KVM: SEV: Configure "ALLOWED_SEV_FEATURES" VMCB Field

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@amd.com>
Signed-off-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20250310201603.1217954-3-kim.phillips@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: kimphillamd

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;

arch/x86/kvm/svm/sev.c

@@ -4451,6 +4451,7 @@ void sev_vcpu_after_set_cpuid(struct vcpu_svm *svm)
 
 static void sev_es_init_vmcb(struct vcpu_svm *svm)
 {
+	struct kvm_sev_info *sev = to_kvm_sev_info(svm->vcpu.kvm);
 	struct vmcb *vmcb = svm->vmcb01.ptr;
 	struct kvm_vcpu *vcpu = &svm->vcpu;
 
@@ -4466,6 +4467,10 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
 	if (svm->sev_es.vmsa && !svm->sev_es.snp_has_guest_vmsa)
 		svm->vmcb->control.vmsa_pa = __pa(svm->sev_es.vmsa);
 
+	if (cpu_feature_enabled(X86_FEATURE_ALLOWED_SEV_FEATURES))
+		svm->vmcb->control.allowed_sev_features = sev->vmsa_features |
+							  VMCB_ALLOWED_SEV_FEATURES_VALID;
+
 	/* Can't intercept CR register access, HV can't modify CR registers */
 	svm_clr_intercept(svm, INTERCEPT_CR0_READ);
 	svm_clr_intercept(svm, INTERCEPT_CR4_READ);

arch/x86/kvm/svm/svm.c

@@ -3452,6 +3452,8 @@ static void dump_vmcb(struct kvm_vcpu *vcpu)
 	pr_err("%-20s%016llx\n", "avic_logical_id:", control->avic_logical_id);
 	pr_err("%-20s%016llx\n", "avic_physical_id:", control->avic_physical_id);
 	pr_err("%-20s%016llx\n", "vmsa_pa:", control->vmsa_pa);
+	pr_err("%-20s%016llx\n", "allowed_sev_features:", control->allowed_sev_features);
+	pr_err("%-20s%016llx\n", "guest_sev_features:", control->guest_sev_features);
 
 	if (sev_es_guest(vcpu->kvm)) {
 		save = sev_decrypt_vmsa(vcpu);