From: Stefan Hajnoczi <stefanha@redhat.com>

Date: Tue, 17 May 2022 09:27:45 +0100

Subject: [PATCH] virtio-scsi: don't waste CPU polling the event virtqueue

Content-type: text/plain

The virtio-scsi event virtqueue is not emptied by its handler function.

This is typical for rx virtqueues where the device uses buffers when

some event occurs (e.g. a packet is received, an error condition

happens, etc).

Polling non-empty virtqueues wastes CPU cycles. We are not waiting for

new buffers to become available, we are waiting for an event to occur,

so it's a misuse of CPU resources to poll for buffers.

Introduce the new virtio_queue_aio_attach_host_notifier_no_poll() API,

which is identical to virtio_queue_aio_attach_host_notifier() except

that it does not poll the virtqueue.

Before this patch the following command-line consumed 100% CPU in the

IOThread polling and calling virtio_scsi_handle_event():

  $ qemu-system-x86_64 -M accel=kvm -m 1G -cpu host \

      --object iothread,id=iothread0 \

      --device virtio-scsi-pci,iothread=iothread0 \

      --blockdev file,filename=test.img,aio=native,cache.direct=on,node-name=drive0 \

      --device scsi-hd,drive=drive0

After this patch CPU is no longer wasted.

Reported-by: Nir Soffer <nsoffer@redhat.com>

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Tested-by: Nir Soffer <nsoffer@redhat.com>

Message-id: 20220427143541.119567-3-stefanha@redhat.com

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

(cherry picked from commit 38738f7dbbda90fbc161757b7f4be35b52205552)

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

---

 hw/scsi/virtio-scsi-dataplane.c |  2 +-

 hw/virtio/virtio.c              | 13 +++++++++++++

 include/hw/virtio/virtio.h      |  1 +

 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c

index 29575cbaf6..8bb6e6acfc 100644

--- a/hw/scsi/virtio-scsi-dataplane.c

+++ b/hw/scsi/virtio-scsi-dataplane.c

@@ -138,7 +138,7 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)

     aio_context_acquire(s->ctx);

     virtio_queue_aio_attach_host_notifier(vs->ctrl_vq, s->ctx);

-    virtio_queue_aio_attach_host_notifier(vs->event_vq, s->ctx);

+    virtio_queue_aio_attach_host_notifier_no_poll(vs->event_vq, s->ctx);

     for (i = 0; i < vs->conf.num_queues; i++) {

         virtio_queue_aio_attach_host_notifier(vs->cmd_vqs[i], s->ctx);

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c

index 9d637e043e..67a873f54a 100644

--- a/hw/virtio/virtio.c

+++ b/hw/virtio/virtio.c

@@ -3534,6 +3534,19 @@ void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx)

                                 virtio_queue_host_notifier_aio_poll_end);

 }

+/*

+ * Same as virtio_queue_aio_attach_host_notifier() but without polling. Use

+ * this for rx virtqueues and similar cases where the virtqueue handler

+ * function does not pop all elements. When the virtqueue is left non-empty

+ * polling consumes CPU cycles and should not be used.

+ */

+void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext *ctx)

+{

+    aio_set_event_notifier(ctx, &vq->host_notifier, true,

+                           virtio_queue_host_notifier_read,

+                           NULL, NULL);

+}

+

 void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx)

 {

     aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL, NULL);

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h

index b31c4507f5..b62a35fdca 100644

--- a/include/hw/virtio/virtio.h

+++ b/include/hw/virtio/virtio.h

@@ -317,6 +317,7 @@ EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq);

 void virtio_queue_set_host_notifier_enabled(VirtQueue *vq, bool enabled);

 void virtio_queue_host_notifier_read(EventNotifier *n);

 void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx);

+void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext *ctx);

 void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx);

 VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector);

 VirtQueue *virtio_vector_next_queue(VirtQueue *vq);