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Dadhania" Subject: Re: [PATCH v15 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests To: Borislav Petkov Cc: linux-kernel@vger.kernel.org, thomas.lendacky@amd.com, 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> Content-Language: en-US In-Reply-To: <20241209155718.GBZ1cTXp2XsgtvUzHm@fat_crate.local> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: PN0PR01CA0033.INDPRD01.PROD.OUTLOOK.COM (2603:1096:c01:4e::23) To DS7PR12MB6309.namprd12.prod.outlook.com (2603:10b6:8:96::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: DS7PR12MB6309:EE_|CH3PR12MB9429:EE_ X-MS-Office365-Filtering-Correlation-Id: 6b1f3e6b-bef4-462d-660e-08dd18d7da7b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|1800799024; 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That is the warning from the APM: 15.36.18 Secure TSC "Guests that run with Secure TSC enabled are not expected to perform writes to the TSC MSR (10h). If such a write occurs, subsequent TSC values read are undefined." What I make out of it is: if a write is performed to the TSC MSR, subsequent reads of TSC is not reliable/trusted. That was the reason to ignore such writes in the #VC handler. > > I'd prefer if we issued a WARN_ONCE() there on the write to catch any > offenders. Do you also want to terminate the offending guest? ES_UNSUPPORTED return will do that. > > *NO ONE* should be writing the TSC MSR but that's a different story. > > IOW, something like this ontop of yours? > > --- > > diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c > index c22cb2ea4b99..050170eb28e6 100644 > --- a/arch/x86/coco/sev/core.c > +++ b/arch/x86/coco/sev/core.c > @@ -1443,9 +1443,15 @@ static enum es_result __vc_handle_msr_tsc(struct pt_regs *regs, bool write) > { > u64 tsc; > > - if (write) > - return ES_OK; > + if (!(sev_status & MSR_AMD64_SNP_SECURE_TSC)) > + goto read_tsc; This is changing the behavior for SEV-ES and SNP guests(non SECURE_TSC), TSC MSR reads are converted to RDTSC. This is a good optimization. But just wanted to bring up the subtle impact. > + > + if (write) { > + WARN_ONCE(1, "TSC MSR writes are verboten!\n"); > + return ES_UNSUPPORTED; Sure, we can add a WARN_ONCE(). > + } > > +read_tsc: > tsc = rdtsc_ordered(); > regs->ax = lower_32_bits(tsc); > regs->dx = upper_32_bits(tsc); > @@ -1462,11 +1468,14 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt) > /* Is it a WRMSR? */ > write = ctxt->insn.opcode.bytes[1] == 0x30; > > - if (regs->cx == MSR_SVSM_CAA) > + switch(regs->cx) { Yes, I was thinking about a switch, as there will be more such instances when we enable newer features. > + case MSR_SVSM_CAA: > return __vc_handle_msr_caa(regs, write); > - > - if (regs->cx == MSR_IA32_TSC && (sev_status & MSR_AMD64_SNP_SECURE_TSC)) > + case MSR_IA32_TSC: > return __vc_handle_msr_tsc(regs, write); > + default: > + break; > + } > > ghcb_set_rcx(ghcb, regs->cx); > if (write) { > Regards, Nikunj