Re: [PATCH v1 2/2] vfio/pci: Emulate PASID/PRI capability for VFs

[Raj, Ashok]

Hi Alex

On Wed, Apr 08, 2020 at 10:19:40AM -0600, Alex Williamson wrote:
> On Tue, 7 Apr 2020 21:00:21 -0700
> "Raj, Ashok" <ashok.raj@xxxxxxxxx> wrote:
> 
> > Hi Alex
> > 
> > + Bjorn
> 
>  + Don
> 
> > FWIW I can't understand why PCI SIG went different ways with ATS, 
> > where its enumerated on PF and VF. But for PASID and PRI its only
> > in PF. 
> > 
> > I'm checking with our internal SIG reps to followup on that.
> > 
> > On Tue, Apr 07, 2020 at 09:58:01AM -0600, Alex Williamson wrote:
> > > > Is there vendor guarantee that hidden registers will locate at the
> > > > same offset between PF and VF config space?   
> > > 
> > > I'm not sure if the spec really precludes hidden registers, but the
> > > fact that these registers are explicitly outside of the capability
> > > chain implies they're only intended for device specific use, so I'd say
> > > there are no guarantees about anything related to these registers.  
> > 
> > As you had suggested in the other thread, we could consider
> > using the same offset as in PF, but even that's a better guess
> > still not reliable.
> > 
> > The other option is to maybe extend driver ops in the PF to expose
> > where the offsets should be. Sort of adding the quirk in the 
> > implementation. 
> > 
> > I'm not sure how prevalent are PASID and PRI in VF devices. If SIG is resisting 
> > making VF's first class citizen, we might ask them to add some verbiage
> > to suggest leave the same offsets as PF open to help emulation software.
> 
> Even if we know where to expose these capabilities on the VF, it's not
> clear to me how we can actually virtualize the capability itself.  If
> the spec defines, for example, an enable bit as r/w then software that
> interacts with that register expects the bit is settable.  There's no
> protocol for "try to set the bit and re-read it to see if the hardware
> accepted it".  Therefore a capability with a fixed enable bit
> representing the state of the PF, not settable by the VF, is
> disingenuous to the spec.
> 
> If what we're trying to do is expose that PASID and PRI are enabled on
> the PF to a VF driver, maybe duplicating the PF capabilities on the VF
> without the ability to control it is not the right approach.  Maybe we
> need new capabilities exposing these as slave features that cannot be
> controlled?  We could define our own vendor capability for this, but of

The other option is to say, VFIO would never emulate these
fake capablities. If exposing a VF with PASID/PRI is required
the PF driver would simply wrap it into a VDCM like model which we do
today for Scalable IOV devices. So PF handles all aspects of this
interface.

I also like the suggestion you propose, maybe an offset where these
capabilities are exposed to VF's. Maybe have an architected DEVCAPx
which exposes these RO capabilities. No control, and the 
offset should be preserved by the SIG, so VMM can have a safe place
to stash them.

> course we have both the where to put it in config space issue, as well
> as the issue of trying to push an ad-hoc standard.  vfio could expose
> these as device features rather than emulating capabilities, but that
> still leaves a big gap between vfio in the hypervisor and the driver in
> the guest VM.  That might still help push the responsibility and policy
> for how to expose it to the VM as a userspace problem though.
> 
> I agree though, I don't know why the SIG would preclude implementing
> per VF control of these features.  Thanks,

Even if we ask SIG for clarification, it might affect today's devices
So might not be useful to solve our current situation.

Ashok