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

[David Hildenbrand]

On 07.04.25 23:20, Michael S. Tsirkin wrote:
> On Mon, Apr 07, 2025 at 08:47:05PM +0200, David Hildenbrand wrote:
> > > In my opinion, it makes the most sense to keep the spec as it is and
> > > change QEMU and the kernel to match, but obviously that's not trivial
> > > to do in a way that doesn't break existing devices and drivers.
> >
> > If only it would be limited to QEMU and Linux ... :)
> >
> > Out of curiosity, assuming we'd make the spec match the current QEMU/Linux
> > implementation at least for the 3 involved features only, would there be a
> > way to adjust crossvm without any disruption?
> >
> > I still have the feeling that it will be rather hard to get that all
> > implementations match the spec ... For new features+queues it will be easy
> > to force the usage of fixed virtqueue numbers, but for free-page-hinting and
> > reporting, it's a mess :(
>
>
> Still thinking about a way to fix drivers... We can discuss this
> theoretically, maybe?

Yes, absolutely. I took the time to do some more digging; regarding 
drivers only Linux seems to be problematic.

virtio-win, FreeBSD, NetBSD and OpenBSD and don't seem to support 
problematic features (free page hinting, free page reporting) in their 
virtio-balloon implementations.

So from the known drivers, only Linux is applicable.

reporting_vq is either at idx 4/3/2
free_page_vq is either at idx 3/2
statsq is at idx2 (only relevant if the feature is offered)

So if we could test for the existence of a virtqueue at an idx easily, 
we could test from highest-to-smallest idx.

But I recall that testing for the existance of a virtqueue on s390x 
resulted in the problem/deadlock in the first place ...

--
Cheers,

David / dhildenb