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

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Message-ID: <25adc196-1070-4d55-b5bb-34dcdd6639d2@redhat.com>
Date: Mon, 7 Apr 2025 10:50:56 +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 10:44, Michael S. Tsirkin wrote:
> Wow great job digging through all these hypervisors!
> 
> On Mon, Apr 07, 2025 at 10:38:59AM +0200, David Hildenbrand wrote:
>> 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
> 
> 
> That's why things must be tied to negotiated features, not to offered
> ones.

cross-vm also has this weird comment:

"
const VIRTIO_BALLOON_F_PAGE_REPORTING: u32 = 5; // Page reporting virtqueue
                                                 // TODO(b/273973298): 
this should maybe be bit 6? to be changed later
"

Not sure why that should be bit 6, the spec says 5 ...

So maybe whatever they run inside the VM is also out of spec ... Really 
hard to tell.

-- 
Cheers,

David / dhildenb