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

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Message-ID: <39a67ca9-966b-40c1-b080-95d8e2cde376@redhat.com>
Date: Mon, 7 Apr 2025 10:38:59 +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 10:17, 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.
> 
> 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.

I'm digging for other virtio-balloon implementations.

virtio-win: 
https://github.com/virtio-win/kvm-guest-drivers-windows/blob/master/Balloon/sys/balloon.c

-> Does not support hinting/reporting -> no problem

libkrun: 
https://github.com/containers/libkrun/blob/main/src/devices/src/virtio/balloon/device.rs

-> Hard-codes queue indexes but always seems to offer all features
  -> Offers VIRTIO_BALLOON_F_FREE_PAGE_HINT and VIRTIO_BALLOON_F_STATS_VQ
     even though it doesn't seem to implement them (device-triggered, so
     nothing to do probably?)
  -> Actually seems to implements VIRTIO_BALLOON_F_REPORTING

crossvm: 
https://github.com/google/crosvm/blob/main/devices/src/virtio/balloon.rs

-> Hard-codes queue numbers; does *not* offer/implement
    VIRTIO_BALLOON_F_STATS_VQ but does offer VIRTIO_BALLOON_F_STATS_VQ
    and VIRTIO_BALLOON_F_DEFLATE_ON_OOM.

-> Implements something that is not in the virtio-spec

const VIRTIO_BALLOON_F_WS_REPORTING: u32 = 8; // Working Set Reporting 
virtqueues

and

const WS_DATA_VQ: usize = 5;
const WS_OP_VQ: usize = 6;

IIUC, Linux inside cross-vm might actually be problematic? They would 
disagree on the virtqueue for free-page-reporting

Maybe I am missing something, it's a mess ...

-- 
Cheers,

David / dhildenb