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

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 01, 2024, Kim Phillips wrote:
> From: Kishon Vijay Abraham I <kvijayab@amd.com>
> 
> 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]. The ALLOWED_SEV_FEATURES feature 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.

How does the host communicate to the guest which features are allowed?  And based
on this blurb:

  Some SEV features can only be used if the Allowed SEV Features Mask is enabled,
  and the mask is configured to permit the corresponding feature. If the Allowed
  SEV Features Mask is not enabled, these features are not available (see SEV_FEATURES
  in Appendix B, Table B-4).

and the appendix, this only applies to PmcVirtualization and SecureAvic.  Adding
that info in the changelog would be *very* helpful.

And I see that SVM_SEV_FEAT_DEBUG_SWAP, a.k.a. DebugVirtualization, is a guest
controlled feature and doesn't honor ALLOWED_SEV_FEATURES.  Doesn't that mean
sev_vcpu_has_debug_swap() is broken, i.e. that KVM must assume the guest can
DebugVirtualization on and off at will?  Or am I missing something?