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

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 10, 2026, Jason Gunthorpe wrote:
> On Thu, Sep 10, 2026 at 03:42:36PM -0700, Sean Christopherson wrote:
> > On Thu, Sep 10, 2026, Jason Gunthorpe wrote:
> > > Sean's initial email was asking for strict never-break ABI compatability
> > > rules on top, and that is what I've been reacting to.
> > 
> > No, I was never asking for that.  I think we just have a different interpretation
> > of ABI, or rather are talking about different pieces of ABI.
> > 
> > I am still asking for never-break serialization format compatibility, i.e. the
> > more literal save/restore ABI.  I'm not asking for full backwards/forwards
> > compatibility across all kernels version, i.e. the higher level "kernel" ABI.
> > 
> > To phrase things differently: I am a-ok if kernels are inherently incompatible
> > because they fundamentally operate differently and/or support different features.
> > I am not ok if we end up with incompatible kernels because the save/restore
> > interfaces and payloads are poorly designed, lack abstraction, etc.  
> 
> I can more agree with this, it is mostly what I thought we'd end up
> doing anyhow with version numbers and growing the structs not
> replacing them.
> 
> So you still end up with a version label (string I guess?) because we
> can break it from time to time, 
   
Why bother with a label/string?  I don't understand what it buys us; I see only
pain.  Make the features discoverable and punt on all of the "is this pile of
insanity my company has created compatible with this other pile of insanity?" to
layers above the kernel.

IMO, trying to capture compatibility in a version string is doomed to fail.  To
be useful, the version would have to be extremely sensitive to changes, otherwise
userspace will have to do its own compatibility analysis/checks anyways.  And at
that point, a verson string will be too restrictive, e.g. will mismatch due to
changes in features the deployment isn't using.

> just the version should have the feature mechanism layered under it? I'm fine
> with that
> 
> Jason