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Date: Fri, 23 Jan 2026 10:24:38 +0100 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: =?utf-8?B?SsO8cmdlbiBHcm/Dnw==?= Cc: Andrew Cooper , "xen-devel@lists.xenproject.org" , Jan Beulich Subject: Re: MTRR init sequence in Xen Message-ID: References: <145af46c-eab5-4d0f-ba35-4ae646c0e4cd@suse.com> <64adf935-1fd8-4d3c-b9b2-cafb0e065463@suse.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <64adf935-1fd8-4d3c-b9b2-cafb0e065463@suse.com> X-ClientProxiedBy: MA3P292CA0060.ESPP292.PROD.OUTLOOK.COM (2603:10a6:250:49::19) To BY1PR03MB7875.namprd03.prod.outlook.com (2603:10b6:a03:5b1::10) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BY1PR03MB7875:EE_|SJ0PR03MB7127:EE_ X-MS-Office365-Filtering-Correlation-Id: 6abb09ed-c068-4726-391c-08de5a613dae X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|7053199007; X-Microsoft-Antispam-Message-Info: =?utf-8?B?TW9EMzBZMU5Cc0hsL1l1R2Z2c1ZRZG9XdTJwWWZqV2N1TDFsTzJ2NGJPT1lZ?= =?utf-8?B?NEU0U2dXeHpGNGJ2RHQwYmN3RHlMUHZLSHdJaHlZNzBRaXlQUEk4cFpYS2Yv?= =?utf-8?B?RmVpWVhMM2NWRHVndXUvVnBSdGg4cmZiSzBBRTRsUXc5bDB5aWloK2lqNDQx?= =?utf-8?B?SHk4aWNvQXVrYURWR0VSRHJVOFp4Q0wwV2krNHM2Y0g5MUZkVFdnRXI0V1d1?= =?utf-8?B?UDhzOWRFTEhReDRZcW5kUWpBbS9jRWRyaHRlU1B3S292ejdqSmpJdXN5ZmFL?= =?utf-8?B?OThlVzJaWHpOMnRudE9BZU1hMTYzOWlYQlVOZEwyVE5ZZUNLYVAwWnRzUjd5?= =?utf-8?B?UStQM0diVVgydDhMZVAvME9jNm9jc2Y1b1RHZFY3TGZzQkpnSlUrS3pMbElI?= =?utf-8?B?TVBxVllENkxkdzZmV1V6YmVoWkNLcm9LMlU2Z0ZleWd6ZlgzWWhKTjdpWW1Z?= =?utf-8?B?Q3d4YWdqRm1DSCtZVWxpVDJvT3Q2dUNUbWx5bitmVzJmejhSdVlQY1l6NTli?= =?utf-8?B?N2ttQnJrd2pXL1NUWnlTbmM5amVUb1pZSFFITlhXTUJBY1h0WGVGODRGME5G?= =?utf-8?B?V3JVYTNVc0NjbUxDSGJvV1crRHp0TUpHSG4yZm53NENpMEw0N0wraithakpi?= =?utf-8?B?aCtaWTVCajk3NXlBWENQUVJyNU5JbHJqUjNqOE91VmYwQUJBVmo1T093OTVQ?= =?utf-8?B?cFZUMXBTVU4weERqdXREbHoxcGR3RlY0UkZEU1krMU1uWGpLL0tKRStPSWZI?= =?utf-8?B?dWxCNVdsV3M0d01tajhnL2lseUZpdG1qUUM1MlA5dnQ1QXlHNlp0aUw3Vkla?= =?utf-8?B?TEloTXFvOVBkRjhOb0tuQUxTNmtIM29xOXpZbncrTjY1Qi9LRVRPTG9iK1Fh?= =?utf-8?B?TC9GYkp2d2VlMk9sR2MvSGhJWFNOWTg3NjJtdXBicm1YZ2ZCdkZpeVpLM3JZ?= =?utf-8?B?M2pwYmI3eHhOdGJHMWhrVTI1WktMUWlqdjNMVGZ6NmMxT2VvUlBqVDN1c0Jt?= =?utf-8?B?T3E3c25TN2kxYUxOOXdFUjFJUFJwSEMyTDFWUlVqUGQzcUFMQmMvL0xrYmF5?= =?utf-8?B?akRuUUxXNys0RC9ZNjNUbDFqYjROOEJjNEVjOEF5QzRXblhMVVJhYWdCUENG?= =?utf-8?B?TkpzN1VXUzlhTk1UZFBBbUo5SVVqQXZ4Vm9ZU2Q3cXNWYUFUd1YrUU9hODlo?= =?utf-8?B?RHJDL2ZiSGxGcUt2NldCM3Q0NWJCZE5MSnV6dU5objZJRHlRMU04UDdCRXh4?= =?utf-8?B?UGRGUnJMUDRjRTBrNXBCTlA5L240YnNpK3lmaGtScG9zbjJYOGpMenZtY0lL?= =?utf-8?B?Mkx2UndCRWpaMEFsd0toem5QaFViM2VBMDVzbWwreXpNV2R4SWdsTjRPakFR?= =?utf-8?B?ZUg5VEdwOWY0Y0h6OW5nRnZoYWJ2NTZVbmp3RzZUcW1TdmRGeG9oZ0dHR3oz?= =?utf-8?B?Q2syZ2RzMGdTcW5uUlRuazdwdmVEdUdMU21hWlBoVm5ZdzdyRnNnT3FUblN4?= =?utf-8?B?cGUzdHd5eDJydHV2cnNVMTNsU3IrYmZZNTFtMjBZZmtrM3FMalMvOGp5QTVi?= =?utf-8?B?YjkzSzVDME1uOXE2MDRGWU1RWTU4OWNvWW1RVDBXR2tzb1VBN1FXWmExRDJE?= =?utf-8?B?cElNOEkvTG1GWEk2djZMUVMrc0F1TFdMVU03aHBPL3NuWFJ4c2pXKzFLR001?= =?utf-8?B?WTI0ZzAyaytralR4RlR1VElEMlBCVmZPWTE2cExxZVBKRlJvSnBMNmx2NDd0?= =?utf-8?B?MUhCQlFvVnJFSDhvT1cvNTJZUEdGY1VaU2pJckdkZ1pFVTRpZWtIbm9BOCtH?= =?utf-8?B?NENML3pvZXgrUU9vQWhtc1JzK0JjVFRUTXhSRHpPUEx4NHhlWU5wenUyMVFu?= =?utf-8?B?NFFiMDlPWDlWSWMyL2lwRU1ydnZ1QXZjZXJ0RlhXNnZSVVltSHg1Q3ltYWFa?= =?utf-8?B?WXJRdEVBRFMrZ1o3MWFway9JTFVOeXFKTDN5cldmdmc2NnlrdFVNSWt5VUFY?= =?utf-8?B?VEtLdnNYdnR1ZFNQbENkZXVydU5sWGRYMUVPRWV0ak9ORXhqVTIySmptRjdk?= =?utf-8?B?RERENXlVdm14d29kYS9kYjd4RXF0aFgrWEEzcFh1QnVoMGtQTHlHdHZhMmx0?= =?utf-8?Q?rqPOJBL7Rk4WzV+YVyaEn7iTA?= X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:BY1PR03MB7875.namprd03.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(1800799024)(376014)(366016)(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. > > > > > > > > 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. > > > > > > I suspect Xen might be insulated by the fact that we don't have parallel > > > AP start (yet), so we don't have the whole system competing on the > > > spinlock at once. > > > > Oh, I think I've misunderstood the issue. Linux is doing MTRR init in > > the AP startup path, and so if it takes too long Linux will report > > that the AP has failed to start. > > No, Linux is deferring the MTRRs until all APs are up, just like Xen > (or Xen does it like Linux). > > > > > This is not an issue on Xen because MTRR initialization is deferred > > until all APs are up, and hence is not part of the timed AP start > > path. This optimization was done in: > > > > 0d22c8d92c6c x86: CPU synchronization while doing MTRR register update > > > > So even if we did parallel AP startup we won't likely be affected, > > because we would still defer the MTRR setup until all APs are up. > > We will be affected, as its the deferred MTRR setup which is the > problem. If it's the watchdog NMI then than won't be possible on Xen, as the watchdog is setup after the MTRR synchronization step. We should however fix it even if it's not a fatal issue on Xen. I assume the avoidance of locking will make a very noticeable performance difference in boot times. Thanks, Roger.