[virtio-dev] Re: [virtio-comment] Re: [PATCH v18] virtio-net: support inner header hash

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

On Thu, Jun 22, 2023 at 04:42:40PM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Thursday, June 22, 2023 12:28 PM
> 
> > > > Not sure what's the implication?
> > > Implication is device needs to store this in always available on-chip memory
> > which is not good.
> > 
> > Oh by devices you mean VFs. Now I get your motivation, at least. Thanks.
> >
> > > >
> > > > > > For example, for migration driver might want to validate that two
> > > > > > devices have same capability. doing it without dma is nicer.
> > > > > >
> > > > > A migration driver for real world scenario, will almost have to use the dma
> > for
> > > > amount of data it needs to exchange.
> > > >
> > > > Not migration itself, provisioning.
> > > >
> > > Provisioning driver usually do not attach to the member device directly.
> > > This requires device reset, followed by reaching _DRIVER stage, querying
> > features etc and config area.
> > > And unbinding it and second reset by member driver. Ugh.
> > > Provisioning driver also needs to get the state or capabilities even when
> > member driver is already attached.
> > > So config space is not much a gain either.
> > >
> > 
> > Absolutely, that's why we have admin commands.  I was hoping for an
> > admin command that basically gets/sets RO fields of the config space.
> >
> Admin command as I recall are not accessible directly by the member driver to the member device.
> So a cmdq or cfgq is needed.

Possible, sure. Or we actually discussed a self group. I took it away until it had a user.


> > > Instead of decision point being RO vs RW,
> > > any new fields via cmdvq and existing fields stays in cfg space, give predictable
> > behavior to size the member devices in the system.
> > > Once the cmdvq is available, we can get rid of GET command used in this
> > version for new future features.
> > > Till that arrives, GET command is the efficient way.
> > 
> > I understand.  I just don't much like these patchwork solutions though.
> > And I don't like that we will pay by not having a single conherent
> > way to provision and query capabilities through config space,
> > instead just for this thing we will have a special thing.
> >
> The single way for every device to query their capabilities is via a cfgvq for all new fields without extending the existing config space.
> (and optionally old fields).

Or adminq with self group. I like this somewhat better because we need
exactly same query from owner.

> > Why don't we focus on a work on a full solution? Just don't implement
> > this thing in your devices meanwhile until we do.
> > 
> Then Heng needs to wait for cfgvq to be defined to be implemented first.
> Doesn't look reasonable to me.

And *everything* has to wait. No, not reasonable. We somehow managed
to release several spec versions and things did not ground to
a halt without cfgvq. Don't see a reason to do it right now,
what's special about now? I feel we should add to config space
and then solve it all.

> Current GET is coherent with the new commands defined such as notification coalescing.
> 
> As community, we should work on defining the cfgvq, till that time have the optimal way to get the config, i.e. using the cvq.

cvq doesn't really work for capabilities though.

-- 
MST


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