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Wed, 19 Feb 2025 23:02:58 +0000 Date: Wed, 19 Feb 2025 15:02:55 -0800 From: Dan Williams To: James Bottomley , Dan Middleton , Dionna Amalie Glaze , Dave Hansen CC: "Xing, Cedric" , Dan Williams , "Kirill A. Shutemov" , Dave Hansen , Thomas Gleixner , Ingo Molnar , Borislav Petkov , , "H. Peter Anvin" , , , Kuppuswamy Sathyanarayanan Subject: Re: [PATCH 0/4] tsm: Unified Measurement Register ABI for TVMs Message-ID: <67b6631fc7054_2d2c294f1@dwillia2-xfh.jf.intel.com.notmuch> References: <01fc0997-a0e7-4086-b0aa-67b4a51b328a@intel.com> <12ed2ab1-e97d-4a20-8370-8c60cabffc77@intel.com> <650b6236-50d3-4932-b5bc-056fd29c93a7@intel.com> <8009a9fd-c7b1-48e3-af8b-8f481328c597@linux.intel.com> <6e773bfd1275d4f8c7df1ca65e22ec42eba12a8e.camel@HansenPartnership.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <6e773bfd1275d4f8c7df1ca65e22ec42eba12a8e.camel@HansenPartnership.com> X-ClientProxiedBy: MW4PR04CA0188.namprd04.prod.outlook.com (2603:10b6:303:86::13) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR11MB8107:EE_|CO1PR11MB4884:EE_ X-MS-Office365-Filtering-Correlation-Id: 0043fce9-d1fa-481a-6415-08dd51398d7f X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016; 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The difference between RTMRs and the > subset of TPM functionality that also provides it is non-existent. > It's like a distinction without a difference. If the SVSM authors had > written for a pure RTMR implementation (just usng a CRB API) would that > have made a difference? That is an interesting hypothetical, "would things be different if the authors, that were forced by SEV-SNP architectural necessity to push runtime measurement functionality into an SVSM layer exclusively, had considered that some architectures would include runtime measurement functionality in the CVM technology directly?". I do not think it helps because that presupposes that vTPM for these other architectures already exists. When I look at the proposed solutions for TDX-vTPM based on service VMs and other complications brought on by architectural differences between TDX and SEV-SNP, and compare that to a potential vTPM that wraps RTMR I see a net reduction in complexity. In other words, a path to a cross-architecture RTMR-backed vTPM without requiring SVSM and approximation of the VMPL mechanism. It follows that userspace, not the kernel, needs to wrap architectural RTMR differences to build that vTPM. So to me the question is less "RTMR vs TPM" and more about vTPM implementation choice where RTMR-backed and SVSM-based vTPM solutions are not mutually exclusive. For the kernel this mean leaking architecture specific RTMR details into its ABI and punting the vTPM interface problem to userspace. It also means that software, in some cases, could forgo vTPM and use raw RTMR. However, I do not think that ultimately fragments the ecosystem. TPM momentum and portability concerns limits how far raw RTMR usage will extend, but in the meantime for use cases that "don't want to have to depend on the vTPM", like the one Dionna mentioned, are enabled. If those use case ultimately melt away and transition to vTPM (whether RTMR backed or SVSM backed), great. If those use cases persist then that is also a useful system evolution signal from the ecosystem.