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Fri, 8 Nov 2024 02:17:25 +0000 Message-ID: Date: Thu, 7 Nov 2024 18:17:24 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/cpufeatures: Free up unused feature bits To: Dave Hansen , "H. Peter Anvin" , , Borislav Petkov , Dave Hansen CC: Thomas Gleixner , Ingo Molnar , "Sean Christopherson" , Tony Luck , References: <20241107233000.2742619-1-sohil.mehta@intel.com> <7ff32a10-1950-434b-8820-bb7c3b408544@intel.com> <3492e85d-4aaa-4dd7-9e6a-c8fdc2aa2178@intel.com> <74338393-4F39-4A04-83B1-BA64CE2D2FDA@zytor.com> <4c01a30a-67d9-4918-8781-240b78944c42@intel.com> Content-Language: en-US From: Sohil Mehta In-Reply-To: <4c01a30a-67d9-4918-8781-240b78944c42@intel.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BYAPR08CA0072.namprd08.prod.outlook.com (2603:10b6:a03:117::49) To BYAPR11MB3320.namprd11.prod.outlook.com (2603:10b6:a03:18::25) 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: BYAPR11MB3320:EE_|BY1PR11MB8056:EE_ X-MS-Office365-Filtering-Correlation-Id: c17411c3-2442-4862-a53b-08dcff9b7cc1 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|1800799024|366016|376014; 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Peter's comment makes sense to me now. > > I sure hope we haven't been using too many of these synthetic features > in MODULE_DEVICE_TABLE()s, because we tend to move them around, but I > guess it's possible. I found at least one recent usage that matches this pattern. Look at commit cbcddaa33d7e ("perf/x86/rapl: Use CPUID bit on AMD and Hygon parts"). It defines a synthetic feature bit X86_FEATURE_RAPL and adds it to the rapl_model_match[] table. MODULE_DEVICE_TABLE(x86cpu, rapl_model_match); It almost seems like some of these bits are now ABI. We probably need to mark them and keep these mappings pinned to avoid future issues. Recycling these bits seems to be very common.