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

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Message-ID: <aEmuKII8FGU4eQZz@google.com>
Date: Wed, 11 Jun 2025 09:26:16 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Xiaoyao Li <xiaoyao.li@...el.com>, 
	"mikko.ylinen@...ux.intel.com" <mikko.ylinen@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, Jiewen Yao <jiewen.yao@...el.com>, 
	Reinette Chatre <reinette.chatre@...el.com>, Tony Lindgren <tony.lindgren@...el.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, Yan Y Zhao <yan.y.zhao@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Kirill Shutemov <kirill.shutemov@...el.com>
Subject: Re: [RFC PATCH 3/4] KVM: TDX: Exit to userspace for GetTdVmCallInfo

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.