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h=Date:From:Subject:To:Cc:References:In-Reply-To:From; b=TXu8NwlRDu8fDbPcGjqill3BNsHgX0GvUITPYr9IBoHH3GCjfMG7577uDy1GKqRSz CS/PEXGldgwejkYOy9WWW9FY4RN3+rytjnTMM2BCv+n6fI/vT6jtINChJ9cYv9FZ1U rTYgdNC2504ShZyWQsoFCIzuAmI7iD+I33HtIroxOjnUymFIKUGH4n22PbuJmS6X2A PVW3daX2eaqMaVJWJ0mLbTJgOU7UQMuL0LZCQ+FOgorfCgtucKKINyDe38/Ji8Ji4D 5UJLjnqKnHzX0ueHBkY7xkagO426UpcGYNzrYHxqA8aiHBVasEmblnP4Km78MlXOOM rKLBIPycWoqEA== Message-ID: <2ed04dff-e778-46c6-bd5f-51295763af06@zytor.com> Date: Mon, 25 Aug 2025 23:54:46 -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 From: Xin Li Subject: Re: [PATCH v13 05/21] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs To: Chao Gao Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, john.allen@amd.com, mingo@redhat.com, minipli@grsecurity.net, mlevitsk@redhat.com, pbonzini@redhat.com, rick.p.edgecombe@intel.com, seanjc@google.com, tglx@linutronix.de, weijiang.yang@intel.com, x86@kernel.org References: <20250821133132.72322-1-chao.gao@intel.com> <20250821133132.72322-6-chao.gao@intel.com> Content-Language: en-US Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 8/24/2025 7:55 PM, Chao Gao wrote: >> static bool is_xstate_managed_msr(u32 index) >> { >> if (!kvm_caps.supported_xss) >> return false; >> >> switch (index) { >> case MSR_IA32_U_CET: >> case MSR_IA32_S_CET: >> case MSR_IA32_PL1_SSP ... MSR_IA32_PL3_SSP: >> return kvm_caps.supported_xss & XFEATURE_MASK_CET_USER && >> kvm_caps.supported_xss & XFEATURE_MASK_CET_KERNEL; >> default: >> return false; > This will duplicate checks in other functions. I slightly prefer to keep this > function super simple and do all capability checks in __kvm_{set,get}_msr() > or kvm_emulate_msr_{write,read}. > >> } >> } >> >> And it would be obvious how to add new MSRs related to other XFEATURE bits. > Just return true for all those MSRs, regardless of host capabilities. If > kvm_caps doesn't support them, those MSRs are not advertised to userspace > either (see kvm_probe_msr_to_save()). Loading or putting the guest FPU when > userspace attempts to read/write those unsupported MSRs shouldn't cause any > performance issues, as userspace is unlikely to access them in hot paths. There is no problem as of now, because there are only two CET related bits set in KVM_SUPPORTED_XSS. So if !CET, the two bits are cleared thus kvm_caps.supported_xss is 0, and kvm_load_guest_fpu() is never executed in __msr_io(). However after any new bit is added to KVM_SUPPORTED_XSS in future, if !CET, kvm_caps.supported_xss could be non-zero. There should still be no problem because we don't expect any access to CET MSRs. The trouble comes with MSR_IA32_PL0_SSP when FRED and !CET, because it will be accessed even !CET. And we need to have to do the following: static bool is_xstate_managed_msr(u32 index) { switch (index) { case MSR_IA32_U_CET: case MSR_IA32_PL1_SSP ... MSR_IA32_PL3_SSP: return true; case MSR_IA32_PL0_SSP: return kvm_caps.supported_xss & XFEATURE_MASK_CET_USER && kvm_caps.supported_xss & XFEATURE_MASK_CET_KERNEL; default: return false; } } Then it makes more sense to handle all CET MSRs consistently. Thanks! Xin