[PATCH v4 2/2] KVM: SEV: Configure "ALLOWED_SEV_FEATURES" VMCB Field

[Kim Phillips]

AMD EPYC 5th generation processors have introduced a feature that allows
the hypervisor to control the SEV_FEATURES that are set for, or by, a
guest [1].  ALLOWED_SEV_FEATURES can be used by the hypervisor to enforce
that SEV-ES and SEV-SNP guests cannot enable features that the
hypervisor does not want to be enabled.

Always enable ALLOWED_SEV_FEATURES.  A VMRUN will fail if any
non-reserved bits are 1 in SEV_FEATURES but are 0 in
ALLOWED_SEV_FEATURES.

Some SEV_FEATURES - currently PmcVirtualization and SecureAvic
(see Appendix B, Table B-4) - require an opt-in via ALLOWED_SEV_FEATURES,
i.e. are off-by-default, whereas all other features are effectively
on-by-default, but still honor ALLOWED_SEV_FEATURES.

[1] Section 15.36.20 "Allowed SEV Features", AMD64 Architecture
    Programmer's Manual, Pub. 24593 Rev. 3.42 - March 2024:
    https://bugzilla.kernel.org/attachment.cgi?id=306250

Co-developed-by: Kishon Vijay Abraham I <kvijayab@xxxxxxx>
Signed-off-by: Kishon Vijay Abraham I <kvijayab@xxxxxxx>
Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>
---
 arch/x86/include/asm/svm.h |  7 ++++++-
 arch/x86/kvm/svm/sev.c     | 13 +++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index 9b7fa99ae951..b382fd251e5b 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -159,7 +159,10 @@ struct __attribute__ ((__packed__)) vmcb_control_area {
 	u64 avic_physical_id;	/* Offset 0xf8 */
 	u8 reserved_7[8];
 	u64 vmsa_pa;		/* Used for an SEV-ES guest */
-	u8 reserved_8[720];
+	u8 reserved_8[40];
+	u64 allowed_sev_features;	/* Offset 0x138 */
+	u64 guest_sev_features;		/* Offset 0x140 */
+	u8 reserved_9[664];
 	/*
 	 * Offset 0x3e0, 32 bytes reserved
 	 * for use by hypervisor/software.
@@ -291,6 +294,8 @@ static_assert((X2AVIC_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AVIC_
 #define SVM_SEV_FEAT_ALTERNATE_INJECTION		BIT(4)
 #define SVM_SEV_FEAT_DEBUG_SWAP				BIT(5)
 
+#define VMCB_ALLOWED_SEV_FEATURES_VALID			BIT_ULL(63)
+
 struct vmcb_seg {
 	u16 selector;
 	u16 attrib;
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 0bc708ee2788..7f6cb950edcf 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -793,6 +793,14 @@ static int sev_launch_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	return ret;
 }
 
+static u64 allowed_sev_features(struct kvm_sev_info *sev)
+{
+	if (cpu_feature_enabled(X86_FEATURE_ALLOWED_SEV_FEATURES))
+		return sev->vmsa_features | VMCB_ALLOWED_SEV_FEATURES_VALID;
+
+	return 0;
+}
+
 static int sev_es_sync_vmsa(struct vcpu_svm *svm)
 {
 	struct kvm_vcpu *vcpu = &svm->vcpu;
@@ -891,6 +899,7 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
 static int __sev_launch_update_vmsa(struct kvm *kvm, struct kvm_vcpu *vcpu,
 				    int *error)
 {
+	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
 	struct sev_data_launch_update_vmsa vmsa;
 	struct vcpu_svm *svm = to_svm(vcpu);
 	int ret;
@@ -900,6 +909,8 @@ static int __sev_launch_update_vmsa(struct kvm *kvm, struct kvm_vcpu *vcpu,
 		return -EINVAL;
 	}
 
+	svm->vmcb->control.allowed_sev_features = allowed_sev_features(sev);
+
 	/* Perform some pre-encryption checks against the VMSA */
 	ret = sev_es_sync_vmsa(svm);
 	if (ret)
@@ -2426,6 +2437,8 @@ static int snp_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		struct vcpu_svm *svm = to_svm(vcpu);
 		u64 pfn = __pa(svm->sev_es.vmsa) >> PAGE_SHIFT;
 
+		svm->vmcb->control.allowed_sev_features = allowed_sev_features(sev);
+
 		ret = sev_es_sync_vmsa(svm);
 		if (ret)
 			return ret;
-- 
2.43.0