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

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Message-ID: <20250407044246-mutt-send-email-mst@kernel.org>
Date: Mon, 7 Apr 2025 04:44:32 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: David Hildenbrand <david@...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

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.


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

Indeed. Let's fix the implementations while keeping work arounds for
existing ones.

-- 
MST