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Thu, 20 Aug 2026 15:54:56 +0000 Message-ID: <7911abea-d508-4401-b6de-2f6f598f08bf@intel.com> Date: Thu, 20 Aug 2026 08:54:54 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/9] x86/resctrl: Require 64-bit x86 for resctrl support To: "Luck, Tony" , "Chen, Yu C" CC: "tglx@kernel.org" , "bp@alien8.de" , "mingo@redhat.com" , "dave.hansen@linux.intel.com" , "hpa@zytor.com" , "fenghuay@nvidia.com" , "babu.moger@amd.com" , "chen.yu@linux.dev" , "x86@kernel.org" , "linux-kernel@vger.kernel.org" References: <980c4d9c15862de0b0dbc535b646f6b96ea46abe.1784968626.git.yu.c.chen@intel.com> <6c933b5c-620d-4684-b651-48a7ebdfc956@intel.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR04CA0200.namprd04.prod.outlook.com (2603:10b6:303:86::25) To SJ2PR11MB8370.namprd11.prod.outlook.com (2603:10b6:a03:540::20) 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: SJ2PR11MB8370:EE_|DS0PR11MB8737:EE_ X-MS-Office365-Filtering-Correlation-Id: b6c7fb61-e983-4f07-bbd3-08defed3617d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014|23010399003|10067099003|56012099006|4143699003|3023799007|22082099003|18002099003|11063799006; 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Are you looking for something like this? > > x86/resctrl: Require 64-bit x86 for resctrl support > > All known hardware that supports Intel RDT or AMD QoS is 64-bit. > > On x86 the maximum supported memory size on a 32-bit kernel is 64GB (when > PAE is enabled). Linux imposes additional constraints since each 4KByte > page requires a 64-byte "struct page" which must reside in the always-mapped > 3-4GByte range of the 32-bit kernel virtual address space. This means the > maximum supported memory size is slightly above 32GB, but at that memory > size there are considerable inefficiencies as over half of kernel virtual > memory is dedicated to the page structures. > > The earliest server processor to support resctrl was Intel Haswell, released in > 2014. It supported four memory channels, and production systems were advised > to populate all channels to provide sufficient memory bandwidth to keep the > cores busy. Best value DIMMs in 2014 were 16GB. So even a single socket Haswell > system would have more memory than could be addressed by a 32-bit kernel. I assume above aims to justify the "no realistic use case for users to build a 32-bit kernel on a server and enable resctrl"? To be specific, my comment about RDT not being a "server" feature was made with Apollo Lake [1] in mind. That system showed up in scenarios where servers do not typically venture and per [1] its max memory size (8GB) is significantly less than what servers can support. Apart from that it was the "no realistic use case" that tripped me. At best I think we can say that we are not aware of 32-bit use cases? > Drop 32-bit support by changing the X86_CPU_RESCTRL dependency from X86 > to X86_64. > > Since X86_CPU_RESCTRL now implies X86_64, remove the redundant X86_64 > dependency from X86_CPU_RESCTRL_INTEL_AET. > Reinette [1] https://www.intel.com/content/www/us/en/products/sku/95594/intel-celeron-processor-j3455-2m-cache-up-to-2-30-ghz/specifications.html