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

[Xin Li <xin@xxxxxxxxx>]

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;