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Fri, 14 Aug 2026 07:37:25 +0000 Date: Fri, 14 Aug 2026 00:37:23 -0700 From: Peter Fang To: Artem Bityutskiy CC: "Edgecombe, Rick P" , "seanjc@google.com" , "linux-coco@lists.linux.dev" , "x86@kernel.org" , "dave.hansen@linux.intel.com" , "kas@kernel.org" , "binbin.wu@linux.intel.com" , "Li, Xiaoyao" , "linux-kernel@vger.kernel.org" , "sathyanarayanan.kuppuswamy@linux.intel.com" , "bp@alien8.de" , "kvm@vger.kernel.org" , "tglx@kernel.org" , "hpa@zytor.com" , "mingo@redhat.com" Subject: Re: [PATCH v3 0/4] tdx-guest: Make Quote buffer size dynamic Message-ID: <20260814073723.GF1013044@pedri> References: <86d532f33816cd4fa3e29c40079a6003abf89324.camel@intel.com> <20260812223707.GD1013044@pedri> <6191a69559e58e04c8e3f1efa776e9639796af3d.camel@intel.com> <98dcc8a12f117745a1cb9981dc8f0f1548e9c96f.camel@gmail.com> <4ad451969522eecb445289ca74a88ee73d51336d.camel@intel.com> <8b432c8731167e97432b2e917d0b1c855fedf47c.camel@gmail.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <8b432c8731167e97432b2e917d0b1c855fedf47c.camel@gmail.com> X-ClientProxiedBy: SJ0P220CA0010.NAMP220.PROD.OUTLOOK.COM (2603:10b6:a03:41b::14) To SJ0PR11MB5645.namprd11.prod.outlook.com (2603:10b6:a03:3b9::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ0PR11MB5645:EE_|SA1PR11MB8374:EE_ X-MS-Office365-Filtering-Correlation-Id: a47591b3-0c15-4bfe-c3fa-08def9d6e247 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|23010399003|1800799024|56012099006|10067099003|22082099003|18002099003|4143699003|11063799006; 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Why freezing TD report size > > > > > > Linux supports 1024-byte TD reports via the `TDX_CMD_GET_REPORT0` > > > ioctl. It is already full, no more TD evidence fits, and changing TD > > > report size would require a new ioctl. > > > > > > Also, as I understand it, based on TDX feature requests from customers, > > > there may be a need to increase TD report size more often and more > > > significantly than one would expect. > > > > > > Therefore, for DICE-based attestation the TDX module adds new TD > > > evidence in the quote instead of expanding the TD report. > > > > > > Is this the cleanest approach? Maybe not.  > > > > > > > I was thinking the cleanest, time-travel facilitated, approach would be to have > > the "report" really just be a replay protection thing and not include any TD > > details. Then the quote could add everything it needed in the final format > > location. So no re-verifying and shuffling things between formats. But since > > that didn't work for SGX based attestation, the report has extra stuff due to > > legacy. But from Linux's POV, going forward we can consider the report as really > > an oversized replay protection blob and forget about the TD details in it. Isn't > > it a pretty clean separation of concerns? And for Linux (guest), the benefit > > shows up in not having to increase TD report size since it has really only one > > job. > > Replay attack protection is a binding value between the remote party > and the final quote. It must sit in final quote. It only needs to exist > in the TD report because of the 2-flow design. > > But why DICE-based attestation uses 2-flow design? Could TD run a > TDCALL[TDG.GET.QUOTE2] or something directly. No concept of TD report > would be needed. That would be my current vision of "cleanest". Yeah, Rick talked about this a bit in an earlier thread [1]. [1] https://lore.kernel.org/kvm/23a9173f6e278ca7dfedce3374626c6ea3e1b47a.camel@intel.com/ > > But the approach that was taken is to minimize software changes. Within > that tradeoff, keeping the 2-step flow, preserving the TD report format > and size absolutely intact, and adding the extra information in the > quote is arguably a clean practical path. > > Is this the cleanest in some absolute sense? No. > Is it acceptable? I would say yes. >