Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()

[Paolo Bonzini]

On 24/05/21 16:59, Christoph Hellwig wrote:
> On Thu, May 20, 2021 at 03:13:05PM +0100, Stefan Hajnoczi wrote:
> > Possible drawbacks of this approach:
> >
> > - Hardware virtio_blk implementations may find virtqueue_disable_cb()
> >    expensive since it requires DMA. If such devices become popular then
> >    the virtio_blk driver could use a similar approach to NVMe when
> >    VIRTIO_F_ACCESS_PLATFORM is detected in the future.
> >
> > - If a blk_poll() thread is descheduled it not only hurts polling
> >    performance but also delays completion of non-REQ_HIPRI requests on
> >    that virtqueue since vq notifications are disabled.
>
> Yes, I think this is a dangerous configuration.  What argument exists
> again just using dedicated poll queues?

There isn't an equivalent of the admin queue in virtio-blk, which would 
allow the guest to configure the desired number of poll queues.  The 
number of queues is fixed.

Could the blk_poll() thread use preempt notifiers to enable/disable 
callbacks, for example using two new .poll_start and .end_poll callbacks 
to struct blk_mq_ops?

Paolo