linux-kernel - Re: [PATCH v3 3/5] KVM: x86: SVM: Pass through shadow stack MSRs

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Message-ID: <aKu6sp1ikRetv/Q1@intel.com>
Date: Mon, 25 Aug 2025 09:21:54 +0800
From: Chao Gao <chao.gao@...el.com>
To: John Allen <john.allen@....com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	<seanjc@...gle.com>, <pbonzini@...hat.com>, <dave.hansen@...el.com>,
	<rick.p.edgecombe@...el.com>, <mlevitsk@...hat.com>,
	<weijiang.yang@...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 v3 3/5] KVM: x86: SVM: Pass through shadow stack MSRs

On Wed, Aug 06, 2025 at 08:45:08PM +0000, John Allen wrote:
>If kvm supports shadow stack, pass through shadow stack MSRs to improve
>guest performance.

The changelog is a bit sparse. Perhaps you could include something similar
to what I did in my v13 version:

Pass through shadow stack MSRs that are managed by XSAVE, as they cannot be
intercepted without also intercepting XSAVE. However, intercepting XSAVE would
likely cause unacceptable performance overhead.

MSR_IA32_INT_SSP_TAB is not managed by XSAVE, so it is intercepted.

>
>Signed-off-by: John Allen <john.allen@....com>
>---
> arch/x86/kvm/svm/svm.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
>diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>index a027d3c37181..82cde3578c96 100644
>--- a/arch/x86/kvm/svm/svm.c
>+++ b/arch/x86/kvm/svm/svm.c
>@@ -838,6 +838,18 @@ static void svm_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
> 	svm_set_intercept_for_msr(vcpu, MSR_IA32_SYSENTER_ESP, MSR_TYPE_RW,
> 				  guest_cpuid_is_intel_compatible(vcpu));
> 
>+	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
>+		bool shstk_enabled = guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
>+
>+		svm_set_intercept_for_msr(vcpu, MSR_IA32_U_CET, MSR_TYPE_RW, !shstk_enabled);
>+		svm_set_intercept_for_msr(vcpu, MSR_IA32_S_CET, MSR_TYPE_RW, !shstk_enabled);
>+		svm_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, !shstk_enabled);
>+		svm_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, !shstk_enabled);
>+		svm_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, !shstk_enabled);
>+		svm_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, !shstk_enabled);
>+		svm_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, !shstk_enabled);

MSR_IA32_INT_SSP_TAB should be intercepted unless there is a justification for
pass-through. See Sean's comment below

https://lore.kernel.org/kvm/aKTGVvOb8PZ7mzVr@google.com/

With above nits fixed,

Reviewed-by: Chao Gao <chao.gao@...el.com>

>+	}
>+
> 	if (sev_es_guest(vcpu->kvm))
> 		sev_es_recalc_msr_intercepts(vcpu);
> 
>-- 
>2.34.1
>