> 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.
