Re: [virtio-comment] RE: [PATCH v5 2/7] Introduce admin command set

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 5/17/2022 2:48 PM, Michael S. Tsirkin wrote:
> On Mon, May 16, 2022 at 09:08:34PM +0000, Parav Pandit wrote:
> > Hi Michael,
> >
> > > From: Michael S. Tsirkin <mst@redhat.com>
> > > Sent: Sunday, May 15, 2022 11:24 AM
> > [..]
> >
> > > > +\subsection{VIRTIO ADMIN DEVICE CAPS ACCEPT
> > > command}\label{sec:Basic
> > > > +Facilities of a Virtio Device / Admin command set / VIRTIO ADMIN
> > > > +DEVICE CAPS ACCEPT command}
> > > > +
> > > > +The VIRTIO_ADMIN_DEVICE_CAPS_ACCEPT command is used by the
> > > driver to acknowledge those admin capabilities it understands and wishes to
> > > use.
> > >
> > >
> > > ok so we have a protocol here, kind of like feature negotiation. Please write
> > > its description.
> > > e.g. is it ok to change accepted caps? when? can device change its caps etc
> > > etc etc.
> > >
> > > Avoiding this kind of spec work is exactly why me and jason keep telling you
> > > to consider just using features instead. Add a 64 bit admin features field to
> > > the PCI transport and be done with it. CCW and MMIO already have feature
> > > selector so it's trivial to add feature bits.
> > As we begin to scale with the device, adding more and more registers like this demands more on-device real estate to comply to the PCI standards.
> >
> > And therefore, things are queried/accessed rare or occasionally, are better accessed via a queue interface.
> >
> > One can argue that admin VQ is proposed only for the mgmt. functions so having this cfg register for PF is enough.
> >
> > However, AQ may find some usage in the VF/SF themselves down the road.
> > Hence, keeping the cap exchange transport this way is more optimal.
> >
> > Max has called out this AQ rationale in 4 or 5 points in the cover letter.
> Hmm. It's kind of a generic claim though. We never put devices on a diet
> trying to conserve registers. There is cost associated with this dance
> and that is driver boot time.
>
> I also don't really understand how you can claim you need to save
> memory like this and at the same time blindly add a more or less
> "just in case" misc config in the config space.
> So, not pretty.
>
> And as I said, you will need much more spec work to reach the level
> to which features are specified - and note we are not yet happy with
> how features are specified either! So it's a moving target.
>
> Maybe put this in features for now, and leave the whole
> capability thing for another day?

If you're not happy on the feature negotiation in the spec so please 
don't insist we use this mechanism for admin capabilities.

I don't want to postpone essential and basic definition to another day.

We need to agree on it today (even yesterday).

>