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

[Jason Gunthorpe]

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.

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.

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

Jason