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

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Message-ID: <3bbad51d-d7d8-46f7-a28c-11cc3af6ef76@redhat.com>
Date: Mon, 7 Apr 2025 20:47:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Daniel Verkamp <dverkamp@...omium.org>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Halil Pasic <pasic@...ux.ibm.com>,
 linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
 virtualization@...ts.linux.dev, kvm@...r.kernel.org,
 Chandra Merla <cmerla@...hat.com>, Stable@...r.kernel.org,
 Cornelia Huck <cohuck@...hat.com>, Thomas Huth <thuth@...hat.com>,
 Eric Farman <farman@...ux.ibm.com>, Heiko Carstens <hca@...ux.ibm.com>,
 Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev
 <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Wei Wang <wei.w.wang@...el.com>
Subject: Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for
 non-existing queues

>>> 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@...omium.org>
>> 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@...omium.org>
>> 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@mail.gmail.com/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