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

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Message-ID: <2b187710-329d-4d36-b2e7-158709ea60d6@redhat.com>
Date: Mon, 7 Apr 2025 10:17:10 +0200
From: David Hildenbrand <david@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, Halil Pasic <pasic@...ux.ibm.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 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.

Whoever adds new feat_X *must be aware* about all previous features, 
otherwise we'd be reusing feature bits and everything falls to pieces.

> 
> 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.

As raised, it's not just qemu+linux, it's *at least* also cloud-hypervisor.

-- 
Cheers,

David / dhildenb