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Mon, 2 Nov 2026 11:05:06 +0000 Message-ID: Date: Mon, 2 Nov 2026 13:04:59 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/6] perf/x86/intel/pt: Stop tracing if VMXON has not yet happened To: Peter Zijlstra CC: Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , "Ian Rogers" , James Clark , "Thomas Gleixner" , Borislav Petkov , Dave Hansen , , "H. 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Wysocki" , Pavel Machek , Greg Kroah-Hartman , "Danilo Krummrich" , Johan Hovold , "Sebastian Andrzej Siewior" , Mathieu Desnoyers , Jiayuan Chen , "Steven Rostedt (Google)" , "Vineeth Pillai (Google)" , Frederic Weisbecker , "Andi Kleen" , , , , References: <20261102074039.26983-1-adrian.hunter@intel.com> <20261102074039.26983-3-adrian.hunter@intel.com> <20261102095750.GS1779021@noisy.programming.kicks-ass.net> 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: <20261102095750.GS1779021@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: DUZPR01CA0063.eurprd01.prod.exchangelabs.com (2603:10a6:10:3c2::13) To IA1PR11MB7198.namprd11.prod.outlook.com (2603:10b6:208:419::15) X-Mailing-List: driver-core@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA1PR11MB7198:EE_|PH7PR11MB6606:EE_ X-MS-Office365-Filtering-Correlation-Id: 9d9de441-94ef-4566-75ff-08df38d12702 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|23010399003|1800799024|366016|376014|7416014|261021121427099003|261021123027099003|261022075727099003|261009223027099003|261026121527099003|4143699003|11063799006|10067099003|56012099006|18002099003|22082099003; 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>> } >> >> +static bool pt_trace_enabled(void) >> +{ >> + u64 ctl; >> + >> + rdmsrq(MSR_IA32_RTIT_CTL, ctl); >> + >> + return ctl & RTIT_CTL_TRACEEN; >> +} >> + >> static void pt_config_stop(struct perf_event *event) >> { >> struct pt *pt = this_cpu_ptr(&pt_ctx); >> @@ -577,7 +586,12 @@ static void pt_config_stop(struct perf_event *event) >> return; >> >> ctl &= ~RTIT_CTL_TRACEEN; >> - if (!READ_ONCE(pt->vmx_on)) >> + /* >> + * vmx_on is set before VMXON, so tracing may not have stopped yet. >> + * RTIT_CTL can be written until VMXON, and after VMXON TraceEn is >> + * clear, so still write RTIT_CTL if TraceEn is set. >> + */ >> + if (!READ_ONCE(pt->vmx_on) || pt_trace_enabled()) >> wrmsrq(MSR_IA32_RTIT_CTL, ctl); >> >> WRITE_ONCE(event->hw.aux_config, ctl); > > It seems rather unfortunate to issue a RDMSR for state we fully control. > Surely we can track this? The RDMSR only happens on BDW and only when VMX is on and someone still tries to use Intel PT (even though no trace can be produced on BDW while VMX is on). There is a window between setting vmx_on and clearing TRACEEN. Dealing with that would be complicated, compared with reading the MSR.