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

[Sean Christopherson <seanjc@google.com>]

On Mon, Aug 19, 2024, Kim Phillips wrote:
> On 8/16/24 5:59 PM, Sean Christopherson wrote:
> > 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?
> 
> I'm not familiar with any future plans to negotiate with the guest directly,

I feel like I'm missing something.  What happens if the guest wants to enable
PmcVirtualization and it's unexpectedly disallowed?  Does the guest simply panic?

> but since commit ac5c48027bac ("KVM: SEV: publish supported VMSA features"),
> userspace can retrieve sev_supported_vmsa_features via an ioctl.
> 
> > 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.
> 
> Ok, how about adding:
> 
> "The PmcVirtualization and SecureAvic features explicitly require
> ALLOWED_SEV_FEATURES to enable them before they can be used."
> 
> > 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?
> 
> My understanding is that users control KVM's DEBUG_SWAP setting
> with a module parameter since commit 4dd5ecacb9a4 ("KVM: SEV: allow
> SEV-ES DebugSwap again").  If the module parameter is not set, with
> this patch, VMRUN will fail since the host doesn't allow DEBUG_SWAP.

But that's just KVM's view of vmsa_features.  With SNP's wonderful
SVM_VMGEXIT_AP_CREATE, can't the guest create a VMSA with whatever sev_features
it wants, so long as they aren't host-controllable, i.e. aren't PmcVirtualization
or SecureAvic?