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
