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

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Message-ID: <16455ceb-ae9c-41ff-b6cf-e3a61645fbac@amd.com>
Date: Thu, 9 Oct 2025 14:46:03 +0530
From: Sairaj Kodilkar <sarunkod@....com>
To: Jason Gunthorpe <jgg@...dia.com>, "Suthikulpanit, Suravee"
	<suravee.suthikulpanit@....com>
CC: <nicolinc@...dia.com>, <linux-kernel@...r.kernel.org>,
	<robin.murphy@....com>, <will@...nel.org>, <joro@...tes.org>,
	<kevin.tian@...el.com>, <jsnitsel@...hat.com>, <vasant.hegde@....com>,
	<iommu@...ts.linux.dev>, <santosh.shukla@....com>,
	<sairaj.arunkodilkar@....com>, <jon.grimm@....com>,
	<prashanthpra@...gle.com>, <wvw@...gle.com>, <wnliu@...gle.com>,
	<gptran@...gle.com>, <kpsingh@...gle.com>, <joao.m.martins@...cle.com>,
	<alejandro.j.jimenez@...cle.com>
Subject: Re: [PATCH v2 10/12] iommu/amd: Add support for nested domain
 allocation



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