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

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Message-ID: <690127bfb4b40_10e9100fe@dwillia2-mobl4.notmuch>
Date: Tue, 28 Oct 2025 13:29:51 -0700
From: <dan.j.williams@...el.com>
To: Vishal Annapurve <vannapurve@...gle.com>, <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

Vishal Annapurve wrote:
> 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.

That gives update a transient error to handle