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Wed, 18 Mar 2026 13:54:49 +0000 Message-ID: Date: Wed, 18 Mar 2026 06:54:46 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/3] x86/cpu: Enable FSGSBASE early in cpu_init_exception_handling() Content-Language: en-US To: Nikunj A Dadhania , , , , , CC: , , , , , , , , References: <20260318075654.1792916-1-nikunj@amd.com> <20260318075654.1792916-2-nikunj@amd.com> From: Sohil Mehta In-Reply-To: <20260318075654.1792916-2-nikunj@amd.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR03CA0189.namprd03.prod.outlook.com (2603:10b6:a03:2ef::14) To DS0PR11MB7997.namprd11.prod.outlook.com (2603:10b6:8:125::14) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7997:EE_|LV4PR11MB9491:EE_ X-MS-Office365-Filtering-Correlation-Id: 80c50e22-32ab-41ff-91f7-08de84f5eba6 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014|56012099003|22082099003|18002099003; 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I would suggest a minor correction to the code comment and the commit log. I found it slightly misleading. It can probably be fixed up while applying as well. On 3/18/2026 12:56 AM, Nikunj A Dadhania wrote: > Move FSGSBASE enablement from identify_cpu() to > cpu_init_exception_handling() to ensure it is enabled before any exceptions > can occur on both boot and secondary CPUs. > It would be more accurate to say ".. before any exceptions that uses paranoid_entry() can occur.." I think early exceptions such #VC can still occur and the bringup_idt_table is set up to handle those. > == Background == ... > + /* > + * On CPUs with FSGSBASE support, paranoid_entry() uses > + * ALTERNATIVE-patched RDGSBASE/WRGSBASE instructions. Secondary CPUs > + * boot after alternatives are patched globally, so early exceptions > + * execute patched code that depends on FSGSBASE. Enable the feature > + * before any exceptions occur. On similar lines. I suggest getting rid of the last line: "Enable the feature before any exceptions occur." The rest looks fine to me. > + */ > + if (cpu_feature_enabled(X86_FEATURE_FSGSBASE)) { > + cr4_set_bits(X86_CR4_FSGSBASE); > + elf_hwcap2 |= HWCAP2_FSGSBASE; > + } > + Reviewed-by: Sohil Mehta