linux-kernel - Re: [PATCH v4 1/5] KVM: x86: SVM: Emulate reads and writes to shadow stack MSRs

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Message-ID: <aMSk3fY7XzScBuOx@google.com>
Date: Fri, 12 Sep 2025 15:55:25 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: John Allen <john.allen@....com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org, 
	pbonzini@...hat.com, dave.hansen@...el.com, rick.p.edgecombe@...el.com, 
	mlevitsk@...hat.com, weijiang.yang@...el.com, chao.gao@...el.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com, 
	tglx@...utronix.de, thomas.lendacky@....com
Subject: Re: [PATCH v4 1/5] KVM: x86: SVM: Emulate reads and writes to shadow
 stack MSRs

On Mon, Sep 08, 2025, John Allen wrote:
> Emulate shadow stack MSR access by reading and writing to the
> corresponding fields in the VMCB.
> 
> Signed-off-by: John Allen <john.allen@....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 e4af4907c7d8..fee60f3378e1 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2767,6 +2767,15 @@ 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;
> @@ -2999,6 +3008,15 @@ 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;

These writes should mark VMCB_CET (the dirty/clean flag) dirty, and obviously
KVM should mark VMCB_CET clean along with everything else on #VMEXIT.

> +		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
> -- 
> 2.47.3
>