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Fri, 25 Apr 2025 08:25:09 +0000 Date: Fri, 25 Apr 2025 16:24:55 +0800 From: Chao Gao To: "Edgecombe, Rick P" CC: "Hansen, Dave" , "seanjc@google.com" , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "tglx@linutronix.de" , "kvm@vger.kernel.org" , "pbonzini@redhat.com" , "ebiggers@google.com" , "dave.hansen@linux.intel.com" , "Spassov, Stanislav" , "levymitchell0@gmail.com" , "samuel.holland@sifive.com" , "Li, Xin3" , "Yang, Weijiang" , "mingo@redhat.com" , "mlevitsk@redhat.com" , "john.allen@amd.com" , "Bae, Chang Seok" , "vigbalas@amd.com" , "hpa@zytor.com" , "peterz@infradead.org" , "aruna.ramakrishna@oracle.com" , "bp@alien8.de" Subject: Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs Message-ID: References: <20250410072605.2358393-1-chao.gao@intel.com> <20250410072605.2358393-4-chao.gao@intel.com> Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: SI1PR02CA0006.apcprd02.prod.outlook.com (2603:1096:4:1f7::11) 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_|CO1PR11MB4804:EE_ X-MS-Office365-Filtering-Correlation-Id: f836eafa-2234-4f01-acd2-08dd83d2b12d 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|376014|7416014|366016; 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Includes all supported user features except independent >> + * managed features and features which have to be requested by >> + * user space before usage. >> + */ >> + unsigned int user_size; >> + >> + /* >> + * @features: >> + * >> + * The default supported features bitmap in guest FPUs. Does not >> + * include independent managed features and features which have to >> + * be requested by user space before usage. >> + */ >> + u64 features; >> + >> + /* >> + * @user_features: >> + * >> + * Same as @features except only user xfeatures are included. >> + */ >> + u64 user_features; >> +}; > >Tracing through the code, it seems that fpu_user_cfg.default_features and >guest_default_cfg.user_features are the same, leading to >fpu_user_cfg.default_size and guest_default_cfg.user_size being also the same. Right. This is primarily for readability and symmetry. I slightly prefer __guest_fpstate_reset() in this series: fpstate->size = guest_default_cfg.size; fpstate->user_size = guest_default_cfg.user_size; fpstate->xfeatures = guest_default_cfg.features; fpstate->user_xfeatures = guest_default_cfg.user_features; over this version: fpstate->size = guest_default_cfg.size; fpstate->xfeatures = guest_default_cfg.features; /* * use fpu_user_cfg for user_* settings for compatibility of exiting * uAPIs. */ fpstate->user_size = fpu_user_cfg.user_size; fpstate->user_xfeatures = fpu_user_cfg.default_features; Referencing different structures for size/xfeatures and their user_* counterparts is not elegant to me. The need for a comment indicates that this chunk may cause confusion. And this pattern will repeat when initializing fpu->guest_perm in fpstate_reset(). > >In the later patches, it doesn't seem to change the "user" parts. These >configurations end up controlling the default size and features that gets copied >to userspace in KVM_SET_XSAVE. I guess today there is only one default size and >feature set for xstate copied to userspace. The suggestion from Chang was that >it makes the code more readable, but it seems like it also breaks apart a >unified concept for no functional benefit. In the future, the feature and size of the uABI buffer for guest FPUs may differ from those of non-guest FPUs. Sean rejected the idea of saving/restoring CET_S xstate in KVM partly because: :Especially because another big negative is that not utilizing XSTATE bleeds into :KVM's ABI. Userspace has to be told to manually save+restore MSRs instead of just :letting KVM_{G,S}ET_XSAVE handle the state. And that will create a bit of a :snafu if Linux does gain support for SSS. *: https://lore.kernel.org/kvm/ZM1jV3UPL0AMpVDI@google.com/ [To be clear, it is not an issue caused by Chang's suggestion. v4 which adds new members @guest_size @guest_default_features to fpu_state_config has the same problem. i.e., fpu_user_cfg.guest_default_feaures is identical to fpu_user_cfg.default_features, adding no functional benefit.] > >Maybe we don't need user_features or user_size here in vcpu_fpu_config? Or did I I don't have a strong opinion on this. I am ok with dropping them. Do you have a strong preference? >get lost somewhere along the way in all the twists and turns that features and >sizes go through. No, your analysis is correct. > >