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

[Jason Wang]

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.

>
> >
> > 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?

Red Hat supports vIOMMU for the guest DPDK path now.

For scalable-mode we need to see some use cases then we can evaluate.
virtio SVA could be a possible use case, but it requires more work e.g
PRS queue.

> If not, do we want to apply this version scheme only when it
> reaches the production quality or also in the experimental phase?

Yes. E.g if we think scalable mode is mature, we can enable 3.0.

Thanks

>
> Thanks
> Kevin