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

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Message-ID: <0c221abf-de20-4ce3-917d-0375c1ec9140@redhat.com>
Date: Mon, 7 Apr 2025 10:44:21 +0200
From: David Hildenbrand <david@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: 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

On 07.04.25 10:34, Michael S. Tsirkin wrote:
> On Mon, Apr 07, 2025 at 10:17:10AM +0200, David Hildenbrand wrote:
>> On 07.04.25 09:52, Michael S. Tsirkin wrote:
>>> On Fri, Apr 04, 2025 at 05:39:10PM +0200, Halil Pasic wrote:
>>>>>
>>>>> Not perfect, but AFAIKS, not horrible.
>>>>
>>>> It is like it is. QEMU does queue exist if the corresponding feature
>>>> is offered by the device, and that is what we have to live with.
>>>
>>> I don't think we can live with this properly though.
>>> It means a guest that does not know about some features
>>> does not know where to find things.
>>
>> Please describe a real scenario, I'm missing the point.
> 
> 
> OK so.
> 
> Device has VIRTIO_BALLOON_F_FREE_PAGE_HINT and VIRTIO_BALLOON_F_REPORTING
> Driver only knows about VIRTIO_BALLOON_F_REPORTING so
> it does not know what does VIRTIO_BALLOON_F_FREE_PAGE_HINT do.
> How does it know which vq to use for reporting?
> It will try to use the free page hint one.

"only knows" -- VIRTIO_BALLOON_F_FREE_PAGE_HINT was proposed + specified 
in the spec.

So I think this is not a very good example.

> 
> 
> 
>> Whoever adds new feat_X *must be aware* about all previous features,
>> otherwise we'd be reusing feature bits and everything falls to pieces.
> 
> 
> The knowledge is supposed be limited to which feature bit to use.

I think we also have to know which virtqueue bits can be used, right?

I mean, I agree that it's all nasty ...

>>>
>>> So now, I am inclined to add linux code to work with current qemu and
>>> with spec compliant one, and add qemu code to work with current linux
>>> and spec compliant one.
>>>
>>> Document the bug in the spec, maybe, in a non conformance section.
>>
>> I'm afraid this results in a lot of churn without really making things
>> better.
> 
>> IMHO, documenting things how they actually behave, and maybe moving towards
>> fixed queue indexes for new features is the low hanging fruit.
> 
> I worry about how to we ensure that?
> If old code is messed up people will just keep propagating that.
> I would like to fix old code so that new code is correct.
> 
>>
>> As raised, it's not just qemu+linux, it's *at least* also cloud-hypervisor.
>>
>> -- 
>> Cheers,
>>
>> David / dhildenb
> 
> There's a slippery slope here in that people will come to us
> with buggy devices and ask to change the spec.

I yet have to fully digest cross-vm: 
https://github.com/google/crosvm/blob/main/devices/src/virtio/balloon.rs

and how it would free-page-reporting work with upstream Linux ... :(

-- 
Cheers,

David / dhildenb