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

[David Hildenbrand]

> > > Heh, but that one said:
> > >
> > > +\item[ VIRTIO_BALLOON_F_WS_REPORTING(6) ] The device has support for
> > > Working Set
> > >
> > > Which does not seem to reflect reality ...
>
> Please feel free to disregard these features and reuse their bits and
> queue indexes; as far as I know, they are not actually enabled
> anywhere currently and the corresponding guest patches were only
> applied to some (no-longer-used) ChromeOS kernel trees, so the
> compatibility impact should be minimal. I will also try to clean up
> the leftover bits on the crosvm side just to clear things up.

Thanks for your reply, and thanks for clarifying+cleaning it up.

> > I dug a bit more into cross-vm, because that one seems to be the only
> > one out there that does not behave like everybody else I found (maybe good,
> > maybe bad :) ).
> >
> >
> > 1) There was temporarily even another feature (VIRTIO_BALLOON_F_EVENTS_VQ)
> > and another queue.
> >
> > It got removed from cross-vm in:
> >
> > commit 9ba634b82b55ba762dc8724676b2cf9419460145
> > Author: Daniel Verkamp <dverkamp@xxxxxxxxxxxx>
> > Date:   Thu Jul 11 11:29:52 2024 -0700
> >
> >       devices: virtio-balloon: remove event queue support
> >
> >       VIRTIO_BALLOON_F_EVENTS_VQ was part of a proposed virtio spec change.
> >
> >       It is not currently supported by upstream Linux, so removing this should
> >       have no effect except for guest kernels that had CHROMIUM patches
> >       applied.
> >
> >       The virtqueue indexes for the ws-related queues are decremented to fill
> >       the hole left by the removal of the event VQ; these are non-standard as
> >       well, so they do not have virtqueue indexes assigned in the virtio spec,
> >       but the proposed spec extension did actually use vq indexes 5 and 6.
> >
> >       BUG=b:214864326
> >
> >
> > 2) cross-vm is aware of the upstream Linux driver
> >
> > They thought your fix would go upstream; it didn't.
> >
> > commit a2fa119e759d0238a42ff15a9aff0dfd122afebd
> > Author: Daniel Verkamp <dverkamp@xxxxxxxxxxxx>
> > Date:   Wed Jul 10 16:16:28 2024 -0700
> >
> >       devices: virtio-balloon: warn about queue index mismatches
> >
> >       The Linux kernel virtio-balloon driver spec non-compliance related to
> >       queue numbering is being fixed; add some diagnostics to our device that
> >       help to check if everything is working as expected.
> >
> >       <https://lore.kernel.org/virtualization/CACGkMEsg0+vpav1Fo8JF1isq4Ef8t4_CFN1scyztDO8bXzRLBQ@xxxxxxxxxxxxxx/T/>
> >
> >       Additionally, replace the num_expected_queues() function with per-queue
> >       checking to avoid the need for the duplicate feature checks and queue
> >       count calculation; each pop_queue() call will be checked using the `?`
> >       operator and return a more useful error message if a particular queue is
> >       missing.
> >
> >       BUG=None
> >       TEST=crosvm run --balloon-page-reporting ...
> >
> >
> > IIRC, in that commit they switched to the "spec" behavior.
> >
> > That's when they started hard-coding the queue indexes.
> >
> > CCing Daniel. All Linux versions should be incompatible with cross-vmm regarding free page reporting.
> > How is that handled?
>
> In practice, it only works because nobody calls crosvm with
> --balloon-page-reporting (it's off by default), so the balloon device
> does not advertise the VIRTIO_BALLOON_F_PAGE_REPORTING feature.
>
> (I just went searching now, and it does seem like there is actually
> one user in Android that does try to enable page reporting[1], which
> I'll have to look into...)
>
> 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 :(

--
Cheers,

David / dhildenb