159 lines
5.4 KiB
Diff
159 lines
5.4 KiB
Diff
From 351dd07d7b5e69cdf47260c9ea848c0c93cd2c8a Mon Sep 17 00:00:00 2001
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From: Stefan Hajnoczi <stefanha@redhat.com>
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Date: Thu, 9 Jan 2020 11:13:25 +0000
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Subject: [PATCH 3/5] virtio: don't enable notifications during polling
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RH-Author: Stefan Hajnoczi <stefanha@redhat.com>
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Message-id: <20200109111325.559557-2-stefanha@redhat.com>
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Patchwork-id: 93298
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O-Subject: [RHEL-AV-8.2.0 qemu-kvm PATCH 1/1] virtio: don't enable notifications during polling
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Bugzilla: 1789301
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: Sergio Lopez Pascual <slp@redhat.com>
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RH-Acked-by: Michael S. Tsirkin <mst@redhat.com>
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Virtqueue notifications are not necessary during polling, so we disable
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them. This allows the guest driver to avoid MMIO vmexits.
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Unfortunately the virtio-blk and virtio-scsi handler functions re-enable
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notifications, defeating this optimization.
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Fix virtio-blk and virtio-scsi emulation so they leave notifications
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disabled. The key thing to remember for correctness is that polling
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always checks one last time after ending its loop, therefore it's safe
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to lose the race when re-enabling notifications at the end of polling.
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There is a measurable performance improvement of 5-10% with the null-co
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block driver. Real-life storage configurations will see a smaller
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improvement because the MMIO vmexit overhead contributes less to
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latency.
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Message-Id: <20191209210957.65087-1-stefanha@redhat.com>
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Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
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Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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(cherry picked from commit d0435bc513e23a4961b6af20164d1c6c219eb4ea)
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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hw/block/virtio-blk.c | 9 +++++++--
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hw/scsi/virtio-scsi.c | 9 +++++++--
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hw/virtio/virtio.c | 12 ++++++------
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include/hw/virtio/virtio.h | 1 +
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4 files changed, 21 insertions(+), 10 deletions(-)
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diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
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index 4c357d2..c4e55fb 100644
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--- a/hw/block/virtio-blk.c
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+++ b/hw/block/virtio-blk.c
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@@ -764,13 +764,16 @@ bool virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
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{
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VirtIOBlockReq *req;
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MultiReqBuffer mrb = {};
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+ bool suppress_notifications = virtio_queue_get_notification(vq);
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bool progress = false;
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aio_context_acquire(blk_get_aio_context(s->blk));
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blk_io_plug(s->blk);
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do {
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- virtio_queue_set_notification(vq, 0);
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+ if (suppress_notifications) {
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+ virtio_queue_set_notification(vq, 0);
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+ }
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while ((req = virtio_blk_get_request(s, vq))) {
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progress = true;
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@@ -781,7 +784,9 @@ bool virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
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}
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}
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- virtio_queue_set_notification(vq, 1);
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+ if (suppress_notifications) {
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+ virtio_queue_set_notification(vq, 1);
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+ }
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} while (!virtio_queue_empty(vq));
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if (mrb.num_reqs) {
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diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
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index 54108c0..e2cd1df 100644
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--- a/hw/scsi/virtio-scsi.c
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+++ b/hw/scsi/virtio-scsi.c
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@@ -597,12 +597,15 @@ bool virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
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{
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VirtIOSCSIReq *req, *next;
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int ret = 0;
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+ bool suppress_notifications = virtio_queue_get_notification(vq);
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bool progress = false;
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QTAILQ_HEAD(, VirtIOSCSIReq) reqs = QTAILQ_HEAD_INITIALIZER(reqs);
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do {
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- virtio_queue_set_notification(vq, 0);
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+ if (suppress_notifications) {
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+ virtio_queue_set_notification(vq, 0);
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+ }
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while ((req = virtio_scsi_pop_req(s, vq))) {
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progress = true;
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@@ -622,7 +625,9 @@ bool virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
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}
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}
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- virtio_queue_set_notification(vq, 1);
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+ if (suppress_notifications) {
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+ virtio_queue_set_notification(vq, 1);
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+ }
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} while (ret != -EINVAL && !virtio_queue_empty(vq));
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QTAILQ_FOREACH_SAFE(req, &reqs, next, next) {
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diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
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index 04716b5..3211135 100644
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--- a/hw/virtio/virtio.c
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+++ b/hw/virtio/virtio.c
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@@ -432,6 +432,11 @@ static void virtio_queue_packed_set_notification(VirtQueue *vq, int enable)
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}
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}
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+bool virtio_queue_get_notification(VirtQueue *vq)
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+{
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+ return vq->notification;
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+}
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+
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void virtio_queue_set_notification(VirtQueue *vq, int enable)
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{
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vq->notification = enable;
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@@ -3384,17 +3389,12 @@ static bool virtio_queue_host_notifier_aio_poll(void *opaque)
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{
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EventNotifier *n = opaque;
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VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
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- bool progress;
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if (!vq->vring.desc || virtio_queue_empty(vq)) {
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return false;
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}
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- progress = virtio_queue_notify_aio_vq(vq);
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-
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- /* In case the handler function re-enabled notifications */
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- virtio_queue_set_notification(vq, 0);
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- return progress;
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+ return virtio_queue_notify_aio_vq(vq);
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}
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static void virtio_queue_host_notifier_aio_poll_end(EventNotifier *n)
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diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
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index c32a815..6a20442 100644
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--- a/include/hw/virtio/virtio.h
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+++ b/include/hw/virtio/virtio.h
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@@ -224,6 +224,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id);
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void virtio_notify_config(VirtIODevice *vdev);
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+bool virtio_queue_get_notification(VirtQueue *vq);
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void virtio_queue_set_notification(VirtQueue *vq, int enable);
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int virtio_queue_ready(VirtQueue *vq);
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--
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1.8.3.1
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