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Mon, 2 Mar 2026 23:53:14 +0000 From: Date: Mon, 2 Mar 2026 15:53:13 -0800 To: Jason Gunthorpe , CC: Robin Murphy , Alexey Kardashevskiy , , , , , Thomas Gleixner , "Ingo Molnar" , Borislav Petkov , Dave Hansen , "H. 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 , , Message-ID: <69a622e92cccf_6423c10092@dwillia2-mobl4.notmuch> In-Reply-To: <20260228002808.GO44359@ziepe.ca> 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> Subject: Re: [PATCH kernel 4/9] dma/swiotlb: Stop forcing SWIOTLB for TDISP devices Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR03CA0065.namprd03.prod.outlook.com (2603:10b6:a03:331::10) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR11MB8107:EE_|PH7PR11MB5941:EE_ X-MS-Office365-Filtering-Correlation-Id: 87bd3eb9-23e4-45ba-aa3e-08de78b6de81 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; 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So the expectation to start is that the device loses access to > > its original shared IOMMU mappings when converted to private operation. > > Yes, the underlying translation changes, but no, it doesn't loose DMA > access to any shared pages, it just goes through the T=1 IOMMU now. Yes, what I meant to say is that Linux may need to be prepared for implementations that do not copy over the shared mappings. At least for early staging / minimum viable implementation for first merge. > The T=1 IOMMU will still have them mapped on all three platforms > AFAIK. Oh, I thought SEV-TIO had trouble with this, if this is indeed the case, great, ignore my first comment. > On TDX/CCA the CPU and IOMMU S2 tables are identical, so of > course the shared pages are mapped. On AMD there is only one IOMMU so > the page must also be mapped or non-TDISP is broken. > > When this TDISP awareness is put in the DMA API it needs to be done in > a way that allows DMA_ATTR_CC_DECRYPTED to keep working for TDISP > devices. > > This is important because we are expecting these sorts of things to > work as part of integrating non-TDISP RDMA devices into CC guests. We > can't loose access to the shared pages that are shared with the > non-TDISP devices... Ok, I need to go look at this DMA_ATTR_CC_DECRYPTED proposal... I have a v2 of a TEE I/O set going out shortly and sounds like it will need a rethink for this attribute proposal for v3. I think it still helps to have combo sets at this stage so the whole lifecycle is visible in one set, but it is nearly at the point of being too big a set to consider in one sitting. > > So on ARM where shared addresses are high, it is future work to figure > > out how an accepted device might also access shared mappings outside the > > device's dma_mask. > > ARM has a "solution" right now. The location of the high bit is > controlled by the VMM and the VMM cannot create a CC VM where the IPA > space exceeds the dma_mask of any assigned device. > > Thus the VMM must limit the total available DRAM to fit within the HW > restrictions. > > Hopefully TDX can do the same. TDX does not have the same problem, but the ARM "solution" seems reasonable for now.