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

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

On Wed, Apr 09, 2025 at 12:46:41PM +0200, David Hildenbrand wrote:
> 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

So let's talk about a new feature bit?

Since vqs are probed after feature negotiation, it looks like
we could have a feature bit trigger sane behaviour, right?

I kind of dislike it that we have a feature bit for bugs though.
What would be a minimal new feature to add so it does not
feel wrong?

Maybe it's in the field of psychology though ...


-- 
MST