Re: [virtio-comment] [PATCH v1 1/8] admin: Add theory of operation for device migration

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 19, 2023 at 08:58:10AM +0000, Parav Pandit wrote:
> It cannot be in the PCI 4K config space for sure.
> It must reside in the virtio config space.

Why? My concern with virtio config space would be
that it's not orthogonal to other things in the
config space. E.g. you need to look at feature bits
to discover presence. How does this work while you are
documenting that device can undergo reset at any time?
With a capability you can discover it without poking at features.

> I am sure that this is used for passthrough mode of #1.
> So, can you please confirm to write this up for mode #2 only?

To me it sounds like a generally useful capability that could be
used as basis e.g. for admin command transport.

> > > A variation of that for the member device, there is owner device, hence
> > admin command on the AQ can be used.
> > >
> > > If we can converge on common virtio interface between #1 and #2, great.
> > > If we cannot be due to technical issues, we shouldn't step on each other's
> > toes, instead build the two interfaces for two different use cases overcoming its
> > own technical challenges.
> > >
> > > And when in future, someone want to implement different kind of bisections,
> > they can propose the extensions.
> > 
> > Not good at all, this means the interface is very narrow.
> > Your "propose an extension" just doesn't work practically.
> > It takes years for things to be widely deployed in the field, by the time they are
> > there are more use-cases.
> 
> We usually see it getting deployed in < 1 year time with new spec advancement pace for many features.
> Building something for unreasonable amount of time without use case results in missing the immediate deployments that happens in 2024 to 2027 of 1.4 spec time frame.
> 
> > We need something universal and admin commands were supposed to be just
> > this.
> I don't see a universal solution for all problems for above #1 and #2.
> 
> Solving above #2 will cover large part of deployments that users are doing.

OK. But additionally, if an interface can cover a couple of use-cases
we can be reasonably sure it's going to cover more going forward.