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

[David Matlack <dmatlack@google.com>]

On 2026-09-04 01:00 PM, Jason Gunthorpe wrote:
> 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.

We received basically the opposite stance from Sean regarding the KVM
LUO ABI:

  https://lore.kernel.org/kvm/aoSCCTTBn9D5hqzk@google.com/

So we're trying to use this series to get some alignment across LUO
ABIs.

> > 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?

The proposal here (which is inspired by the KVM UAPI) is to ensure every
LUO ABI struct has 2 properties:

 1. A field to encode options/features (e.g. u64 flags).
 2. A way way to grow without breaking backward compatibility (e.g. so
    we can add new fields).

Each flag can mean whatever it needs to. e.g. It can indicate the
precence of one or more fields (i.e. new fields in the struct), or it
can mean a field now has a different meaning (i.e. union in the struct).

This would enable adding support for new features without breaking
backward compatibility. Downstream users would have to ensure their
kernel does not start using a new feature while it can still rollback to
a version that does not support the new feature.