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

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Message-ID: <b331a780-a9db-4d76-af7c-e9e8e7d1cc10@redhat.com>
Date: Mon, 7 Apr 2025 10:54:00 +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 10:49, 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.

Assume cross-vm as an example. It would make use of virtqueue indexes 
5+6 with their VIRTIO_BALLOON_F_WS_REPORTING.

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)

-- 
Cheers,

David / dhildenb