[PATCH v5 6/8] KVM: SEV: Advertise SVM_SEV_FEAT_SNP_ACTIVE

[Kim Phillips <kim.phillips@amd.com>]

Allow userspace to set the feature in kvm_sev_init.vmsa_features.

KVM still needs to set the flag for backwards compatibility, but
disallowing SVM_SEV_FEAT_SNP_ACTIVE for an SNP guest is "bizarre."

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/kvm/aaWog_UjW-M3412C@google.com/
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Assisted-by: ClaudeCode:claude-opus-4-7
---
 arch/x86/include/asm/svm.h                        | 3 ++-
 arch/x86/kvm/svm/sev.c                            | 8 ++++++--
 tools/testing/selftests/kvm/x86/sev_init2_tests.c | 4 +++-
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index a206a0ed2c58..facabba26a91 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -311,7 +311,8 @@ static_assert((X2AVIC_4K_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AV
 #define SVM_SEV_FEAT_DEBUG_SWAP				BIT_ULL(5)
 #define SVM_SEV_FEAT_SECURE_TSC				BIT_ULL(9)
 
-#define SVM_SEV_FEAT_SNP_ONLY_MASK			(SVM_SEV_FEAT_SECURE_TSC)
+#define SVM_SEV_FEAT_SNP_ONLY_MASK	(SVM_SEV_FEAT_SNP_ACTIVE | \
+					 SVM_SEV_FEAT_SECURE_TSC)
 
 #define VMCB_ALLOWED_SEV_FEATURES_VALID			BIT_ULL(63)
 
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 3c9483733865..6cfb7496415e 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3257,8 +3257,12 @@ void __init sev_hardware_setup(void)
 	    cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP))
 		sev_supported_vmsa_features |= SVM_SEV_FEAT_DEBUG_SWAP;
 
-	if (sev_snp_enabled && tsc_khz && cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
-		sev_supported_vmsa_features |= SVM_SEV_FEAT_SECURE_TSC;
+	if (sev_snp_enabled) {
+		sev_supported_vmsa_features |= SVM_SEV_FEAT_SNP_ACTIVE;
+
+		if (tsc_khz && cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
+			sev_supported_vmsa_features |= SVM_SEV_FEAT_SECURE_TSC;
+	}
 }
 
 void sev_hardware_unsetup(void)
diff --git a/tools/testing/selftests/kvm/x86/sev_init2_tests.c b/tools/testing/selftests/kvm/x86/sev_init2_tests.c
index 8269f146b1f5..d70482e07141 100644
--- a/tools/testing/selftests/kvm/x86/sev_init2_tests.c
+++ b/tools/testing/selftests/kvm/x86/sev_init2_tests.c
@@ -13,11 +13,13 @@
 #include "svm_util.h"
 #include "kselftest.h"
 
+#define SVM_SEV_FEAT_SNP_ACTIVE		BIT_ULL(0)
 #define SVM_SEV_FEAT_DEBUG_SWAP		BIT_ULL(5)
 #define SVM_SEV_FEAT_SECURE_TSC		BIT_ULL(9)
 
 /* Features valid only for SNP guests, rejected for SEV-ES and below. */
-#define SNP_ONLY_FEATURES		(SVM_SEV_FEAT_SECURE_TSC)
+#define SNP_ONLY_FEATURES		(SVM_SEV_FEAT_SNP_ACTIVE | \
+					 SVM_SEV_FEAT_SECURE_TSC)
 
 /*
  * Some features may have hidden dependencies, or may only work
-- 
2.43.0