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

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Message-ID: <9126bfbf-9461-4959-bd38-1d7bc36d7701@redhat.com>
Date: Mon, 7 Apr 2025 15:17:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Halil Pasic <pasic@...ux.ibm.com>, "Michael S. Tsirkin" <mst@...hat.com>
Cc: 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 15:12, Halil Pasic wrote:
> On Mon, 7 Apr 2025 04:34:29 -0400
> "Michael S. Tsirkin" <mst@...hat.com> 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.
> 
> First, sorry for not catching up again with the discussion earlier.
> 
> I think David's point is based on the assumption that by the time feature
> with the feature bit N+1 is specified and allocates a queue Q, all
> queues with indexes smaller than Q are allocated and possibly associated
> with features that were previously specified (and probably have feature
> bits smaller than N+1).
> 
> I.e. that we can mandate, even if you don't want to care about other
> optional features, you have to, because we say so, for the matter of
> virtqueue existence. And anything in the future, you don't have to care
> about because the queue index associated with future features is larger
> than Q, so it does not affect our position.
> 
> I think that argument can fall a part if:
> * future features reference optional queues defined in the past
> * somebody managed to introduce a limbo where a feature is reserved, and
>    they can not decide if they want a queue or not, or make the existence
>    of the queue depend on something else than a feature bit.

Staring at the cross-vmm, including the adding+removing of features and 
queues that are not in the spec, I am wondering if (in a world with 
fixed virtqueues)

1) Feature bits must be reserved before used.

2) Queue indices must be reserved before used.

It all smells like a problem similar to device IDs ...

-- 
Cheers,

David / dhildenb