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Dadhania" , linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org, mingo@redhat.com, tglx@linutronix.de, dave.hansen@linux.intel.com, pgonda@google.com, seanjc@google.com, pbonzini@redhat.com References: <20241203090045.942078-1-nikunj@amd.com> <20241203090045.942078-5-nikunj@amd.com> <20241209155718.GBZ1cTXp2XsgtvUzHm@fat_crate.local> <0477b378-aa35-4a68-9ff6-308aada2e790@amd.com> <15e82ca3-9166-cdb4-7d66-e1c6600919d7@amd.com> <20241211190023.GGZ1nhR7YQWGysKeEW@fat_crate.local> <984f8f36-8492-9278-81b3-f87b9b193597@amd.com> <20241211222257.GKZ1oQwZcSXSMXPvoY@fat_crate.local> From: Tom Lendacky Subject: Re: [PATCH v15 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests In-Reply-To: <20241211222257.GKZ1oQwZcSXSMXPvoY@fat_crate.local> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SA9PR13CA0012.namprd13.prod.outlook.com (2603:10b6:806:21::17) To DM4PR12MB5070.namprd12.prod.outlook.com (2603:10b6:5:389::22) 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: DM4PR12MB5070:EE_|SA1PR12MB7247:EE_ X-MS-Office365-Filtering-Correlation-Id: aec25154-92fb-4280-3beb-08dd1a353480 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|7416014; 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Writing >> to or reading from that MSR is not that common, so I would think we want >> to keep the same behavior that has been in effect. > > Ah, I thought you're gonna say something along the lines of, yeah, we must use > the HV GHCB protocol because of and there's no other way this could > work. > >> But if we do want to make this change, maybe do it separate from the >> Secure TSC series since it alters the behavior of SEV-ES guests and >> SEV-SNP guests without Secure TSC. > > Already suggested so - this should be a separate patch. > > It would be interesting to see if it brings any improvement by avoiding the HV > call... especially since RDTSC is used a *lot* and prominently at that in > sched_clock, for example. I doubt you would notice anything since it doesn't look like Linux ever reads from or writes to MSR_IA32_TSC, preferring to just use RDTSC or RDTSCP which aren't usually intercepted at all and so never goes out to the HV. Thanks, Tom >