linux-kernel - [PATCH 13/28] KVM: SVM: Pass through GHCB MSR if and only if VM is an SEV-ES guest

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Message-ID: <20250529234013.3826933-14-seanjc@google.com>
Date: Thu, 29 May 2025 16:39:58 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Borislav Petkov <bp@...en8.de>, Xin Li <xin@...or.com>, Chao Gao <chao.gao@...el.com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: [PATCH 13/28] KVM: SVM: Pass through GHCB MSR if and only if VM is an
 SEV-ES guest

Disable interception of the GHCB MSR if and only if the VM is an SEV-ES
guest.  While the exact behavior is completely undocumented in the APM,
common sense and testing on SEV-ES capable CPUs says that accesses to the
GHCB from non-SEV-ES guests will #GP.  I.e. from the guest's perspective,
no functional change intended.

Fixes: 376c6d285017 ("KVM: SVM: Provide support for SEV-ES vCPU creation/loading")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/sev.c | 3 ++-
 arch/x86/kvm/svm/svm.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index bb0ec029b3d4..694d38a2327c 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4512,7 +4512,8 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
 	/* Can't intercept XSETBV, HV can't modify XCR0 directly */
 	svm_clr_intercept(svm, INTERCEPT_XSETBV);
 
-	/* Clear intercepts on selected MSRs */
+	/* Clear intercepts on MSRs that are context switched by hardware. */
+	svm_disable_intercept_for_msr(vcpu, MSR_AMD64_SEV_ES_GHCB, MSR_TYPE_RW);
 	svm_disable_intercept_for_msr(vcpu, MSR_EFER, MSR_TYPE_RW);
 	svm_disable_intercept_for_msr(vcpu, MSR_IA32_CR_PAT, MSR_TYPE_RW);
 }
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 56460413eca6..fa1a1b9b2d59 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -110,7 +110,7 @@ static const struct svm_direct_access_msrs {
 	{ .index = MSR_IA32_XSS,			.always = false },
 	{ .index = MSR_EFER,				.always = false },
 	{ .index = MSR_IA32_CR_PAT,			.always = false },
-	{ .index = MSR_AMD64_SEV_ES_GHCB,		.always = true  },
+	{ .index = MSR_AMD64_SEV_ES_GHCB,		.always = false },
 	{ .index = MSR_TSC_AUX,				.always = false },
 	{ .index = X2APIC_MSR(APIC_ID),			.always = false },
 	{ .index = X2APIC_MSR(APIC_LVR),		.always = false },
-- 
2.49.0.1204.g71687c7c1d-goog