: [PATCH] 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 +++++++-- 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 d62e6377c2..b12157b5eb 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 e8b2b64d09..f080545f48 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 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) diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h index 777772475c..b69d517496 100644 --- 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);