RE: [PATCH V2 1/4] intel-iommu: don't warn guest errors when getting rid2pasid entry

[Tian, Kevin]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Wednesday, April 6, 2022 11:33 AM
> To: Tian, Kevin <kevin.tian@intel.com>
> Cc: Liu, Yi L <yi.l.liu@intel.com>; mst@redhat.com; peterx@redhat.com;
> yi.y.sun@linux.intel.com; qemu-devel@nongnu.org
> Subject: Re: [PATCH V2 1/4] intel-iommu: don't warn guest errors when
> getting rid2pasid entry
> 
> On Sat, Apr 2, 2022 at 3:34 PM Tian, Kevin <kevin.tian@intel.com> wrote:
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Wednesday, March 30, 2022 4:37 PM
> > > On Wed, Mar 30, 2022 at 4:16 PM Tian, Kevin <kevin.tian@intel.com>
> wrote:
> > > >
> > > > > From: Jason Wang <jasowang@redhat.com>
> > > > > Sent: Tuesday, March 29, 2022 12:52 PM
> > > > > >
> > > > > >>>
> > > > > >>> Currently the implementation of vtd_ce_get_rid2pasid_entry() is
> also
> > > > > >>> problematic. According to VT-d spec, RID2PASID field is effective
> only
> > > > > >>> when ecap.rps is true otherwise PASID#0 is used for RID2PASID. I
> > > didn't
> > > > > >>> see ecap.rps is set, neither is it checked in that function. It
> > > > > >>> works possibly
> > > > > >>> just because Linux currently programs 0 to RID2PASID...
> > > > > >>
> > > > > >> This seems to be another issue since the introduction of scalable
> mode.
> > > > > >
> > > > > > yes. this is not introduced in this series. The current scalable 
> > > > > > mode
> > > > > > vIOMMU support was following 3.0 spec, while RPS is added in 3.1.
> > > Needs
> > > > > > to be fixed.
> > > > >
> > > > >
> > > > > Interesting, so this is more complicated when dealing with migration
> > > > > compatibility. So what I suggest is probably something like:
> > > > >
> > > > > -device intel-iommu,version=$version
> > > > >
> > > > > Then we can maintain migration compatibility correctly. For 3.0 we
> can
> > > > > go without RPS and 3.1 and above we need to implement RPS.
> > > >
> > > > This is sensible. Probably a new version number is created only when
> > > > it breaks compatibility with an old version, i.e. not necessarily to 
> > > > follow
> > > > every release from VT-d spec. In this case we definitely need one from
> > > > 3.0 to 3.1+ given RID2PASID working on a 3.0 implementation will
> > > > trigger a reserved fault due to RPS not set on a 3.1 implementation.
> > >
> > > 3.0 should be fine, but I need to check whether there's another
> > > difference for PASID mode.
> > >
> > > It would be helpful if there's a chapter in the spec to describe the
> > > difference of behaviours.
> >
> > There is a section called 'Revision History' in the start of the VT-d spec.
> > It talks about changes in each revision, e.g.:
> > --
> >   June 2019, 3.1:
> >
> >   Added support for RID-PASID capability (RPS field in ECAP_REG).
> 
> Good to know that, does it mean, except for this revision history, all
> the other semantics keep backward compatibility across the version?

Yes and if you find anything not clarified properly I can help forward
to the spec owner.

Thanks
Kevin