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

Tom Lendacky
2025-02-10T18:08:32Z

On 2/7/25 17:34, Kim Phillips wrote:
> 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)

Hmmm... I believe it is safe to define this bit value, as the Allowed
SEV features VMCB field shows bits 61:0 being used for the allowed
features mask and we know that the SEV_FEATURES field is used in the SEV
Features MSR left-shifted 2 bits, so we only expect bits 61:0 to be used
and bits 62 and 63 will always be reserved. But, given that I think we
need two functions:

- get_allowed_sev_features()
keeping it as you have it below, where it returns the
sev->vmsa_features bitmap if SVM_SEV_FEAT_ALLOWED_SEV_FEATURES is set
or 0 if SVM_SEV_FEAT_ALLOWED_SEV_FEATURES is not set.

- get_vmsa_sev_features()
which removes the SVM_SEV_FEAT_ALLOWED_SEV_FEATURES bit, since it is
not defined in the VMSA SEV_FEATURES definition.

>
> #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) &&

Not sure if the cpu_feature_enabled() check is necessary, as init should
have failed if SVM_SEV_FEAT_ALLOWED_SEV_FEATURES wasn't set in
sev_supported_vmsa_features.

Thanks,
Tom

> + (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)