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

[Sean Christopherson <seanjc@google.com>]

On Tue, Oct 28, 2025, dan.j.williams@intel.com wrote:
> Sean Christopherson wrote:
> [..]
> > > IMO, It is something userspace should decide, kernel's job is to
> > > provide the necessary interface about it.
> > 
> > I disagree, I don't think userspace should even get the option.  IMO, not setting
> > AVOID_COMPAT_SENSITIVE is all kinds of crazy.
> 
> Do see Table 4.4: "Comparison of Update Incompatibility Detection and/or
> Avoidance Methods" from the latest base architecture specification [1].
> It lists out the pros and cons of not setting AVOID_COMPAT_SENSITIVE.
> This thread has only argued the merits of "None" and "Avoid updates
> during update- sensitive times". It has not discussed "Detect
> incompatibility after update", but let us not do that.

But we already are discussing that, because the "None" option is just punting
"Detect incompatibility after update" to something other than the VMM.  Doing
literally nothing isn't an option.  The fact that it's even listed in the table,
not to mention has "Simplest." listed as a pro, makes me question whether or not
the authors actually understand how software built around the TDX-Module is used
in practice.

If an update causes a TD build to fail, or to generate the wrong measurement, or
whatever "Failures due to incompatibilities" means, *something* eventually needs
to take action.  Doing nothing is certainly the simplest option for the hypervisor
and VMM, but when looking at the entire stack/ecosystem, it's the most complex
option as it bleeds the damage into multiple, potentially-unknown components of
the stack.  Either that, or I'm grossly misunderstanding what "Failures" means.

That section also states:

  Future TDX Module versions may have different or additional update-sensitive cases.

Which means that from an ABI perspective, "Avoid updates during update-sensitive
times" is the _ONLY_ viable option.  My read of that is that future TDX-Modules
can effectively change the failure modes for a existing KVM ioctls.  That is an
ABI change and will break userspace, e.g. if userspace is sane and expects certain
operations to succeed.