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Tue, 29 Apr 2025 03:22:41 +0000 Message-ID: Date: Mon, 28 Apr 2025 20:22:38 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs To: "Edgecombe, Rick P" CC: "ebiggers@google.com" , "kvm@vger.kernel.org" , "Hansen, Dave" , "dave.hansen@linux.intel.com" , "Spassov, Stanislav" , "levymitchell0@gmail.com" , "samuel.holland@sifive.com" , "Li, Xin3" , "linux-kernel@vger.kernel.org" , "mingo@redhat.com" , "Yang, Weijiang" , "pbonzini@redhat.com" , "tglx@linutronix.de" , "john.allen@amd.com" , "mlevitsk@redhat.com" , "vigbalas@amd.com" , "hpa@zytor.com" , "peterz@infradead.org" , "aruna.ramakrishna@oracle.com" , "Gao, Chao" , "bp@alien8.de" , "seanjc@google.com" , "x86@kernel.org" References: <20250410072605.2358393-1-chao.gao@intel.com> <20250410072605.2358393-4-chao.gao@intel.com> <4a4b1f18d585c7799e5262453e4cfa2cf47c3175.camel@intel.com> <6ca20733644279373227f1f9633527c4a96e30ef.camel@intel.com> <9925d172-94e1-4e7a-947e-46261ac83864@intel.com> Content-Language: en-US From: "Chang S. 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Bae wrote: >> On 4/28/2025 8:42 AM, Edgecombe, Rick P wrote: >>> >>> Right, so there should be no need to keep a separate features and buffer size >>> for KVM's xsave UABI, as this patch does. Let's just leave it using the core >>> kernels UABI version. >> >> Hmm, why so? >> >> As I see it, the vcpu->arch.guest_fpu structure is already exposed to >> KVM. This series doesn’t modify those structures (fpu_guest and >> fpstate), other than removing a dead field (patch 2). >> >> Both ->usersize and ->user_xfeatures fields are already exposed -- >> currently KVM just doesn’t reference them at all. >> >> All the changes introduced here are transparent to KVM. Organizing the >> initial values and wiring up guest_perm and fpstate are entirely >> internal to the x86 core, no? > > This patch adds struct vcpu_fpu_config, with new fields user_size, > user_features. Then those fields are used to configure the guest FPU, where > today it just uses fpu_user_cfg.default_features, etc. > > KVM doesn't refer to any of those fields specifically, but since they are used > to configure struct fpu_guest they become part of KVM's uABI. Today, fpu_alloc_guest_fpstate() -> __fpstate_reset() sets vcpu->arch.guest_fpu.fpstate->user_xfeatures using fpu_user_cfg.default_features. Are you really saying that switching this to guest_default_cfg.user_features would constitute a uABI change? Do you consider fpu_user_cfg.default_features to be part of the uABI or anything else?