Re: [PATCH v3 1/5] KVM: x86: SVM: Emulate reads and writes to shadow stack MSRs

[Chao Gao <chao.gao@intel.com>]

On Wed, Aug 06, 2025 at 08:45:06PM +0000, John Allen wrote:
>Set up interception of shadow stack MSRs. In the event that shadow stack
>is unsupported on the host or the MSRs are otherwise inaccessible, the
>interception code will return an error. In certain circumstances such as
>host initiated MSR reads or writes, the interception code will get or
>set the requested MSR value.

The changelog does not match the code. This patch does not set up interception
for shadow stack MSRs; instead, it emulates shadow stack MSR read/write by
accessing the corresponding fields in the VMCB.

>
>Signed-off-by: John Allen <john.allen@amd.com>
>---
> arch/x86/kvm/svm/svm.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
>diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>index 6375695ce285..d4e27e70b926 100644
>--- a/arch/x86/kvm/svm/svm.c
>+++ b/arch/x86/kvm/svm/svm.c
>@@ -2776,6 +2776,15 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> 		if (guest_cpuid_is_intel_compatible(vcpu))
> 			msr_info->data |= (u64)svm->sysenter_esp_hi << 32;
> 		break;
>+	case MSR_IA32_S_CET:
>+		msr_info->data = svm->vmcb->save.s_cet;
>+		break;
>+	case MSR_IA32_INT_SSP_TAB:
>+		msr_info->data = svm->vmcb->save.isst_addr;
>+		break;
>+	case MSR_KVM_INTERNAL_GUEST_SSP:
>+		msr_info->data = svm->vmcb->save.ssp;
>+		break;
> 	case MSR_TSC_AUX:
> 		msr_info->data = svm->tsc_aux;
> 		break;
>@@ -3008,6 +3017,15 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
> 		svm->vmcb01.ptr->save.sysenter_esp = (u32)data;
> 		svm->sysenter_esp_hi = guest_cpuid_is_intel_compatible(vcpu) ? (data >> 32) : 0;
> 		break;
>+	case MSR_IA32_S_CET:
>+		svm->vmcb->save.s_cet = data;
>+		break;
>+	case MSR_IA32_INT_SSP_TAB:
>+		svm->vmcb->save.isst_addr = data;
>+		break;
>+	case MSR_KVM_INTERNAL_GUEST_SSP:
>+		svm->vmcb->save.ssp = data;
>+		break;
> 	case MSR_TSC_AUX:
> 		/*
> 		 * TSC_AUX is always virtualized for SEV-ES guests when the
>-- 
>2.34.1
>