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

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Message-ID: <26a0929e0c2ac697ad37d24ad6ef252cfc140289.camel@intel.com>
Date: Fri, 29 Aug 2025 22:17:33 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "Huang, Kai" <kai.huang@...el.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "Weiny, Ira" <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, 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.

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

> 
> > But I don't think we do for security.

But, actually they are some of the same paths. So same pattern.

> > 
> > Speaking for Yan here, I think she was a little more worried about this scenario
> > then me, so I read this verbiage and thought to try to close it out.