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

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

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@intel.com> 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.

Would it be OK to implement a lock down mechanism for the PF PASID
capability, preventing changes to the PF cap when the VF is in use by
VFIO?  The emulation would still break the spec: since the PF cap would
always be enabled the VF configuration bits would have no effect, but it
seems preferable to having the Enable bit not enable anything.

> 
> 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
> 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.

Userspace might have more difficulty working around the issues mentioned
in this thread. They would still need a guarantee that the PF PASID
configuration doesn't change at runtime, and they wouldn't have the
ability to talk to a vendor driver to figure out where to place the fake
PASID capability.

Thanks,
Jean

> 
> I agree though, I don't know why the SIG would preclude implementing
> per VF control of these features.  Thanks,
> 
> Alex
> 
> > > FWIW, vfio started out being more strict about restricting config space
> > > access to defined capabilities, until...
> > > 
> > > commit a7d1ea1c11b33bda2691f3294b4d735ed635535a
> > > Author: Alex Williamson <alex.williamson@redhat.com>
> > > Date:   Mon Apr 1 09:04:12 2013 -0600
> > >   
> > 
> > Cheers,
> > Ashok
> > 
>