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Mon, 6 Oct 2025 17:56:49 +0000 Message-ID: Date: Mon, 6 Oct 2025 10:56:47 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] fs/resctrl: Fix MBM events being unconditionally enabled in mbm_event mode To: Babu Moger , , , , , CC: , , , , , References: <6082147693739c4514e4a650a62f805956331d51.1759263540.git.babu.moger@amd.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <6082147693739c4514e4a650a62f805956331d51.1759263540.git.babu.moger@amd.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR04CA0344.namprd04.prod.outlook.com (2603:10b6:303:8a::19) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR11MB7573:EE_|CH3PR11MB7769:EE_ X-MS-Office365-Filtering-Correlation-Id: 9afccecd-1f33-4219-c1e7-08de0501b937 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|1800799024; 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To turn off the memory bandwidth events (mbmtotal and > mbmlocal), users need to pass the following parameter to the kernel: > "rdt=!mbmtotal,!mbmlocal". ah, indeed ... although, the intention behind the mbmtotal and mbmlocal kernel parameters was to connect them to the actual hardware features identified by X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL respectively. > Found that memory bandwidth events (mbmtotal and mbmlocal) cannot be > disabled when mbm_event mode is enabled. resctrl_mon_resource_init() > unconditionally enables these events without checking if the underlying > hardware supports them. Technically this is correct since if hardware supports ABMC then the hardware is no longer required to support X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL in order to provide mbm_total_bytes and mbm_local_bytes. I can see how this may be confusing to user space though ... > > Remove the unconditional enablement of MBM features in > resctrl_mon_resource_init() to fix the problem. The hardware support > verification is already done in get_rdt_mon_resources(). I believe by "hardware support" you mean hardware support for X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL. Wouldn't a fix like this then require any system that supports ABMC to also support X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL to be able to support mbm_total_bytes and mbm_local_bytes? This problem seems to be similar to the one solved by [1] since by supporting ABMC there is no "hardware does not support mbmtotal/mbmlocal" but instead there only needs to be a check if the feature has been disabled by command line. That is, add a rdt_is_feature_enabled() check to the existing "!resctrl_is_mon_event_enabled()" check? But wait ... I think there may be a bigger problem when considering systems that support ABMC but not X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL. Shouldn't resctrl prevent such a system from switching to "default" mbm_assign_mode? Otherwise resctrl will happily let such a system switch to default mode and when user attempts to read an event file resctrl will attempt to read it via MSRs that are not supported. Looks like ABMC may need something similar to CONFIG_RESCTRL_ASSIGN_FIXED to handle this case in show() while preventing user space from switching to "default" mode on write()? Reinette [1] https://lore.kernel.org/lkml/20250925200328.64155-23-tony.luck@intel.com/