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Fri, 16 May 2025 09:02:53 +0000 Date: Fri, 16 May 2025 17:02:37 +0800 From: Chao Gao To: Uros Bizjak CC: , , , , , , , , , , , , , , Aruna Ramakrishna , Dave Hansen , Eric Biggers , "H. Peter Anvin" , "Ingo Molnar" , Kees Cook , Maxim Levitsky , Mitchell Levy , "Nikolay Borisov" , Oleg Nesterov , "Samuel Holland" , Sohil Mehta , Stanislav Spassov , Vignesh Balasubramanian , Zhao Liu Subject: Re: [PATCH v7 0/6] Introduce CET supervisor state support Message-ID: References: <20250512085735.564475-1-chao.gao@intel.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: KL1PR02CA0032.apcprd02.prod.outlook.com (2603:1096:820:d::19) To CH3PR11MB8660.namprd11.prod.outlook.com (2603:10b6:610:1ce::13) 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: CH3PR11MB8660:EE_|PH7PR11MB6546:EE_ X-MS-Office365-Filtering-Correlation-Id: 06368eb0-c130-4906-af5e-08dd94587148 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|376014|7416014|1800799024|366016; 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Most of >> patches have received Reviewed-by/Acked-by tags. >> >> Thanks Chang, Rick, Xin, Sean and Dave for their help with this series. >> >> == Changelog == >> v6->v7: >> - Collect reviews from Rick >> - Tweak __fpstate_reset() to handle guest fpstate rather than adding a >> guest-specific reset function (Sean & Dave) >> - Fold xfd initialization into __fpstate_reset() (Sean) >> - v6: https://lore.kernel.org/all/20250506093740.2864458-1-chao.gao@intel.com/ >> >> == Background == >> >> CET defines two register states: CET user, which includes user-mode control >> registers, and CET supervisor, which consists of shadow-stack pointers for >> privilege levels 0-2. >> >> Current kernel disables shadow stacks in kernel mode, making the CET >> supervisor state unused and eliminating the need for context switching. >> >> == Problem == >> >> To virtualize CET for guests, KVM must accurately emulate hardware >> behavior. A key challenge arises because there is no CPUID flag to indicate >> that shadow stack is supported only in user mode. Therefore, KVM cannot >> assume guests will not enable shadow stacks in kernel mode and must >> preserve the CET supervisor state of vCPUs. >> >> == Solution == >> >> An initial proposal to manually save and restore CET supervisor states >> using raw RDMSR/WRMSR in KVM was rejected due to performance concerns and >> its impact on KVM's ABI. Instead, leveraging the kernel's FPU >> infrastructure for context switching was favored [1]. > >Dear Chao, > >I wonder if the same approach can be used to optimize switching of >Intel PT configuration context. There was a patch series [1] posted >some time ago that showed substantial reduction of overhead when >switching Intel PT configuration context on VM-Entry/Exit using >XSAVES/XRSTORS instructions: No, the guest-only infrastructure utilizes the FPU core to switch states during context switches, whereas Intel PT state is switched at different points, i.e., on VM entry/exit. Switching Intel PT state on VM entry/exit is necessary only for the "host-guest" mode, which is currently marked as BROKEN. Unless functional issues are addressed first, there's no point in optimizing its state switching. If we ever reinstate support for the "host-guest" mode, I think Intel PT state probably could be implemented as an independent feature, similar to LBR state.