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Thu, 5 Nov 2026 10:40:27 -0800 Received: from nvidia.com (10.127.8.13) by mail.nvidia.com (10.129.68.7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.56 via Frontend Transport; Thu, 5 Nov 2026 10:40:19 -0800 Date: Thu, 5 Nov 2026 10:40:16 -0800 From: Nicolin Chen To: Jason Gunthorpe CC: Aneesh Kumar K.V , , "Alex Williamson" , Alexey Kardashevskiy , "Bjorn Helgaas" , Catalin Marinas , Jacob Pan , Joerg Roedel , Jonathan Cameron , Jonathan Hunter , Kevin Tian , Krishna Reddy , "Lukas Wunner" , Robin Murphy , Samuel Ortiz , Shameer Kolothum , Steven Price , Suravee Suthikulpanit , Suzuki K Poulose , Thierry Reding , "Vasant Hegde" , Will Deacon , Xu Yilun , , , , , , , Jonathan Corbet , Shuah Khan , Randy Dunlap Subject: Re: [PATCH v8 13/15] iommufd/viommu: Allow vIOMMUs without a parent HWPT Message-ID: References: <20261104161632.3547423-1-aneesh.kumar@kernel.org> <20261104161632.3547423-14-aneesh.kumar@kernel.org> <20261104193139.GB3889730@ziepe.ca> <20261105182106.GD663069@ziepe.ca> X-Mailing-List: linux-tegra@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: FFmEDHaAvae7GJATKzAttTK72XtjmrPnh2uZ6WImvfroImHRNZmR6Wl5iQ8Q2KUVstodoPm4gGlsH0psXoc22Ip3gfcxpYZdNP9b19ev+WOmULcF86g6YJeaDvUt3CuU+Fsfn4u0ShfMvFwvFuLOu1Qh+i+29qW+7kRQtjJC6pshxQmFjTWt///CYortlFVhQC4vE0HawFBMGM6suqUnp54o8yNeaOLipJbasIHP+RP4Pvm4M84gBKQW8pPHLgBuhDF1T7YwVuJMEscJ1nCIecbLjT3SLC7IBI9MtBbu1g0oUAFpzsnkF8y3xDQfvMysKdAzK0KVdfFOs//wLA1xRoUSzxB4bCBpQA4tgZguG68Auy8txjc9BeTWIIEI33oOH2UugCbyCfkBxoOkMYgr9ZXtRCGzVkTsU1M2DMt3BCxXc9Fxh28bZ3PhGZ5Xd4MY X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 05 Nov 2026 18:40:53.1246 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: a4d941dc-f517-4cc0-48e3-08df3b6c523f X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.160];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: DS8PEPF000000E3.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH8PR12MB6745 On Thu, Nov 05, 2026 at 02:21:06PM -0400, Jason Gunthorpe wrote: > On Thu, Nov 05, 2026 at 09:44:42AM -0800, Nicolin Chen wrote: > > On Thu, Nov 05, 2026 at 10:58:06AM +0530, Aneesh Kumar K.V wrote: > > > Jason Gunthorpe writes: > > > > On Wed, Nov 04, 2026 at 11:03:41AM -0800, Nicolin Chen wrote: > > > >> By dropping the nesting parent from VIOMMU, it just simplifies the > > > >> attachment to an S1 paging hwpt, right? > > > > > > > > Yes. > > > > > > > > The VMM must always maintain a S1 paging attachment that is the > > > > identity map of the VM on all devices, same action as when there is no > > > > viommu. > > > > > > > > > > I am currently creating an S2 paging attachment. This results in a > > > configuration with S1 disabled and S2 enabled, which is used by all > > > untrusted devices. > > > > For validation, that would work. Yet "no viommu" should would be S1 > > paging, i.e. hwpt allocated without IOMMU_HWPT_ALLOC_NEST_PARENT. > > It doesn't matter, consistently using a single S2 for everything is > probably better.. Hmm, I am confused: if we use a nesting parent, why do we need to allow a viommu without a parent HWPT here? Nicolin