[PATCH v3 2/2] KVM: SEV: Configure "ALLOWED_SEV_FEATURES" VMCB Field

Kim Phillips
2025-02-07T23:34:52Z

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.

When ALLOWED_SEV_FEATURES is enabled, 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>
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
arch/x86/include/asm/svm.h | 5 ++++-
arch/x86/kvm/svm/sev.c | 17 +++++++++++++++++
2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index e2fac21471f5..6d94a727cc1a 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -158,7 +158,9 @@ 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 */
+ u8 reserved_9[672];
/*
* Offset 0x3e0, 32 bytes reserved
* for use by hypervisor/software.
@@ -289,6 +291,7 @@ static_assert((X2AVIC_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AVIC_
#define SVM_SEV_FEAT_RESTRICTED_INJECTION BIT(3)
#define SVM_SEV_FEAT_ALTERNATE_INJECTION BIT(4)
#define SVM_SEV_FEAT_DEBUG_SWAP BIT(5)
+#define SVM_SEV_FEAT_ALLOWED_SEV_FEATURES BIT_ULL(63)

#define SVM_SEV_FEAT_INT_INJ_MODES \
(SVM_SEV_FEAT_RESTRICTED_INJECTION | \
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index a2a794c32050..a9e16792cac0 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -894,9 +894,19 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
return 0;
}

+static u64 allowed_sev_features(struct kvm_sev_info *sev)
+{
+ if (cpu_feature_enabled(X86_FEATURE_ALLOWED_SEV_FEATURES) &&
+ (sev->vmsa_features & SVM_SEV_FEAT_ALLOWED_SEV_FEATURES))
+ return sev->vmsa_features;
+
+ return 0;
+}
+
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;
@@ -906,6 +916,8 @@ 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)
@@ -2447,6 +2459,8 @@ 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;
@@ -3069,6 +3083,9 @@ void __init sev_hardware_setup(void)
sev_supported_vmsa_features = 0;
if (sev_es_debug_swap_enabled)
sev_supported_vmsa_features |= SVM_SEV_FEAT_DEBUG_SWAP;
+
+ if (sev_es_enabled && cpu_feature_enabled(X86_FEATURE_ALLOWED_SEV_FEATURES))
+ sev_supported_vmsa_features |= SVM_SEV_FEAT_ALLOWED_SEV_FEATURES;
}

void sev_hardware_unsetup(void)
--
2.43.0