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

[Halil Pasic]

On Thu, 3 Apr 2025 10:35:33 -0400
"Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:

> On Thu, Apr 03, 2025 at 04:18:36PM +0200, Halil Pasic wrote:
> > On Wed,  2 Apr 2025 22:36:21 +0200
[..]
> > 
> > 5.5.2 Virtqueues
> > 
> > 0
> >     inflateq 
> > 1
> >     deflateq 
> > 2
> >     statsq 
> > 3
> >     free_page_vq 
> > 4
> >     reporting_vq  
> 
> Indeed. Unfortunately, no one at all implemented this properly as
> per spec :(.
> 
> Balloon is the worst offender here but other devices are broken
> too in some configurations.
> 
> 
> Given it has been like this for many years I'm inclined in this
> instance to fix the spec not the implementations.
> 

I understand! For me at least knowing if we are going to change the
spec or the implementations is pretty important. It would probably
make sense to spin a patch for the spec, just for the unlikely case that
somebody did end up implementing this by the spec and wants to protest.

I think, a consequence of this design is that all queues need to be
created and allocated at initialization time.

I mean in the spec we have something like 
"""
5.1.6.5.6.1 Driver Requirements: Automatic receive steering in multiqueue mode
The driver MUST configure the virtqueues before enabling them with the VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command. 
"""

For example one could want to hotplug 2 more vCPUs and still have a
queue-pair per cpu (and a controlq). Those 2 extra queue-pairs could
in theory be allocated on-demand instead of having to allocate memory
for the rings for all the queues corresponding to the maxed out setup.
I've had a look at the Linux virtio-net and it does allocate all the
queues up front.

Also with this design, I believe we would effectively prohibit "holes".
Now if holes are prohibited, IMHO it does not make a whole lot of
sense for the virtio_find_vqs() to support holes. Because the holes
and a fair amount of complexity. Of course that would make sense as a
cleanup on top.

Regards,
Halil