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McKenney" , Boqun Feng , Jonathan Corbet , Prakash Sangappa , Madadi Vineeth Reddy , K Prateek Nayak , Steven Rostedt , Sebastian Andrzej Siewior , Arnd Bergmann , linux-arch@vger.kernel.org, Randy Dunlap , Ron Geva , Waiman Long , "carlos@redhat.com" , Michael Jeanson References: <20251215155615.870031952@linutronix.de> <20251215155708.669472597@linutronix.de> <87jyyjbclh.ffs@tglx> <225b9868-4ab7-4a90-8acb-8d965626f6a7@efficios.com> <87ldi4gjm3.ffs@tglx> <45fd706a-86be-42b8-879e-11bbe262e159@efficios.com> <20260119102138.GQ830755@noisy.programming.kicks-ass.net> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <20260119102138.GQ830755@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: GV3PEPF0001DBED.SWEP280.PROD.OUTLOOK.COM (2603:10a6:158:400::307) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YT3PR01MB10954:EE_ X-MS-Office365-Filtering-Correlation-Id: 1b6c5a00-ae49-40a3-6248-08de5745d66f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|1800799024; 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I recall that doing an additional rseq system call at thread >> creation was analyzed thoroughly for performance regressions at the >> libc level. I would not want to start requiring libc to issue a >> handful of additional prctl system calls per thread creation for no good >> reason. > > A wee something like so? > > That would allow registering rseq with RSEQ_FLAG_SLICE_EXT_DEFAULT_ON > set and if all the stars align, it will then have it on at the end. That's a very good step in the right direction. I just wonder how userspace is expected to learn that it runs on a kernel which accepts the RSEQ_FLAG_SLICE_EXT_DEFAULT_ON flag ? I think it could expect it when getauxval for AT_RSEQ_FEATURE_SIZE includes the slice ext field. This gives us a cheap way to know from userspace whether this new flag is supported or not. One nit below: [...] > - if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) > + if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) { > rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE; > + if (rseq_slice_extension_enabled() && > + flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON) I think you want to surround flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON with parentheses () to have the expected operator priority. Thanks! Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com