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Fri, 4 Sep 2026 15:50:11 +0000 Message-ID: Date: Fri, 4 Sep 2026 08:50:08 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/5] fs/resctrl: Drop support for different bandwidth delay mappings To: Chen Yu CC: , , , , , , , , , , , , , , , References: <9d39435847fb26fa7b467c563cf8bc45c314b917.1788452481.git.reinette.chatre@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: MW4PR02CA0012.namprd02.prod.outlook.com (2603:10b6:303:16d::35) 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_|PH8PR11MB8037:EE_ X-MS-Office365-Filtering-Correlation-Id: b3ac320b-0aa9-4e09-ca15-08df0a9c33a9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|23010399003|376014|7416014|1800799024|4143699003|56012099006|10067099003|5023799004|11063799006|6133799003|22082099003|18002099003; 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>> - >> - pr_warn_once("Non Linear delay-bw map not supported but queried\n"); >> - return MAX_MBA_BW; >> -} >> - >> static void mba_wrmsr_intel(struct msr_param *m) >> { >> struct rdt_hw_ctrl_domain *hw_dom = resctrl_to_arch_ctrl_dom(m->dom); >> struct rdt_hw_resource *hw_res = resctrl_to_arch_res(m->res); >> unsigned int i; >> >> - /* Write the delay values for mba. */ >> + if (!m->res->membw.delay_linear) { >> + pr_warn_once("Non-linear bandwidth delay not supported\n"); >> + return; > > Previously if linear delay is not supported, we still write MAX_MBA_BW > to MSR, and here we return with MSR untouched, this should be the expected > behavior? I believe so, yes. Two reasons: a) This is not a supported scenario. This code path should never execute and already complains loudly if encountered. b) In the unlikely case that this unsupported scenario is encountered I find leaving the hardware as-is to be the safer option. > Besides, in patch 2, __get_mem_config_intel() returns false for > non-linear-delay platform thus MB resource's alloc_capable is false, > do we still need to do the linear check in mba_wrmsr_intel() in patch 1 > as the code path will not be triggered? You are right that the check is not required. I view this check more like why we use "lockdep_assert_held()". More specifically, it uses the code to document the requirements instead of comments. For Intel this is valuable since the transition to controls and their flags means that when a developer builds on the new resctrl capabilities to initialize multiple controls it is required for *each* control to have the "this scalar is linear" flag set. This test makes that requirement clear. Reinette