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

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Message-ID: <20250407045456-mutt-send-email-mst@kernel.org>
Date: Mon, 7 Apr 2025 04:58:18 -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 10:54:00AM +0200, David Hildenbrand wrote:
> On 07.04.25 10:49, Michael S. Tsirkin wrote:
> > On Mon, Apr 07, 2025 at 10:44:21AM +0200, David Hildenbrand wrote:
> > > > 
> > > > 
> > > > 
> > > > > 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.
> > > 
> > > I think we also have to know which virtqueue bits can be used, right?
> > > 
> > 
> > what are virtqueue bits? vq number?
> 
> Yes, sorry.

I got confused myself, it's vq index actually now, we made the spec
consistent with that terminology. used to be number/index
interchangeably.

> Assume cross-vm as an example. It would make use of virtqueue indexes 5+6
> with their VIRTIO_BALLOON_F_WS_REPORTING.


crossvm guys really should have reserved the feature bit even if they
did not bother specifying it. Let's reserve it now at least?


> So whatever feature another device implements couldn't use this feature bit
> or these virtqueue indexes.
> 
> (as long the other device never intends to implement
> VIRTIO_BALLOON_F_WS_REPORTING, the virtqueue indexes could be reused. But
> the spec will also be a mess, because virtqueue indexes could also have
> duplicate meanings ... ugh)

what do they do with vq indices btw?



> -- 
> Cheers,
> 
> David / dhildenb