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

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Message-ID: <440de313-e470-4afa-9f8a-59598fe8dc21@redhat.com>
Date: Wed, 9 Apr 2025 12:46:41 +0200
From: David Hildenbrand <david@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Daniel Verkamp <dverkamp@...omium.org>, 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 23:20, Michael S. Tsirkin wrote:
> On Mon, Apr 07, 2025 at 08:47:05PM +0200, David Hildenbrand wrote:
>>> In my opinion, it makes the most sense to keep the spec as it is and
>>> change QEMU and the kernel to match, but obviously that's not trivial
>>> to do in a way that doesn't break existing devices and drivers.
>>
>> If only it would be limited to QEMU and Linux ... :)
>>
>> Out of curiosity, assuming we'd make the spec match the current QEMU/Linux
>> implementation at least for the 3 involved features only, would there be a
>> way to adjust crossvm without any disruption?
>>
>> I still have the feeling that it will be rather hard to get that all
>> implementations match the spec ... For new features+queues it will be easy
>> to force the usage of fixed virtqueue numbers, but for free-page-hinting and
>> reporting, it's a mess :(
> 
> 
> Still thinking about a way to fix drivers... We can discuss this
> theoretically, maybe?

Yes, absolutely. I took the time to do some more digging; regarding 
drivers only Linux seems to be problematic.

virtio-win, FreeBSD, NetBSD and OpenBSD and don't seem to support 
problematic features (free page hinting, free page reporting) in their 
virtio-balloon implementations.

So from the known drivers, only Linux is applicable.

reporting_vq is either at idx 4/3/2
free_page_vq is either at idx 3/2
statsq is at idx2 (only relevant if the feature is offered)

So if we could test for the existence of a virtqueue at an idx easily, 
we could test from highest-to-smallest idx.

But I recall that testing for the existance of a virtqueue on s390x 
resulted in the problem/deadlock in the first place ...

-- 
Cheers,

David / dhildenb