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Tue, 21 Oct 2025 16:36:23 +0000 Message-ID: <4841c40b-47b0-4b1b-985f-d1a16cbe81fa@intel.com> Date: Tue, 21 Oct 2025 19:36:19 +0300 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/4] KVM: TDX: Synchronize user-return MSRs immediately after VP.ENTER To: Sean Christopherson CC: Rick P Edgecombe , "pbonzini@redhat.com" , "kas@kernel.org" , Xiaoyao Li , "kvm@vger.kernel.org" , "linux-coco@lists.linux.dev" , "linux-kernel@vger.kernel.org" , Yan Y Zhao , "x86@kernel.org" , wenlong hou References: <20251016222816.141523-1-seanjc@google.com> <20251016222816.141523-2-seanjc@google.com> Content-Language: en-US From: Adrian Hunter Organization: Intel Finland Oy, Registered Address: c/o Alberga Business Park, 6 krs, Bertel Jungin Aukio 5, 02600 Espoo, Business Identity Code: 0357606 - 4, Domiciled in Helsinki In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: DU6P191CA0021.EURP191.PROD.OUTLOOK.COM (2603:10a6:10:540::26) To IA1PR11MB7198.namprd11.prod.outlook.com (2603:10b6:208:419::15) 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: IA1PR11MB7198:EE_|PH3PPF91320C45A:EE_ X-MS-Office365-Filtering-Correlation-Id: 5bad4dc9-fe35-4de3-568d-08de10bff8ab 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|366016|376014|1800799024; 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Mark those MSRs as needing an >>>> + * update to synchronize the "current" value in KVM's cache with the >>>> + * value in hardware (loaded by the TDX-Module). >>>> + */ >>> >>> I think we should be synchronizing only after a successful VP.ENTER with a real >>> TD exit, but today instead we synchronize after any attempt to VP.ENTER. > > Well this is all completely @#($*#. Looking at the TDX-Module source, if the > TDX-Module synthesizes an exit, e.g. because it suspects a zero-step attack, it > will signal a "normal" exit but not "restore" VMM state. > >> If the MSR's do not get clobbered, does it matter whether or not they get >> restored. > > It matters because KVM needs to know the actual value in hardware. If KVM thinks > an MSR is 'X', but it's actually 'Y', then KVM could fail to write the correct > value into hardware when returning to userspace and/or when running a different > vCPU. I don't quite follow: if an MSR does not get clobbered, where does the incorrect value come from?