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Wed, 4 Feb 2026 18:53:33 +0000 Message-ID: <722b53a7-7560-4a1b-ab26-73eeed3dffa5@intel.com> Date: Wed, 4 Feb 2026 10:53:28 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] x86/x2apic: disable x2apic on resume if the kernel expects so Content-Language: en-US To: Shashank Balaji CC: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , , "H. Peter Anvin" , Suresh Siddha , "K. Y. Srinivasan" , "Haiyang Zhang" , Wei Liu , Dexuan Cui , Long Li , Ajay Kaher , Alexey Makhalov , Broadcom internal kernel review list , Jan Kiszka , Paolo Bonzini , Vitaly Kuznetsov , Juergen Gross , Boris Ostrovsky , Ingo Molnar , , , , , , , Rahul Bukte , Daniel Palmer , Tim Bird , References: <20260202-x2apic-fix-v1-0-71c8f488a88b@sony.com> <20260202-x2apic-fix-v1-1-71c8f488a88b@sony.com> <0149c37d-7065-4c72-ab56-4cea1a6c15d0@intel.com> From: Sohil Mehta In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR03CA0064.namprd03.prod.outlook.com (2603:10b6:a03:331::9) To DS0PR11MB7997.namprd11.prod.outlook.com (2603:10b6:8:125::14) Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7997:EE_|DS0PR11MB8163:EE_ X-MS-Office365-Filtering-Correlation-Id: 8057de85-c57d-46e6-3aab-08de641eb22b X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014; 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I meant the X86_FEATURE_X2APIC and not X86_FEATURE_APIC. But, thinking about it more, checking that the CPU is really in X2APIC mode by reading the MSR is good enough. > x2apic_enabled also does the same checks, > the only difference being, it uses rdmsrq_safe instead of just rdmsrq, > which is what __x2apic_disable uses. The safe version is because of > Boris' suggestion [1]. If that's applicable here as well, then rdmsrq in > __x2apic_disable should be changed to rdmsrq_safe. I don't know if there is a strong justification for changing to rdmsrq_safe() over here. Also, that would be beyond the scope of this patch. In general, it's better to avoid such changes unless an actual issue pops up. > >> I considered if an error message should be printed along with this. But, >> I am not sure if it can really be called a firmware issue. It's probably >> just that newer CPUs might have started defaulting to x2apic on. >> >> Can you specify what platform you are encountering this? > > > I'm not sure it's the CPU defaulting to x2apic on. As per Section > 12.12.5.1 of the Intel SDM: > > On coming out of reset, the local APIC unit is enabled and is in > the xAPIC mode: IA32_APIC_BASE[EN]=1 and IA32_APIC_BASE[EXTD]=0. > > So, the CPU should be turning on in xapic mode. In fact, when x2apic is > disabled in the firmware, this problem doesn't happen. > It's a bit odd then that the firmware chooses to enable x2apic without the OS requesting it. Linux maintains a concept of X2APIC_ON_LOCKED in x2apic_state which is based on the hardware preference to keep the apic in X2APIC mode. When you have x2apic enabled in firmware, but the system is in XAPIC mode, can you read the values in MSR_IA32_ARCH_CAPABILITIES and MSR_IA32_XAPIC_DISABLE_STATUS? XAPIC shouldn't be disabled because you are running in that mode. But, it would be good to confirm. > Either way, a pr_warn maybe helpful. How about "x2apic re-enabled by the > firmware during resume. Disabling\n"? I mainly want to make sure the firmware is really at fault before we add such a print. But it seems likely now that the firmware messed up.