Re: [PATCH 1/2] virtio-scsi: Attach event vq notifier with no_poll

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Jan 24, 2024 at 06:38:29PM +0100, Hanna Czenczek wrote:
> As of commit 38738f7dbbda90fbc161757b7f4be35b52205552 ("virtio-scsi:
> don't waste CPU polling the event virtqueue"), we only attach an io_read
> notifier for the virtio-scsi event virtqueue instead, and no polling
> notifiers.  During operation, the event virtqueue is typically
> non-empty, but none of the buffers are intended to be used immediately.
> Instead, they only get used when certain events occur.  Therefore, it
> makes no sense to continuously poll it when non-empty, because it is
> supposed to be and stay non-empty.
> 
> We do this by using virtio_queue_aio_attach_host_notifier_no_poll()
> instead of virtio_queue_aio_attach_host_notifier() for the event
> virtqueue.
> 
> Commit 766aa2de0f29b657148e04599320d771c36fd126 ("virtio-scsi: implement
> BlockDevOps->drained_begin()") however has virtio_scsi_drained_end() use
> virtio_queue_aio_attach_host_notifier() for all virtqueues, including
> the event virtqueue.  This can lead to it being polled again, undoing
> the benefit of commit 38738f7dbbda90fbc161757b7f4be35b52205552.
> 
> Fix it by using virtio_queue_aio_attach_host_notifier_no_poll() for the
> event virtqueue.
> 
> Reported-by: Fiona Ebner <f.ebner@proxmox.com>
> Fixes: 766aa2de0f29b657148e04599320d771c36fd126
>        ("virtio-scsi: implement BlockDevOps->drained_begin()")
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> ---
>  hw/scsi/virtio-scsi.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

Thank you!

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

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