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

[Parav Pandit <parav@nvidia.com>]

> From: virtio-dev@lists.oasis-open.org <virtio-dev@lists.oasis-open.org> On
> Behalf Of Michael S. Tsirkin
> Sent: Thursday, June 22, 2023 1:38 PM
> 
> On Thu, Jun 22, 2023 at 05:15:50PM +0000, Parav Pandit wrote:
> >
> >
> > > From: Michael S. Tsirkin <mst@redhat.com>
> > > Sent: Thursday, June 22, 2023 1:11 PM
> >
> > > > 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.
> > >
> > > Actually it's RO so you *can* read it without any issues:
> >
> > It is RO but not same across all devices.
> 
> If you provision VFs differently. I got it.
> 
> > > - block guest access to status
> > > - check DRIVER.
> > > If set:
> > > 	- read features, config
> > > If not set:
> > > 	- read features, config
> > > 	- reset
> > >
> > This is what I explained.
> > It is more messy if you equate to GET command has mess.
> 
> At least it works.
>
And new GET command also works when CVQ is trapped.
And also works when AQ is querying capabilities of a member device using WIP cmd.
 
> > > I am not saying it is elegant but then all of vdpa pile of hacks is not elegant.
> > >
> > I don't want to comment for vdpa. But it is not part of the spec...
> 
> Neither is QEMU.  It's one of spec implementations. Yes, we care about not
> adding blockers for features that, superficially, might make sense for them.
> 
> > > And I am all for building something better but we didn't build it yet.
> >
> > The proposal for 1.4 is literally very simple as below.
> > 1. All existing fields of cfg space stays in cfg space 2. Any new
> > capabilities to be queried, query using a vq (aq, cfgvq, whatevervq).
> > 3. Optionally existing fields can be queries over vq of #2 Once this
> > arrive, no need for new GET commands.
> > Till that time, don't keep infinitely grow the cfg space.
> > Any next addition to cfg space, should work on defining the cfgvq.
> 
> Simple, but short sighted. I know you guys don't support your hardware for 10-
> 20 years but for software people do.
> And so "All existing fields of cfg space stays in cfg space" is a bad idea simply
> because this does not allow removing things from config space not in 10 not in
> 20 years not ever.
>
#1 is for backward compat for existing drivers.
 You missed about #3. Existing cfg space fields can be queries using the cfgvq too.

> 
> Instead we need to allow two ways to access config space.  Teach drivers about
> both, actually mandate supporting both.  And then devices will make their own
> cost/benefit decision about which features they want to support in MMIO.

If both method is mandated, I don't see benefit at all of two methods.
VQ is generic part of the spec for slow and fast operation, so it is not at all a cost for config reading.