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

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 20, 2024, Kim Phillips wrote:
> On 8/19/24 5:23 PM, Sean Christopherson wrote:
> > On Mon, Aug 19, 2024, Kim Phillips wrote:
> > > 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?
> 
> No, as above, if the guest tries any silly business the host will
> get a VMEXIT_INVALID, no matter if using the feature *requires*
> ALLOWED_SEV_FEATURES to be enabled and explicitly allow it (currently
> PmcVirtualization or SecureAvic).

Oooh, I finally get it.  PmcVirtualization and SecureAvic 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.