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Date: Wed, 24 Sep 2025 15:40:43 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Andrew Cooper Cc: "xen-devel@lists.xenproject.org" , "oleksii.kurochko@gmail.com" , Jan Beulich Subject: Re: [PATCH for-4.21] x86/cpu: populate CPUID 0x1.edx features early for self-snoop detection Message-ID: References: <20250924110051.2160-1-roger.pau@citrix.com> <2d17d2d502df489f97b51e43a2d86535@DM6PR03MB5227.namprd03.prod.outlook.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2d17d2d502df489f97b51e43a2d86535@DM6PR03MB5227.namprd03.prod.outlook.com> X-ClientProxiedBy: PAZP264CA0071.FRAP264.PROD.OUTLOOK.COM (2603:10a6:102:1fd::6) To DM6PR03MB5227.namprd03.prod.outlook.com (2603:10b6:5:247::22) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM6PR03MB5227:EE_|DS7PR03MB8242:EE_ X-MS-Office365-Filtering-Correlation-Id: a1c8af9f-afb7-48b1-9efc-08ddfb700175 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014; 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Well, that's the reason for the rant here. The reset at the top of identify_cpu() has been there since 2005. It's arguably to make sure the BSP and the APs have the same empty state in the passed cpuinfo_x86 struct, as for the BSP this would be already partially initialized due to what's done in early_cpu_init(). The underlying question is whether we would rather prefer to not do the reset for the BSP, but that would lead to differences in the contents of cpuinfo_x86 struct between the BSP and the APs. In the past we have arranged for leaves needed early to be populated in generic_identify(), like FEATURESET_e21a, hence the proposed patch does that for FEATURESET_1d. > > However that > > creates an imbalance on the state of the BSP versus the APs in the > > identify_cpu() code. > > > > I've opted for the less controversial solution of populating FEATURESET_1d > > in generic_identify(), as the value is already there. The same is done for > > the AMD faulting probe code. > > > > Fixes: f2663ca2e520 ("x86/cpu/intel: Clear cache self-snoop capability in CPUs with known errata") > > Signed-off-by: Roger Pau Monné > > ... this Fixes tag is incorrect? I think the Fixes tag is accurate; the code was OK before that change. Nothing in c_early_init hooks depended on (some of) the x86_capability fields being populated, which is required after the change. Thanks, Roger.