Re: [virtio-comment] RE: [PATCH 1/2] virtio: introduce selective queue enabling

[Eugenio Perez Martin <eperezma@redhat.com>]

On Tue, Jun 6, 2023 at 9:25âPM Parav Pandit <parav@nvidia.com> wrote:
>
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Tuesday, June 6, 2023 3:20 PM
> > >
> > > Can you please highlight, current spec normative which indicates that
> > > all queues must be enabled before DRIVER_OK and more queues cannot be
> > > enabled after DRIVER_OK?
> >
> >
> > This one:
> > \drivernormative{\subsection}{Device Initialization}{General Initialization And
> > Device Operation / Device Initialization}
> >
> > it does not say "all queues" if you want to split hairs but I think assuming it's
> > safe to enable queues later is dangerous.
> >
> Yes, I have read it few times and every time I concluded that:
> it does not say that it must be done before DRIVER_OK and cannot be done after DRIVER_OK.
>

I also read it this way to be honest. From a POV, to allow it if a
device does not handle it could be considered a fix, rather than an
addition.

I'm assuming we need to modify the standard just because it was not
explicitly allowed. To explicitly allow it before DRIVER_OK was not
needed because it was already widespread.

It would be great for me if we agree the device must support it, but
maybe it is just too late for that.

Thanks!

> > E.g. vhost user backends tend to assume no new queues appear after
> > DRIVER_OK.
>
> This backend implementation can be improved without introducing new bits on the guest side.
> Q reset is already a second example that demonstrates that queues will be disabled/enabled after DRIVER_OK.
>
> And for PCI transport, PCI device and driver do not restrict queue_enable access after DRIVER_OK.
>
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