Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for non-existing queues

[Halil Pasic <pasic@linux.ibm.com>]

On Fri, 4 Apr 2025 01:33:28 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> > 
> > I think, a consequence of this design is that all queues need to be
> > created and allocated at initialization time.  
> 
> Why? after feature negotiation.

What I mean is, with this change having queues that exist but are not
set up before the device becomes operational is not viable any more.

Let me use the virtio-net example again. I assume by the current spec
it would be OK to have max_virtqueue_pairs quite big e.g. 64, just in
case the guest ends up having many vCPUs hotplugged. But start out with
2 vcpus, 2 queue pairs and initially doing VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET with 2.

And then grow the guest to 8 vcpus, 'discover' virtqueues
2(I-1) receiveqI, 2(I-1)+1 transmitqI for 2 < I < 9 (I is a natural number)
and do another VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET with 8.

Please notice that the controlq would sit at index 128 (64*2) all along. That
is in the old world. In the new world we don't do holes, so we need to
allocate all the virtqueues up to controlq up-front. To avoid having
holes. Or any queue-pairs that are discoverd after the initial vq discovery
would need to have an index larger than controlq has.

Regards,
Halil