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

[David Hildenbrand]

On 07.04.25 10:58, Michael S. Tsirkin wrote:
> On Mon, Apr 07, 2025 at 10:54:00AM +0200, David Hildenbrand wrote:
> > On 07.04.25 10:49, Michael S. Tsirkin wrote:
> > > On Mon, Apr 07, 2025 at 10:44:21AM +0200, David Hildenbrand wrote:
> > > > > > Whoever adds new feat_X *must be aware* about all previous features,
> > > > > > otherwise we'd be reusing feature bits and everything falls to pieces.
> > > > >
> > > > >
> > > > > The knowledge is supposed be limited to which feature bit to use.
> > > >
> > > > I think we also have to know which virtqueue bits can be used, right?
> > >
> > > what are virtqueue bits? vq number?
> >
> > Yes, sorry.
>
> I got confused myself, it's vq index actually now, we made the spec
> consistent with that terminology. used to be number/index
> interchangeably.
>
> > Assume cross-vm as an example. It would make use of virtqueue indexes 5+6
> > with their VIRTIO_BALLOON_F_WS_REPORTING.
>
>
> crossvm guys really should have reserved the feature bit even if they
> did not bother specifying it. Let's reserve it now at least?

Along with the virtqueue indices, right?

Note that there was

https://lists.gnu.org/archive/html/qemu-devel/2023-05/msg02503.html

and

https://groups.oasis-open.org/communities/community-home/digestviewer/viewthread?GroupId=3973&MessageKey=afb07613-f56c-4d40-8981-2fad1c723998&CommunityKey=2f26be99-3aa1-48f6-93a5-018dce262226&hlmlt=VT

But it only was RFC, and as the QEMU implementation didn't materialize, 
nobody seemed to care ...

> > So whatever feature another device implements couldn't use this feature bit
> > or these virtqueue indexes.
> >
> > (as long the other device never intends to implement
> > VIRTIO_BALLOON_F_WS_REPORTING, the virtqueue indexes could be reused. But
> > the spec will also be a mess, because virtqueue indexes could also have
> > duplicate meanings ... ugh)
>
> what do they do with vq indices btw?

See above links, they use the two for "s_vq and notification_vq".

--
Cheers,

David / dhildenb