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

[Sairaj Kodilkar <sarunkod@amd.com>]

On 10/9/2025 11:52 AM, Sairaj Kodilkar wrote:
>
>
> On 10/8/2025 5:09 AM, Jason Gunthorpe wrote:
>> 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.
> Hi Jason,
> The guest will only see V2 page table when we are using hardware vIOMMU.
> Since IOMMU driver allocates per device domains when it is using V2 
> page table,
> the mappings are still N:N and invalidations will work similar to V2 
> page table mode in
> host.
Small correction --> "mappings are still 1:1" because guest IOMMU driver 
allocates per device domain.

Thanks
sairaj