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

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Message-ID: <a86240bc-8417-48a6-bf13-01dd7ace5ae9@redhat.com>
Date: Mon, 7 Apr 2025 11:11:34 +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 10:58, Michael S. Tsirkin wrote:
> On Mon, Apr 07, 2025 at 10:54:00AM +0200, David Hildenbrand wrote:
>> On 07.04.25 10:49, Michael S. Tsirkin wrote:
>>> On Mon, Apr 07, 2025 at 10:44:21AM +0200, David Hildenbrand wrote:
>>>>>
>>>>>
>>>>>
>>>>>> 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?
>>>>
>>>
>>> what are virtqueue bits? vq number?
>>
>> Yes, sorry.
> 
> I got confused myself, it's vq index actually now, we made the spec
> consistent with that terminology. used to be number/index
> interchangeably.
> 
>> Assume cross-vm as an example. It would make use of virtqueue indexes 5+6
>> with their VIRTIO_BALLOON_F_WS_REPORTING.
> 
> 
> crossvm guys really should have reserved the feature bit even if they
> did not bother specifying it. Let's reserve it now at least?

Along with the virtqueue indices, right?

Note that there was

https://lists.gnu.org/archive/html/qemu-devel/2023-05/msg02503.html

and

https://groups.oasis-open.org/communities/community-home/digestviewer/viewthread?GroupId=3973&MessageKey=afb07613-f56c-4d40-8981-2fad1c723998&CommunityKey=2f26be99-3aa1-48f6-93a5-018dce262226&hlmlt=VT

But it only was RFC, and as the QEMU implementation didn't materialize, 
nobody seemed to care ...

> 
> 
>> So whatever feature another device implements couldn't use this feature bit
>> or these virtqueue indexes.
>>
>> (as long the other device never intends to implement
>> VIRTIO_BALLOON_F_WS_REPORTING, the virtqueue indexes could be reused. But
>> the spec will also be a mess, because virtqueue indexes could also have
>> duplicate meanings ... ugh)
> 
> what do they do with vq indices btw?

See above links, they use the two for "s_vq and notification_vq".

-- 
Cheers,

David / dhildenb