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

[Kim Phillips <kim.phillips@amd.com>]

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,
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.

Thanks for your review!

Kim