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: 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.

> 
> Since most of the advanced features has not been implemented, we may
> probably start just from 3.4 (assuming it's the latest version). And all
> of the following effort should be done for 3.4 in order to productize it.
> 

Agree. btw in your understanding is intel-iommu in a production quality
now? If not, do we want to apply this version scheme only when it
reaches the production quality or also in the experimental phase?

Thanks
Kevin