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

[David Hildenbrand]

On 07.04.25 15:12, Halil Pasic wrote:
> On Mon, 7 Apr 2025 04:34:29 -0400
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
>
> > On Mon, Apr 07, 2025 at 10:17:10AM +0200, David Hildenbrand wrote:
> > > On 07.04.25 09:52, Michael S. Tsirkin wrote:
> > > > On Fri, Apr 04, 2025 at 05:39:10PM +0200, Halil Pasic wrote:
> > > > > > Not perfect, but AFAIKS, not horrible.
> > > > >
> > > > > It is like it is. QEMU does queue exist if the corresponding feature
> > > > > is offered by the device, and that is what we have to live with.
> > > >
> > > > I don't think we can live with this properly though.
> > > > It means a guest that does not know about some features
> > > > does not know where to find things.
> > >
> > > Please describe a real scenario, I'm missing the point.
> >
> >
> > OK so.
> >
> > Device has VIRTIO_BALLOON_F_FREE_PAGE_HINT and VIRTIO_BALLOON_F_REPORTING
> > Driver only knows about VIRTIO_BALLOON_F_REPORTING so
> > it does not know what does VIRTIO_BALLOON_F_FREE_PAGE_HINT do.
> > How does it know which vq to use for reporting?
> > It will try to use the free page hint one.
>
> First, sorry for not catching up again with the discussion earlier.
>
> I think David's point is based on the assumption that by the time feature
> with the feature bit N+1 is specified and allocates a queue Q, all
> queues with indexes smaller than Q are allocated and possibly associated
> with features that were previously specified (and probably have feature
> bits smaller than N+1).
>
> I.e. that we can mandate, even if you don't want to care about other
> optional features, you have to, because we say so, for the matter of
> virtqueue existence. And anything in the future, you don't have to care
> about because the queue index associated with future features is larger
> than Q, so it does not affect our position.
>
> I think that argument can fall a part if:
> * future features reference optional queues defined in the past
> * somebody managed to introduce a limbo where a feature is reserved, and
>    they can not decide if they want a queue or not, or make the existence
>    of the queue depend on something else than a feature bit.

Staring at the cross-vmm, including the adding+removing of features and 
queues that are not in the spec, I am wondering if (in a world with 
fixed virtqueues)

1) Feature bits must be reserved before used.

2) Queue indices must be reserved before used.

It all smells like a problem similar to device IDs ...

--
Cheers,

David / dhildenb