Re: [PATCH v7 1/3] iommufd: Add data structure for Intel VT-d stage-1 cache invalidation

[Nicolin Chen]

On Fri, Dec 15, 2023 at 01:50:07AM +0000, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > Sent: Thursday, December 14, 2023 7:27 PM
> >
> > On 2023/11/17 21:18, Yi Liu wrote:> This adds the data structure for
> > flushing iotlb for the nested domain
> >
> > +struct iommu_hwpt_vtd_s1_invalidate {
> > +     __aligned_u64 addr;
> > +     __aligned_u64 npages;
> > +     __u32 flags;
> > +     __u32 __reserved;
> > +     __u32 error;
> > +     __u32 dev_id;
> > +};
> >
> > dev_id is used to report the failed device, userspace should be able to map
> > it to a vRID, and inject it to VM as part of ITE/ICE error.
> >
> > However, I got a problem when trying to get dev_id in cache invalidation
> > path, since this is filled in intel iommu driver. Seems like there is no
> > good way for it. I've below alternatives to move forward, wish you have
> > a look.

> >
> > - Reuse Nicolin's vRID->pRID mapping. If thevRID->pRID mapping is
> > maintained, then intel iommu can report a vRID back to user. But intel
> > iommu driver does not have viommu context, no place to hold the vRID-
> > >pRID
> > mapping. TBH. It may require other reasons to introduce it other than the
> > error reporting need. Anyhow, this requires more thinking and also has
> > dependency even if it is doable in intel side.
> 
> this sounds like a cleaner way to inject knowledge which iommu driver
> requires to find out the user tag. but yes it's a bit weird to introduce
> viommu awareness in intel iommu driver when there is no such thing
> in real hardware.

I think a viommu is defined more like a software object representing
the virtual IOMMU in a VM. Since VT-d has a vIOMMU in a nesting case,
there could be an object for it too?

> and for this error reporting case what we actually require is the
> reverse map i.e. pRID->vRID. Not sure whether we can leverage the
> same RID mapping uAPI as for ARM/AMD but ignore viommu_id
> and then store vRID under device_domain_info. a bit tricky on
> life cycle management and also incompatible with SIOV...

One thing that I am not very clear here: since both vRID and dev_id
are given by the VMM, shouldn't it already know the mapping if the
point is to translate (pRID->)dev_id->vRID?

Thanks
Nicolin