RE: [PATCH v2 00/11] iommufd: Add nesting infrastructure

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Wednesday, June 21, 2023 8:05 PM
> 
> On Wed, Jun 21, 2023 at 06:02:21AM +0000, Tian, Kevin wrote:
> > > > My understanding of ARM SMMU is that from host p.o.v. the CD is the
> > > > S1 in the nested configuration. 'identity' is one configuration in the CD
> > > > then it's in the business of nesting.
> > >
> > > I think it is the same. A CD doesn't come into the picture until the
> > > guest installs a CD pointing STE. Until that time the S2 is being used
> > > as identity.
> > >
> > > It sounds like the same basic flow.
> >
> > After a CD table is installed in a STE I assume the SMMU still allows to
> > configure an individual CD entry as identity? e.g. while vSVA is enabled
> > on a device the guest can continue to keep CD#0 as identity when the
> > default domain of the device is set as 'passthrough'. In this case the
> > IOAS still needs to gain reserved regions even though S2 is not directly
> > attached from host p.o.v.
> 
> In any nesting configuration the hypervisor cannot directly restrict
> what IOVA the guest will use. The VM could make a normal nest and try
> to use unusable IOVA. Identity is not really special.

Sure. What I talked is the end result e.g. after the user explicitly requests
to load reserved regions into an IOAS.

> 
> The VMM should construct the guest memory map so that an identity
> iommu_domain can meet the reserved requirements - it needs to do this
> anyhow for the initial boot part. It shouuld try to forward the
> reserved regions to the guest via ACPI/etc.

Yes.

> 
> Being able to explicitly load reserved regions into an IOAS seems like
> a useful way to help construct this.
> 

And it's correct in concept because the IOAS is 'implicitly' accessed by
the device when the guest domain is identity in this case.