Re: [RFC PATCH 0/3] liveupdate: Move to feature flags for LUO and memfd ABI compatibility

[Jason Gunthorpe]

On Wed, Sep 02, 2026 at 07:34:49PM -0700, Logan Odell wrote:
> We're including maintainers from all subsystems that currently is or
> will be expected to participate in live update to ensure alignment on
> the path forward for compatibility.
> 
> Currently, Live Update Orchestrator (LUO) and its file handlers rely on
> monolithic compatibility strings (such as "luo-v5" and "memfd-v1") to
> validate ABI compatibility across kexec live updates. Any modification
> to serialized structures requires bumping the version string, which
> strictly breaks compatibility between adjacent kernels even when changes
> are additive, backwards-compatible, or optional.

That was the intention, I aruged strongly that upstream does not want
to maintain a CSP matrix of endless kernel version combinations. That
is far too much work to push on maintainers. Upstream would do much
less, maybe only same-version, depending.

> This RFC series transitions LUO and subsystem file handlers to use
> granular feature bitmasks instead of compatibility strings. We replace
> the compatibility string with a header structure that includes some
> reserved space to define the features that are included after the
> feature.

The compatability string is only a small part of it, you also need a
serializing ABI that can handle some random mixmash of these
features. Ie the various TLV schemes that were all proposed. What is
the plan here?

Jason