Re: [PATCH v2 10/12] iommu/amd: Add support for nested domain allocation

[Jason Gunthorpe]

On Tue, Oct 07, 2025 at 03:36:58PM -0500, Suthikulpanit, Suravee wrote:
> The gDTE[DomainID] field contains guest Domain ID (gDomID). The host IOMMU
> driver uses the gDomId and guest ID (gid) to index the Domain ID mapping
> table, and store the host Domain ID (hDomID) in the table entry. This data
> structure is required by hw to translation gDomID->hDomID to virtualize
> guest invalidation command. This will be part of the upcoming series to
> enable hw-vIOMMU.

Sure, this translation is part of viommu

> This ndom->id is the hDomID, which is currently allocated per-device to
> avoid TLB aliasing i.e. A guest w/ multiple pass-through devices w/ the same
> hDomID (same stage 2 table) and different stage-1 tables with same PASID.
> IOMMU would use the same TLB tag, which results in TLB aliasing issue.
> Therefore, we workaround the issue by allocating per-device hDomID for
> nested domain.

But this is what I mean here, the gDomId should be 1:1 with the hDomId
and here you are making it 1:N.

It has to be like this or you cannot manage invalidation.

Given this series is not really functional it is OK to leave a little
hack I guess, but it is worth noting how it is supposed to work.

It also probably means we should see the viommu series pretty quickly
with a goal to merge them all together in one cycle.

Jason