Re: [PATCH v6 0/6] iommufd: Add nesting infrastructure (part 2/2)

[Nicolin Chen <nicolinc@nvidia.com>]

On Mon, Dec 11, 2023 at 09:20:41AM -0400, Jason Gunthorpe wrote:
> On Mon, Dec 11, 2023 at 08:35:09PM +0800, Yi Liu wrote:
> > > So.. In short.. Invalidation is a PITA. The idea is the same but
> > > annoying little details interfere with actually having a compltely
> > > common API here. IMHO the uAPI in this series is fine. It will support
> > > Intel invalidation and non-ATC invalidation on AMD/ARM. It should be
> > > setup to allow that the target domain object can be any HWPT.
> > 
> > This HWPT is still nested domain. Is it? But it can represent a guest I/O
> > page table (VT-d), guest CD table (ARM), guest CR3 Table (AMD, it seems to
> > be a set of guest CR3 table pointers). May ARM and AMD guys keep me honest
> > here.
> 
> I was thinking ARM would not want to use a nested domain because
> really the invalidation is global to the entire nesting parent.
> 
> But, there is an issue with that - the nesting parent could be
> attached to multiple iommu instances but we only want to invalidate a
> single instance. 

I am still not sure about attaching an S2 domain to multiple
SMMUs. An S2 domain is created per SMMU, and we have such a
rejection in arm_smmu_attach_dev():
	} else if (smmu_domain->smmu != smmu)
		ret = -EINVAL;

I understand that it would be probably ideal to share the S2
iopt among the SMMUs. But in the driver the objects (domain)
holding a shared S2 iopt must be different to allocate their
own VMIDs, right?

Thanks
Nicolin