[PATCH v2 15/32] KVM: SVM: Pass through GHCB MSR if and only if VM is an SEV-ES guest

[Sean Christopherson <seanjc@google.com>]

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@google.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 74dab69fb69e..a020aa755a7e 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4448,7 +4448,8 @@ 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 93d66109f495..7747f9bc3e9d 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 },
-- 
2.50.0.rc0.642.g800a2b2222-goog