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
