Re: [RFC PATCH v2 3/4] liveupdate: Replace compatibility string with a 2-bit pair feature header

[Pasha Tatashin]

On Wed, Oct 21, 2026 at 1:40 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> On Wed, Oct 21, 2026 at 01:27:22PM -0400, Pasha Tatashin wrote:
> > On Wed, Oct 21, 2026 at 11:25 AM Tarun Sahu <tarunsahu@xxxxxxxxxx> wrote:
> > >
> > > LUO and its file handlers validate handed-over state with monolithic
> > > compatibility strings ("luo-v5", "memfd-v2"). Any change to a serialized
> > > structure, even an optional additive one, must bump the string, which
> > > breaks live update between adjacent kernels. Replace the string in
> > > struct luo_ser with a feature header, so that the target kernel can
> > > accept or reject the source kernel state per feature.
> > >
> > > struct luo_feature_hdr is a fixed 128-byte header: the serialized
> >
> > This patch is all wrong; 128 bytes is far too large. For every _ser
> > struct, we only need 8 bytes instead.
> >
> > The first 4 bytes are the size, and the next 4 bytes are flags (i.e.,
> > 30 feature flags + 2 reserved bits for flags extension). The last two
> > bits of the flags indicate whether an extended flags field is present.
> > Each extended flags field is 8 bytes, and its last two bits indicate
> > that yet another extended flags field follows.
>
> I thought we agreed not to loose the string too, it was a tree
>  string -> feature flags -> individual values
>
> The string is the emergency if we really badly mess it up.

Yes, we'll still have the file-handlers/flbs compatibility strings.

>
> Jason