virtio-scsi: don't waste CPU polling the event virtqueue

virtio-scsi: don't waste CPU polling the event virtqueue

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>