On 24/05/21 16:59, Christoph Hellwig wrote:

On Thu, May 20, 2021 at 03:13:05PM +0100, Stefan Hajnoczi wrote:

quoted

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

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