linux-kernel - Re: [RFC PATCH v2 05/18] KVM: TDX: Drop superfluous page pinning in S-EPT management

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Message-ID: <aLIwlZWdyhBp6I0Z@google.com>
Date: Fri, 29 Aug 2025 15:58:29 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Kai Huang <kai.huang@...el.com>, "ackerleytng@...gle.com" <ackerleytng@...gle.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Yan Y Zhao <yan.y.zhao@...el.com>, 
	Ira Weiny <ira.weiny@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "michael.roth@....com" <michael.roth@....com>
Subject: Re: [RFC PATCH v2 05/18] KVM: TDX: Drop superfluous page pinning in
 S-EPT management

On Fri, Aug 29, 2025, Rick P Edgecombe wrote:
> On Fri, 2025-08-29 at 15:02 -0700, Sean Christopherson wrote:
> > On Fri, Aug 29, 2025, Rick P Edgecombe wrote:
> > > On Fri, 2025-08-29 at 13:19 -0700, Sean Christopherson wrote:
> > > > I'm happy to include more context in the changelog, but I really don't want
> > > > anyone to walk away from this thinking that pinning pages in random KVM code
> > > > is at all encouraged.
> > > 
> > > Sorry for going on a tangent. Defensive programming inside the kernel is a
> > > little more settled. But for defensive programming against the TDX module, there
> > > are various schools of thought internally. Currently we rely on some
> > > undocumented behavior of the TDX module (as in not in the spec) for correctness.
> > 
> > Examples?
> 
> I was thinking about the BUSY error code avoidance logic that is now called
> tdh_do_no_vcpus(). We assume no new conditions will appear that cause a
> TDX_OPERAND_BUSY. Like a guest opt-in or something.

Ah, gotcha.  If that happens, that's a TDX-Module ABI break.  Probably a good
idea to drill it into the TDX-Module authors/designers that ABI is established
when behavior is visible to the user, regardless of whether or not that behavior
is formally defined.

Note, breaking ABI _can_ be fine, e.g. if the behavior of some SEAMCALL changes,
but KVM doesn't care.  But if the TDX-Module suddenly starts failing a SEAMCALL
that previously succeeded, then we're going to have a problem.

> It's on our todo list to transition those assumptions to promises. We just need
> to formalize them.