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

[Tom Lendacky]

On 3/5/25 18:38, 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.
> 
> 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);

I think you can move this to sev_es_init_vmcb() and have it just in that
one place instead of each launch update routine.

Thanks,
Tom

> +
>  	/* 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;