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Peter Anvin" , Sean Christopherson , Paolo Bonzini , Andy Lutomirski , Peter Zijlstra , Bjorn Helgaas , Marek Szyprowski , Andrew Morton , Catalin Marinas , Michael Ellerman , Mike Rapoport , Tom Lendacky , Ard Biesheuvel , Neeraj Upadhyay , Ashish Kalra , Stefano Garzarella , Melody Wang , Seongman Lee , Joerg Roedel , Nikunj A Dadhania , Michael Roth , Suravee Suthikulpanit , Andi Kleen , Kuppuswamy Sathyanarayanan , Tony Luck , David Woodhouse , Greg Kroah-Hartman , Denis Efremov , Geliang Tang , Piotr Gregor , "Michael S. Tsirkin" , Alex Williamson , Arnd Bergmann , Jesse Barnes , Jacob Pan , Yinghai Lu , Kevin Brodsky , Jonathan Cameron , "Aneesh Kumar K.V (Arm)" , Xu Yilun , Herbert Xu , Kim Phillips , Konrad Rzeszutek Wilk , Stefano Stabellini , Claire Chang , linux-coco@lists.linux.dev, iommu@lists.linux.dev Subject: Re: [PATCH kernel 4/9] dma/swiotlb: Stop forcing SWIOTLB for TDISP devices Message-ID: <20260303124306.GA1002356@nvidia.com> References: <20260225053806.3311234-1-aik@amd.com> <20260225053806.3311234-5-aik@amd.com> <699f238873ae7_1cc5100b6@dwillia2-mobl4.notmuch> <04b06a53-769c-44f1-a157-34591b9f8439@arm.com> <699f621daab02_2f4a1008f@dwillia2-mobl4.notmuch> <20260228002808.GO44359@ziepe.ca> <69a622e92cccf_6423c10092@dwillia2-mobl4.notmuch> <20260303001911.GA964116@ziepe.ca> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260303001911.GA964116@ziepe.ca> X-ClientProxiedBy: MN0P223CA0010.NAMP223.PROD.OUTLOOK.COM (2603:10b6:208:52b::16) To LV8PR12MB9620.namprd12.prod.outlook.com (2603:10b6:408:2a1::19) Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LV8PR12MB9620:EE_|DM6PR12MB4251:EE_ X-MS-Office365-Filtering-Correlation-Id: d08467d2-aeaa-43cf-aea2-08de79226b6d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014; 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I think Alexey has clarified this in the other thread, and probably AMD has some work to do here. The issue is AMD does not have seperate address spaces for shared/private like ARM does, instead it relies on a C bit in the *PTE* to determine shared/private. The S2 IOMMU page table *does* have the full mapping of all shared & private pages but the HW requires a matching C bit to permit access. If there is a S1 IOMMU then the IOPTEs of the VM can provide the C bit, so no problem. If there is no S1 then the sDTE of the hypervisor controls the C bit, and it sounds like currently AMD sets this globally which effectively locks TDISP RUN devices to *only* access private memory. I suspect AMD needs to use their vTOM feature to allow shared memory to remain available to TDISP RUN with a high/low address split. Alexey, did I capture this properly? Jason