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b=WPdzXwhbeKhLUq9an2hsf1RZtk5ulrWBMxm761uKxvGQGGE8821oZaYLfPRZHEk2Bm/FUII9oAHNcTexinYN0IEbVh72M/UxdrH8RQFKLvwnKhMa57PxZMrIMXXdcyaLvPc+XvdDigocsWycn6Kda1u45nVroCcN5dgv8kGw97s= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from DS5PPFA3734E4BA.namprd12.prod.outlook.com (2603:10b6:f:fc00::65c) by BN7PPF0D942FA9A.namprd12.prod.outlook.com (2603:10b6:40f:fc02::6c7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9137.13; Tue, 7 Oct 2025 17:36:21 +0000 Received: from DS5PPFA3734E4BA.namprd12.prod.outlook.com ([fe80::1370:cd3b:4c30:5a57]) by DS5PPFA3734E4BA.namprd12.prod.outlook.com ([fe80::1370:cd3b:4c30:5a57%7]) with mapi id 15.20.9115.020; Tue, 7 Oct 2025 17:36:20 +0000 Message-ID: <0cd2c8ac-8dee-4280-b726-af0119baa4a1@amd.com> Date: Tue, 7 Oct 2025 12:36:18 -0500 User-Agent: Mozilla Thunderbird From: Babu Moger Subject: Re: [PATCH] fs/resctrl: Fix MBM events being unconditionally enabled in mbm_event mode To: Reinette Chatre , babu.moger@amd.com, tony.luck@intel.com, Dave.Martin@arm.com, james.morse@arm.com, dave.hansen@linux.intel.com, bp@alien8.de Cc: kas@kernel.org, rick.p.edgecombe@intel.com, linux-kernel@vger.kernel.org, x86@kernel.org, linux-coco@lists.linux.dev, kvm@vger.kernel.org References: <6082147693739c4514e4a650a62f805956331d51.1759263540.git.babu.moger@amd.com> <2cdc5b52-a00c-4772-8221-8d98b787722a@intel.com> Content-Language: en-US In-Reply-To: <2cdc5b52-a00c-4772-8221-8d98b787722a@intel.com> Content-Type: text/plain; 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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? >> >> Yes. That is correct. Right now, ABMC and X86_FEATURE_CQM_MBM_TOTAL/ >> X86_FEATURE_CQM_MBM_LOCAL are kind of tightly coupled. We have not clearly >> separated the that. > > Are you speaking from resctrl side since from what I understand these are > independent features from the hardware side? It is independent from hardware side. I meant we still use legacy events from "default" mode. > >>> 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? >> >> Enable or disable needs to be done at get_rdt_mon_resources(). It needs to >> be done early in the initialization before calling domain_add_cpu() where >> event data structures (mbm_states aarch_mbm_states) are allocated. > > Good point. My mistake to suggest the event should be enabled by > resctrl fs. How about adding another check in get_rdt_mon_resources()? if (rdt_cpu_has(X86_FEATURE_CQM_MBM_TOTAL) || rdt_is_feature_enabled(mbmtotal)) { resctrl_enable_mon_event(QOS_L3_MBM_TOTAL_EVENT_ID); ret = true; } I need to take Tony's patch for this. > >> >>> >>> 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()? >> >> This may not be an issue right now. When X86_FEATURE_CQM_MBM_TOTAL and >> X86_FEATURE_CQM_MBM_LOCAL are not supported then mon_data files of these >> events are not created. > > By "right now" I assume you mean the current implementation? I think your statement > assumes that no CPUs come or go after resctrl_mon_resource_init() enables the MBM events? > Current implementation will enable MBM events if ABMC is supported. When the > first CPU of a domain comes online after that then resctrl will create the mon_data > files. These files will remain if a user then switches to default mode and if > the user then attempts to read one of these counters then I expect problems. Yes. It will be a problem in the that case. I am not clear on using config option you mentioned above. What about using the check resctrl_is_mon_event_enabled() in resctrl_mbm_assign_mode_show() and resctrl_mbm_assign_mode_write() ? Thanks Babu