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Wed, 30 Apr 2025 15:01:47 +0000 Message-ID: <281354d3-1f04-483d-a6d0-baf6fdcec376@intel.com> Date: Wed, 30 Apr 2025 08:01:44 -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" , "vigbalas@amd.com" , "pbonzini@redhat.com" , "mlevitsk@redhat.com" , "john.allen@amd.com" , "Yang, Weijiang" , "tglx@linutronix.de" , "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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It uses fields configured in struct > fpu_guest. If fpu_user_cfg.default_features changes value (in the current code) > it would change KVM's uABI. Not quite. The ABI reflects the XSAVE format directly. The XSAVE header indicates which feature states are present, so while the _contents_ of the buffer may vary depending on the feature set, the _format_ itself remains unchanged. That doesn't constitute a uABI change. > It should be simple. Two new configuration fields are added in this patch that > match the existing concept and values of existing configurations fields. Per > Sean, there are no plans to have them diverge. So why add them. I'm fine with dropping them -- as long as the resulting code remains clear and avoids unnecessary complexity around VCPU allocation. Here are some of the considerations that led me to suggest them in the first place: * The guest-only feature model should be established in a clean and structured way. * The initialization logic should stay explicit -- especially to make it clear what constitutes guest features, even when they match host features. That naturally led to introducing a dedicated data structure. * Since the VCPU FPU container includes struct fpstate, it felt appropriate to mirror relevant fields where useful. * Including user_size and user_xfeatures made the VCPU allocation logic more straightforward and self-contained. And to clarify -- this addition doesn’t necessarily imply divergence from fpu_guest_cfg. Its usage is local to setting up the guest fpstate, and nothing more.