linux-kernel - Re: [PATCH v2 00/21] Runtime TDX Module update support

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Message-ID: <CAGtprH-rv9T1ano+ti=3eU4FO2APCOcR06buPALggAwUnka3Dg@mail.gmail.com>
Date: Mon, 27 Oct 2025 17:42:12 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: dan.j.williams@...el.com
Cc: Dave Hansen <dave.hansen@...el.com>, Chao Gao <chao.gao@...el.com>, 
	"Reshetova, Elena" <elena.reshetova@...el.com>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, 
	"Chatre, Reinette" <reinette.chatre@...el.com>, "Weiny, Ira" <ira.weiny@...el.com>, 
	"Huang, Kai" <kai.huang@...el.com>, "yilun.xu@...ux.intel.com" <yilun.xu@...ux.intel.com>, 
	"sagis@...gle.com" <sagis@...gle.com>, "paulmck@...nel.org" <paulmck@...nel.org>, 
	"nik.borisov@...e.com" <nik.borisov@...e.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Ingo Molnar <mingo@...hat.com>, "Kirill A. Shutemov" <kas@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 00/21] Runtime TDX Module update support

On Mon, Oct 27, 2025 at 11:53 AM <dan.j.williams@...el.com> wrote:
>
> Vishal Annapurve wrote:
> [..]
> > > A theoretical TDX Module change could ensure that atomicity.
> >
> > IIUC TDX module already supports avoiding this clobber based on the
> > TDH.SYS.SHUTDOWN documentation from section 5.4.73 of TDX ABI Spec
> > [1].
> >
> > Host kernel needs to set bit 16 of rcx when invoking TDH.SYS.SHUTDOWN
> > is available.
> >
> > "If supported by the TDX Module, the host VMM can set the
> > AVOID_COMPAT_SENSITIVE flag to request the TDX Module to fail
> > TDH.SYS.UPDATE if any of the TDs are currently in a state that is
> > impacted by the update-sensitive cases."
>
> That is not a fix. That just shifts the complexity from build to update.
> It still leaves update in a state where it is not guaranteed to make

IMO, there are two problems here:
1) Giving a consistent ABI that leaves the responsibility of ensuring
forward progress by sequencing TD update with TD build steps with
userspace.
2) Ensuring that userspace can't screw up the in-progress TD VM
metadata if userspace doesn't adhere to the sequence above.

Problem 2 should be solved in the TDX module as it is the state owner
and should be given a chance to ensure that nothing else can affect
it's state. Kernel is just opting-in to toggle the already provided
TDX module ABI. I don't think this is adding complexity to the kernel.

> forward progress. The way to ensure forward progress is the same as
> ensuring build consistency, i.e. sequence build with respect to update.
> The kernel sheds complexity by ether making userspace solve that
> problem, or motivating a real fix in the TDX Module that obviates the
> AVOID_COMPAT_SENSITIVE case.