Re: [RFC PATCH 3/4] KVM: TDX: Exit to userspace for GetTdVmCallInfo

[Sean Christopherson <seanjc@google.com>]

On Wed, Jun 11, 2025, Rick P Edgecombe wrote:
> On Wed, 2025-06-11 at 07:54 -0700, Sean Christopherson wrote:
> > > Let's see what Paolo and Sean will say.
> > 
> > Kicking this to userspace seems premature.  AIUI, no "optional" VMCALL
> > features
> > are defined at this time, i.e. there's nothing to enumerate.  And there's no
> > guarantee that there will ever be capabilties that require enumeration from 
> > *userspace*.  E.g. if fancy feature XYZ requires enumeration, but that feature
> > requires explicit KVM support, then forcing userspace will be messy.
> > 
> > So I don't see why KVM should anything other than return '0' to the guest (or
> > whatever value says "there's nothing here").
> 
> GetQuote is not part of the "Base" TDVMCALLs and so has a bit in
> GetTdVmCallInfo. We could move it to base?

Is GetQuote actually optional?  TDX without attestation seems rather pointless.

> Paolo seemed keen on GetTdVmCallInfo exiting to userspace, but this was before
> the spec overhaul.

If GetQuote is truly optional, then exiting to userspace makes sense.  But as
above, that seems odd to me.