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

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Message-ID: <20250407042058-mutt-send-email-mst@kernel.org>
Date: Mon, 7 Apr 2025 04:34:29 -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

On Mon, Apr 07, 2025 at 10:17:10AM +0200, 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.


OK so.

Device has VIRTIO_BALLOON_F_FREE_PAGE_HINT and VIRTIO_BALLOON_F_REPORTING
Driver only knows about VIRTIO_BALLOON_F_REPORTING so
it does not know what does VIRTIO_BALLOON_F_FREE_PAGE_HINT do.
How does it know which vq to use for reporting?
It will try to use the free page hint one.



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


The knowledge is supposed be limited to which feature bit to use.



> > 
> > 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.

I worry about how to we ensure that?
If old code is messed up people will just keep propagating that.
I would like to fix old code so that new code is correct.

> 
> As raised, it's not just qemu+linux, it's *at least* also cloud-hypervisor.
> 
> -- 
> Cheers,
> 
> David / dhildenb

There's a slippery slope here in that people will come to us
with buggy devices and ask to change the spec.




-- 
MST