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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=RFeRNuA3VITJtjtCWgnIR1gSeyfuKcFlaE9dfPbek+ei/rtJ6ELFEox78CBB3bzOK 41LaibC4KCT1SwB6sJF4gQjRa6xA2MLNTn6+mLbk3i9Em5US8txbCR18bOwjsvqbZl yIdXK9FCN68rSPUICMMOTILqvW6O8zL4KL8ztDXzOEeXUfIvAJIS2yrfyHDei1Cr5u VTWrH/q2bNR9mq3uIst28+AHUch6ra6e+3+eep6a6kOl2yeeuZWoL9Y14ZLOAw6roO JJLicnkGRn3I/+qPwc6Te+887dVQiOciXlHKThq9Xo7ykEmsWT7e58GB5aZcK8CQOW sHY8T5/tXyoSw== Message-ID: <15fd8eaf-ba71-4f0e-9aed-5f7a6f5a4ea7@zytor.com> Date: Mon, 28 Apr 2025 00:44:06 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs To: Chao Gao , Sean Christopherson Cc: Rick P Edgecombe , "kvm@vger.kernel.org" , "ebiggers@google.com" , Dave Hansen , "dave.hansen@linux.intel.com" , Stanislav Spassov , "levymitchell0@gmail.com" , "samuel.holland@sifive.com" , Xin3 Li , "linux-kernel@vger.kernel.org" , Weijiang Yang , "mingo@redhat.com" , "pbonzini@redhat.com" , "tglx@linutronix.de" , "john.allen@amd.com" , "mlevitsk@redhat.com" , Chang Seok Bae , "vigbalas@amd.com" , "peterz@infradead.org" , "hpa@zytor.com" , "bp@alien8.de" , "aruna.ramakrishna@oracle.com" , "x86@kernel.org" References: <20250410072605.2358393-1-chao.gao@intel.com> <20250410072605.2358393-4-chao.gao@intel.com> <4a4b1f18d585c7799e5262453e4cfa2cf47c3175.camel@intel.com> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 4/27/2025 8:26 PM, Chao Gao wrote: >> For KVM, it's just the one MSR, and KVM needs to support save/restore of that MSR >> no matter what, so supporting it via XSAVE would be more work, a bit sketchy, and >> create yet another way for userspace to do weird things when saving/restoring vCPU >> state. > Agreed. One more issue of including CET_S into KVM_GET/SET_XSAVE{2} is: > > XSAVE UABI buffers adhere to the standard format defined by the SDM, which > never includes supervisor states. Attempting to incorporate supervisor states > into UABI buffers would lead to many issues, such as deviating from the > standard format and the need to define offsets for each supervisor state. Good point. FRED RSPs are in the supervisor state and are not defined within the XSAVE area. Their save/restore operations need to be explicitly added one by one. Would it be sensible to introduce a KVM supervisor MSR state that includes new MSRs? This approach is similar to what KVM has with its CPUID API, allowing batch MSR set/get operations and improving MSR code sharing. Since the FRED state is entirely defined in MSRs (except for CR4.FRED), this should simplify the context save and restore for FRED. Thanks! Xin