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Date: Thu, 22 Jan 2026 18:18:45 +0100 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: =?utf-8?B?SsO8cmdlbiBHcm/Dnw==?= Cc: "xen-devel@lists.xenproject.org" , Jan Beulich , Andrew Cooper Subject: Re: MTRR init sequence in Xen Message-ID: References: <145af46c-eab5-4d0f-ba35-4ae646c0e4cd@suse.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <145af46c-eab5-4d0f-ba35-4ae646c0e4cd@suse.com> X-ClientProxiedBy: PR3P189CA0070.EURP189.PROD.OUTLOOK.COM (2603:10a6:102:b4::15) To CH7PR03MB7860.namprd03.prod.outlook.com (2603:10b6:610:24e::14) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH7PR03MB7860:EE_|MW5PR03MB6905:EE_ X-MS-Office365-Filtering-Correlation-Id: 36be5ecd-92f2-460a-c426-08de59da4f6d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024|7053199007; X-Microsoft-Antispam-Message-Info: =?utf-8?B?cHJrNXoxTFFFOUt0dDlnUkpta2lsaWQ5cmVIRDFNaXlDc3JsaGdBNk1FOFNa?= =?utf-8?B?REswcDQ5cmVyb1VHcUdGemNxdkhPRGUyRHhQYlE2T0pKa21GbElhMHNLdmpG?= =?utf-8?B?QUlmaHVEUEw0YkwyRWhSZVhOcGpqSDN5dDNHVEpVcWFwaDUvOHBzdGtJRWd0?= =?utf-8?B?dVdhVzE3UkcwU1RYMEQvOHViK1VqYllaeitzdzhRWGVieWxYV2x3d0Y1UU1M?= =?utf-8?B?UEhpYkE4cCtDclJKWjNNZVNHcW9LSjA1ZVRSK0p0UWFDTmMwRGEzY0pvQ0VM?= =?utf-8?B?STVwbUN6Ynh3ckhvdkQ4S2pZZU1yK1RiQ0U0c3p5anVyWk5CTTQ0WHB4eEhs?= =?utf-8?B?R0FVZ0dQS1N5OERaVlBNOWhUSndoTTNaU3FSRG1rVWt1bVE0eWRWb0RBYVhs?= =?utf-8?B?bkdKdDVZbUo5ZjhvRnBNblc5dDRaRXU0eVBPSG51LzA4dmJXZlVkZnVxVFhT?= =?utf-8?B?cHppQmo4cFNKTStyMnJPcFdReFVSMjdicGEzU003Zld4S2lMUWprSDNzMlNW?= =?utf-8?B?dllaSWxCekhqNGpOSlRsblE5a1RxczNjb2hhUlI0c3hJVytpS3FYdUdKVThy?= =?utf-8?B?N3FsaURPeno2UGlHT3JXN2NvMWE3U1RBOTViTzBQZWtLdzRtSTdXYzg4R0pl?= =?utf-8?B?QnRpcUFVbUI1dTlXS3R5RWhGb1I0QTdDdEd1REJBdUx6cjJBYk5UN0xmbE1H?= =?utf-8?B?ams5UWl6c25LY0dpYmVqZlVQa0ZEVFJXekZDa3VheGxzTHdENWd6cGZuQWMx?= =?utf-8?B?YU1mR3E3aHN3MWtWVldXUXlyWWpab01YeC9yblhObFo1bG9tWEUxY0lzRXE2?= =?utf-8?B?dVZ5TEY3clNwT3I1Uy9EZkVMM3U1WTViR0NsRFZGMTFSemhid3dWZXptT2lj?= =?utf-8?B?OGwwdFRTTFhlUzlDcWZUc0wyeThtSmNTQ1hFczJxd3hPTk0xYUc1ay9NZGtR?= =?utf-8?B?djE0TUdFOWUzNy9jREhIZG95MlZHUzVPdUl0Q0FUS1dNam96aGlIK0F2YmVm?= =?utf-8?B?YlcyTkR0UFB0MWNIMVQyeUNQOE95QnpCY2pJMHY5OTdWcmRrYjZ3U3dYNWtJ?= =?utf-8?B?SnFoWCs2Wlc1cEhEdzVEUnRqajFBdlFiY1lDVm9Wdk1oRjRhbzFGdy9QMUFU?= =?utf-8?B?MEdFQ3piSWdMK2E2RHJ5RUxWSklOVldyQ05uU0lDUnYvcWNsaWpFY2hxcXBs?= =?utf-8?B?UHI5ZmNpcDJSbnpWZDd0WkkraU9MOGlsdnROb2lTU09GSW4zOHRuVzdJWHdw?= =?utf-8?B?YSt6T1lUdFZNMlVHYVNSWXpOcHJsZUhEcTgzYm5nZWdzLzFvQW5RbUdZMzBB?= =?utf-8?B?T2RiaXZVdGFJSjVqRnpLU0xKbFcxWGg1N0tQRWgxODN2K1AzMTQ5dnVBOFNF?= =?utf-8?B?TCswRlV2NWNOTVI3SWZHZTZ0SkYva3lkbGo3T1BFT3g0c0R2Uk1wQWIyV2lG?= =?utf-8?B?WGlqclMwNkcxeDlPNTczNkRrMGtjL3d3YWNadUl3TUhCTlVNcXZqNnRLL29P?= =?utf-8?B?cE1jL3Uxd1B4Y3htUExheXpVTzliT3FXRjNYMlFURmtLbWhUWTFrS0JNRy9x?= =?utf-8?B?ZDQvMjZVM3ZpTk4xQzJVNFRLUDFBQjdReFRUSkFac05RemFhSzZHL3JZVjBk?= =?utf-8?B?NGNxa01ZT3BGYkE1OHZPWUIxUFlZSTgwVEdUWGRnSnZvRnNiSjArbVJUczNr?= =?utf-8?B?SkY0VmFvcXcvSURWZkNpMEtmWjB5cEE1Nkp5MzcvdnQxZGFEeFhGRlkwbHJr?= =?utf-8?B?dVpCRXFLOW5uRERHL0FZTHh5Tko0VXZDOHBpNDc5QWxHODYxQTFML2NNb3Rp?= =?utf-8?B?Ym1LZmV5ZWNhTHlFdWdBUWJpdnlVRkdNdzg5TnloV0pPdjYyQmFZM0ltL1A4?= =?utf-8?B?dUx5d052U2JuY0Q2WkhERGE5b1NkazRoYmpWZGt5OEZMWUZZYUMrNXk1cDhM?= =?utf-8?B?NGszUUpFYVdCT1lYaHhtdVA4VnZwL1A1bzVNOXBmV0MzblhsbThqODEyN21W?= =?utf-8?B?cFBadUtWaStJWU9abm9TS1JBZXhveFRSeEkwcEFaWkxvMUhuZjM5ekFheWVV?= =?utf-8?B?bVlaM2NrL0tHYmg2YVRXSHVDczhPRUZ0WEVmVlV3alFnSENoMVA3d2pOTVJH?= =?utf-8?Q?IxZo+nvhp7UOh4Ke4CQcbIGdL?= X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:CH7PR03MB7860.namprd03.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(376014)(366016)(1800799024)(7053199007);DIR:OUT;SFP:1101; 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This machine has > 8 NUMA nodes and 960 CPUs. The hang occurs in roughly 1.5% of the boot > attempts in MTRR initialization of the APs. Do you know why you get hard lockups? Is some watchdog triggering on Linux? Otherwise I think it should just be slow, but ultimately succeed? > I have sent a small patch series to LKML which seems to fix the problem: > https://lore.kernel.org/lkml/20260121141106.755458-1-jgross@suse.com/ > > As Xen MTRR handling is taken from the Linux kernel, I guess the same > problem could happen in Xen, too. > > As the hang always occurred while waiting for the lock, which is > serializing the single CPUs doing MTRR initialization, my solution was > to eliminate the lock, allowing all APs to init MTRRs in parallel. > > Maybe we want to do the same in Xen. Hm, yes, I think Xen would be equally affected with regards to being contented on a lock while updating MTRRs. The MTRR initialization is deferred until all APs are up, and serialized on the set_atomicity_lock lock. Regards, Roger.