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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Apr 07, 2025 at 03:28:13PM +0200, Cornelia Huck wrote:
> On Mon, Apr 07 2025, David Hildenbrand <david@redhat.com> wrote:
> 
> > On 07.04.25 15:12, Halil Pasic wrote:
> >> On Mon, 7 Apr 2025 04:34:29 -0400
> >> "Michael S. Tsirkin" <mst@redhat.com> 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 ...
> 
> Indeed, we need a rule "reserve a feature bit/queue index before using
> it, even if you do not plan to spec it properly".


Reserving feature bits is something I do my best to advocate for
in all presentations I do.


-- 
MST