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X-CSE-ConnectionGUID: ZwAmHwZQT9yuxof3dZlypQ== X-CSE-MsgGUID: w8wrm8QdTZWrKcWt0JbBbw== X-IronPort-AV: E=McAfee;i="6800,10657,11885"; a="113565785" X-IronPort-AV: E=Sophos;i="6.25,240,1779174000"; d="scan'208";a="113565785" Received: from fmviesa003.fm.intel.com ([10.60.135.143]) by fmvoesa101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Aug 2026 07:18:54 -0700 X-CSE-ConnectionGUID: jD3zytn1Tb2Ab77NJb6JJw== X-CSE-MsgGUID: kouJlBQnSqGYtiYSnZS9wQ== X-ExtLoop1: 1 Received: from jmaxwel1-mobl.amr.corp.intel.com (HELO [10.125.108.191]) ([10.125.108.191]) by fmviesa003-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Aug 2026 07:18:53 -0700 Message-ID: Date: Mon, 24 Aug 2026 07:18:50 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 2/9] x86/resctrl: Require 64-bit x86 for resctrl support To: Chen Yu , Reinette Chatre Cc: Borislav Petkov , "Luck, Tony" , "Chen, Yu C" , "tglx@kernel.org" , "mingo@redhat.com" , "dave.hansen@linux.intel.com" , "hpa@zytor.com" , "fenghuay@nvidia.com" , "babu.moger@amd.com" , "x86@kernel.org" , "linux-kernel@vger.kernel.org" References: <980c4d9c15862de0b0dbc535b646f6b96ea46abe.1784968626.git.yu.c.chen@intel.com> <6c933b5c-620d-4684-b651-48a7ebdfc956@intel.com> <7911abea-d508-4401-b6de-2f6f598f08bf@intel.com> <20260821023700.GAaoe5zEH173SJwxga@fat_crate.local> <03e10b76-01d7-47cf-b139-a29f41e0c47f@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/25/26 06:12, Chen Yu wrote: > "32-bit x86 is maintained for legacy reasons. New CPU features are generally > not enabled for 32-bit system, its test coverage keeps shrinking, and modern > 32-bit distributions have become rare." What would folks think about plopping something like the following text in to Documentation/arch/x86/32bit-kernels.rst ? Would that clear things up? --- ========================================= Supporting new features on 32-bit kernels ========================================= While it is possible to compile and run a 32-bit kernel on 64-bit capable hardware, it is impractical and discouraged. Processors capable of running 64-bit kernels are expected to do so. The last 32-bit processor to come out of Intel or AMD was a Quark CPU in 2013. Before that the most recent were some Atom CPUs in 2008. Distros supporting 32-bit kernels and userspace are disappearing. For example, in 2026 Ubuntu and Fedora no longer have support, with Debian being the most notable holdout still maintaining 32-bit options. New Processor Features ====================== When adding support for a new processor feature in 2026 and beyond it should, by default, be: config NEW_SNAZZY_CPU_FEATURE depends X86_64 This ensures that the folks developing this feature do not waste resources making it work on 32-bit kernels running on 64-bit hardware. It is also acceptable to add this restriction to existing processor features if 32-bit kernel support is adding maintenance burden, especially if the feature was only ever available on 64-bit processors. New Software Features ===================== Let's say you are enhancing ftrace or adding a debugging feature that has no strict dependency on processor features. It does not need to be restricted to 64-bit only. But you might want to consider doing so if it simplifies your implementation. For instance, sometimes it adds complexity to support HIGHMEM. You may add a dependency on X86_64 in cases where it is impractical or simply challenging to support 32-bit kernels.