linux-kernel - Re: [PATCH v6 06/20] KVM: VMX: Set FRED MSR intercepts

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Message-ID: <5b1c5f80-bbe1-4294-8ede-5e097e8feda1@zytor.com>
Date: Wed, 27 Aug 2025 15:24:58 -0700
From: Xin Li <xin@...or.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, pbonzini@...hat.com, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        luto@...nel.org, peterz@...radead.org, andrew.cooper3@...rix.com,
        chao.gao@...el.com, hch@...radead.org
Subject: Re: [PATCH v6 06/20] KVM: VMX: Set FRED MSR intercepts

On 8/26/2025 3:17 PM, Sean Christopherson wrote:
>> +		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
>> +			wrmsrns(MSR_IA32_FRED_SSP0, vmx->msr_guest_fred_ssp0);
> FWIW, if we can't get an SDM change, don't bother with RDMSR/WRMSRNS, just
> configure KVM to intercept accesses.  Then in kvm_set_msr_common(), pivot on
> X86_FEATURE_SHSTK, e.g.


Intercepting is a solid approach: it ensures the guest value is fully
virtual and does not affect the hardware FRED SSP0 MSR.  Of course the code
is also simplified.


> 
> 	case MSR_IA32_U_CET:
> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> 		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> 			WARN_ON_ONCE(msr != MSR_IA32_FRED_SSP0);
> 			vcpu->arch.fred_rsp0_fallback = data;
> 			break;
> 		}
> 
> 		kvm_set_xstate_msr(vcpu, msr_info);
> 		break;
> 
> and
> 
> 	case MSR_IA32_U_CET:
> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> 		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> 			WARN_ON_ONCE(msr_info->index != MSR_IA32_FRED_SSP0);
> 			vcpu->arch.fred_rsp0_fallback = msr_info->data;
> 			break;
> 		}
> 
> 		kvm_get_xstate_msr(vcpu, msr_info);
> 		break;