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Mon, 26 Oct 2026 17:22:33 +0000 Message-ID: <20d0feae-9aa0-43cf-8357-fcdf617a5a97@intel.com> Date: Mon, 26 Oct 2026 10:22:30 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 00/28] KVM: Enable FRED support with KVM VMX Content-Language: en-US To: Sean Christopherson , "H. Peter Anvin" CC: Xin Li , , , "Paolo Bonzini" , Jonathan Corbet , Shuah Khan , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Andy Lutomirski , "Peter Zijlstra" , Andrew Cooper , Tom Lendacky , Nikunj A Dadhania , Shivansh Dhiman , David Woodhouse , Chao Gao , Binbin Wu , Zhao Liu , Yosry Ahmed , David Matlack , , , References: <20260911213659.2025974-1-sohil.mehta@intel.com> <9467c7b6-d797-4462-bee1-1406bb8469cd@zytor.com> From: Sohil Mehta In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR13CA0076.namprd13.prod.outlook.com (2603:10b6:a03:2c4::21) To CY8PR11MB7846.namprd11.prod.outlook.com (2603:10b6:930:79::14) X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CY8PR11MB7846:EE_|SA0PR11MB4544:EE_ X-MS-Office365-Filtering-Correlation-Id: 35be7fe1-cffa-4960-ba48-08df3385b8b0 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|23010399003|376014|7416014|261026140227099003|11063799006|4143699003|56012099006|261022105027099003|261022075727099003|261021123027099003|18002099003|22082099003|10067099003; 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So, compile tested only... > Will test this out on real hardware with the MSR_STAR change below. > > +static bool kvm_is_msr_used_in_host(u32 msr) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_FRED)) > + return true; > + > + switch (msr) { > + case MSR_STAR: AFAIU, MSR_STAR is still used with FRED so it shouldn't be included here. The Spec says in section 4.4: IA32_STAR: - Existing use: this MSR is used by the existing operation of SYSCALL and SYSRET. - FRED use: FRED event delivery loads the CS and SS selectors with values derived from IA32_STAR[47:32] (see Section 5.3). ERETU uses the value of IA32_STAR[63:48] to determine how to load the CS and SS registers (see Section 6.2.1). It is expected that operating systems will set IA32_STAR[33:32] to 00B and IA32_STAR[49:48] to 11B. Likewise, the kernel also writes to MSR_STAR during syscall_init() and it has a comment saying: "Except the IA32_STAR MSR, there is NO need to setup SYSCALL and SYSENTER MSRs for FRED, .." > + case MSR_LSTAR: > + case MSR_CSTAR: > + case MSR_SYSCALL_MASK: > + return false; > + default: > + break; > + } > + return true; > +} > +