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([fe80::b6cf:ce77:3cdf:7cc]) by SJ2PR11MB8370.namprd11.prod.outlook.com ([fe80::b6cf:ce77:3cdf:7cc%5]) with mapi id 15.21.0451.014; Fri, 25 Sep 2026 16:39:15 +0000 Message-ID: Date: Fri, 25 Sep 2026 09:39:13 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v12 15/25] x86,fs/resctrl: Handle systems where AET is the only resource To: "Luck, Tony" CC: Fenghua Yu , Maciej Wieczor-Retman , Peter Newman , James Morse , Babu Moger , "Drew Fustini" , Dave Martin , Chen Yu , David E Box , , Christoph Hellwig , , References: <20260916231320.14502-1-tony.luck@intel.com> <20260916231320.14502-16-tony.luck@intel.com> <39582d24-0fc1-4d5e-bc05-01c2f29ddfb1@intel.com> <110d2c7f-8354-4d25-aff8-cd5ceb46334b@intel.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MW4PR04CA0079.namprd04.prod.outlook.com (2603:10b6:303:6b::24) To SJ2PR11MB8370.namprd11.prod.outlook.com (2603:10b6:a03:540::20) 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=?utf-8?B?djFiTXljVkNvOWMzdG56S2c2RjJsVXpSZnp0Uk5EcGRkd0I0anpyeWhBbHVi?= =?utf-8?Q?MXTlaHQ25yakLpmM=3D?= X-Exchange-RoutingPolicyChecked: oy2SeYaWq71MZ8LC4eeQo5E75qPpWKkSAM53rrJxrPK8oOE7OOvYHPbHGJ3QrVe+iWqEx/W5N86awdOx0Uaef9OYvcjm6OcmRPM2DAXYiEExOE5ZSwmqTPsxvSWGb/5HP/JM3sXDVR7qgs619vr9H/r+N7YHXM7g9cb5yx1pjSuPL1GnsCuPfhnNiVffGjgXlJujTHRqyTHiIpY1p9dLDZpMtNcc7WcdMK2bN5KurHB5m+D0A592j17z18QcICBP1bzRlqPDcmkC+/+I1i03w+i+xGqs2DqyBN3qYh71gLxJIiGUACY03fAmYjT/ArDMD/mkc13PRfFyrRIMov5ayA== X-MS-Exchange-CrossTenant-Network-Message-Id: 1fc76aba-1d2a-44b7-ce44-08df1b23899e X-MS-Exchange-CrossTenant-AuthSource: SJ2PR11MB8370.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Sep 2026 16:39:15.8233 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: A+H844x6q2ggHivoup0Mo+jSf10DRIT4pXBSaK9jAuaXJpzYkwfa16fwB7azp3L498+NuHVrkNZbTwY4FtWdrH4PcZIKtAPsRovmo/uUyTc= X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA3PR11MB186312 X-OriginatorOrg: intel.com Hi Tony, On 9/25/26 8:50 AM, Luck, Tony wrote: > On Fri, Sep 25, 2026 at 07:37:20AM -0700, Reinette Chatre wrote: >> Hi Tony, >> >> On 9/24/26 5:06 PM, Luck, Tony wrote: >>> On Thu, Sep 24, 2026 at 01:41:48PM -0700, Reinette Chatre wrote: >>>> On 9/16/26 4:13 PM, Tony Luck wrote: >> >>>>> @@ -1001,27 +1001,29 @@ static __init bool get_rdt_mon_resources(void) >>>>> >>>>> /* Any of the L3 monitoring features? */ >>>>> if (!cpu_feature_enabled(X86_FEATURE_CQM_LLC)) >>>>> - return false; >>>>> + goto skip_l3_feature_checks; >>>> >>>> This goto looks unnecessary. Why not just "return true"? Even so, this >>>> also changes behavior from the previous version in a way that is not clear >>>> to me. I was expecting this to consider the number of RMIDs supported by the system, >>>> now that this function added: >>>> >>>> pqr_assoc_num_rmid = cpuid_ebx(0xf) + 1; >>>> >>>> Should this be: >>>> >>>> if (!cpu_feature_enabled(X86_FEATURE_CQM_LLC)) >>>> return pqr_assoc_num_rmid > 1; >>>> >>>> Although ... looking at this closer it does look strange for pqr_assoc_num_rmid >>>> to be 1, thus reflecting that the system supports one RMID, when zero may be more >>>> accurate? >>> >>> If X86_FEATURE_CQM_LLC is set, then monitoring is supported and the system >>> must support at least one RMID. I don't see a need to insist on more than one >>> RMID as a precondition for using resctrl. It wouldn't be super interesting >> >> My suggestion was not to use "more than one RMID as a precondition". By copying the >> relevant code not shown in the hunk >> >> pqr_assoc_num_rmid = cpuid_ebx(0xf) + 1; >> >> I aimed to highlight that if the system returns zero then pqr_assoc_num_rmid >> will contain 1. The "pqr_assoc_num_rmid > 1" is thus not a check for "more >> than one RMID" but instead a check that the system did not return zero. > > Architecturally the cpuid_ebx(0xf) can legally return 0 meaning that the > system supports one RMID. Here's the description from SDM Volume 1 Table > 21-38 "Leaf 0FH.00H Intel® Resource Director Technology (Intel® RDT) Monitoring": > > Register Field Name Description > EBX[31:0] MAX_RMID Maximum range (zero-based) of RMID within > this physical processor of all types. > > This code path is only executed if the test for X86_FEATURE_CQM > succeeded. That should guarantee that leaf 0xF exists[1]. There's > no way for leaf 0xF to indicate zero RMIDs because the MAX_RMID > field gives a zero-based result. > > So a test for "pqr_assoc_num_rmid > 1" really is checking that two or > more RMIDs are supported. > > If you want resctrl to refuse to support a crazy system with only one > RMID, then the check should as soon as pqr_assoc_num_rmid is set: > > pqr_assoc_num_rmid = cpuid_ebx(0xf) + 1; > > /* Systems with only one RMID can't usefully support resctrl */ > if (pqr_assoc_num_rmid == 1) > return false; Up to you. I already highlighted in https://lore.kernel.org/lkml/39582d24-0fc1-4d5e-bc05-01c2f29ddfb1@intel.com/ that my goal is simply to align this version to the previous https://lore.kernel.org/lkml/20260831174421.13921-14-tony.luck@intel.com/ that used return resctrl_arch_system_max_rmid_idx() > 0 to determine if AET can supported. Ignoring this simple question in https://lore.kernel.org/lkml/arW6xlQWVWkixiWe@agluck-desk3/ you instead use the SDM to argue against me for some reason. Reinette