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Wed, 19 Aug 2026 05:03:07 +0000 Message-ID: <2f8e907e-1c77-416d-90be-9428ce8242f6@intel.com> Date: Tue, 18 Aug 2026 22:03:03 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 2/7] KVM: x86: Use linear_read_system() to read the TSS I/O bitmap Content-Language: en-US To: Binbin Wu CC: , , Sean Christopherson , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H . Peter Anvin" , Shuah Khan , Peter Zijlstra , "Chang S . Bae" , Kai Huang , Fuad Tabba , Chao Gao , Yosry Ahmed , Claudio Imbrenda , David Matlack , Bala-Vignesh-Reddy , Kishen Maloor , Rick Edgecombe , , References: <20260806011536.4172258-1-sohil.mehta@intel.com> <20260806011536.4172258-3-sohil.mehta@intel.com> <41a327bd-cbbd-4530-9f30-9175c5840014@linux.intel.com> From: Sohil Mehta In-Reply-To: <41a327bd-cbbd-4530-9f30-9175c5840014@linux.intel.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR03CA0007.namprd03.prod.outlook.com (2603:10b6:a03:33a::12) 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_|DM4PR11MB6407:EE_ X-MS-Office365-Filtering-Correlation-Id: a5cb5997-2a98-4cc7-62dc-08defdaf281d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|376014|7416014|366016|1800799024|18002099003|3023799007|22082099003|56012099006|11063799006|10067099003|4143699003; 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On 8/18/2026 8:19 PM, Binbin Wu wrote: > On 8/6/2026 9:15 AM, Sohil Mehta wrote: >> TSS I/O permission bitmap reads are implicit supervisor accesses which >> are subject to Linear Address Space Separation (LASS) enforcement. > > I am not sure about it. > > According to SDM vol 3a (253668-089US), Chapter "LINEAR-ADDRESS PRE-PROCESSING", > section "MODE-BASED ACCESSES AND LINEAR-ADDRESS-SPACE PARTITIONING", it defines > and limits "implicit supervisor-mode accesses" to the following operations: The way I understand it, this list isn't exhaustive. The wording in those sections, "Such accesses include the following.." and "Examples of such accesses include.." suggests that this is not limiting. > - accesses to the global descriptor table (GDT) or local descriptor table (LDT) > to load a segment descriptor > - accesses to the interrupt descriptor table (IDT) when delivering an interrupt > or exception > - accesses to the task-state segment (TSS) as part of a task switch or change of > CPL > - accesses to a user posted-interrupt descriptor (UPID) during user-interrupt > notification processing > For example, this doesn't list the SENDUIPI instruction's access to the UITT (IA32_UINTR_TT). That one is an implicit supervisor-mode access to a kernel address at CPL = 3. In section 5.6.1 (Determination of Access Rights), immediately following the above examples the SDM says: "All these accesses are called implicit supervisor-mode accesses regardless of CPL." > IIUC, the TSS I/O bitmap read during ordinary instruction execution > (under CPL = 3) rather than during a task switch or CPL transition, I don't > think they fall under the classification of implicit supervisor-mode accesses. > AFAIU, the entire TSS struct (including the I/O bitmap portion) is mapped at struct cpu_entry_area using CPU_ENTRY_AREA_BASE with PAGE_KERNEL_RO permissions. So bit 63 will always be 1 and U/S = 0. Yes, the TSS I/O bitmap access happens at CPL = 3 but it still has to be an implicit supervisor access. If it weren't, any user instruction access to the I/O bitmap would always generate a fault (even without LASS), right? Sohil