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Tue, 25 Aug 2026 16:38:31 +0000 Date: Tue, 25 Aug 2026 09:38:30 -0700 From: "Luck, Tony" To: "Chen, Yu C" CC: Reinette Chatre , , , , , , , , , , Subject: Re: [PATCH v6 5/9] x86/resctrl: Parse ACPI CMRC table Message-ID: References: <01d48e87-eade-438d-9d33-b1110b6bbfcd@intel.com> <64ac0eaf-c2da-4683-9859-81b46ff325e7@intel.com> <4314ce95-baa9-43cd-ad1c-4e56c07b9556@intel.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4314ce95-baa9-43cd-ad1c-4e56c07b9556@intel.com> X-ClientProxiedBy: SJ2P220CA0007.NAMP220.PROD.OUTLOOK.COM (2603:10b6:a03:5da::13) To SJ1PR11MB6083.namprd11.prod.outlook.com (2603:10b6:a03:48a::9) 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: SJ1PR11MB6083:EE_|SJ0PR11MB6672:EE_ X-MS-Office365-Filtering-Correlation-Id: fd56010e-bd92-480a-b3ab-08df02c74c3e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|23010399003|376014|1800799024|366016|4143699003|10067099003|11063799006|22082099003|18002099003|56012099006; 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The existing code can thus not accommodate the new values and need to > > > > be updated? > > > > > > > > > > Yes, the legacy CPUID reports it as 32 bits, while CMRC is declared as 64 bits. > > > > > > In theory, we should change the scale type from unsigned int to u64 to accommodate > > > both the legacy CPUID and CMRC. However, it seems unlikely that the scale would > > > exceed 32 bits. If the scale were 32 bits, the L3 occupancy would be at least > > > 2^32 − 1, which is about 4 GB. We have not yet seen platform with 4 GB of L3 cache. > > > So perhaps we can keep erdt_scale as unsigned int for now IMO. > > > > > > It sounds like you are saying that the spec you are enabling should not be using 64 bits for > > this value. It is unexpected to me that work on enabling of a spec adds such constraint without > > clear motivation. If you find that the spec is not correct then I think it would be better to have > > it changed there instead of building what appears to be unsupported assumptions into resctrl. > > > > OK, let me check with the team. Looking at the RDT architecture spec that defines the CMRC table it looks to me like the "CMT Counter Upscaling Factor" was made 8-bytes just to round the table out to a whole number or quad-words. This value represents the granularity of cache occupancy reporting. The feature would be worthless if this was >=4GB no matter how large the LLC cache may be in future generations. If large values are a big concern then perhaps include a sanity check in the code: if (cmrc->upscale >= SZ_4G) pr_warn(FW_BUG "insane CMRC upscale value 0x%llx\n", cmrc->upscale); > > thanks, > Chenyu -Tony