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

[David Matlack]

On 2026-10-27 12:21 PM, Pasha Tatashin wrote:

> My main requirement is that DECLARE_KHO_SER() in include/linux/kho/abi/ 
> must be enforced by the compiler rather than being just a bare 
> static_assert() that someone could forget to add. Since kho/abi/ headers 
> contain both versioned structs and unversioned sub-structs, it must be 
> mandatory to declare versioned structs in kho/abi/ with their 
> static_assert size check before the kho_ser_*() helpers can be used on 
> them.
> 
> So, how about adding enum names like this, that are going to be required 
> in accessor functions later:
> 
> #define DECLARE_KHO_SER(name, payload_size, nr_feats)             \
>     static_assert(sizeof(struct name) == (payload_size),          \
>                   "struct " #name " size mismatch");              \
>     enum {                                                        \
>         __KHO_SER_SIZE_##name = (payload_size),                   \
>         __KHO_SER_NR_FEATS_##name = (nr_feats),                   \
>     }

Sounds good. Since you're also on board with splitting required
and optional features [1], this would take both counts:

#define DECLARE_KHO_SER(name, payload_size, nr_required, nr_optional) \
    static_assert(sizeof(struct name) == (payload_size),              \
                  "struct " #name " size mismatch");                  \
    enum {                                                            \
        __KHO_SER_SIZE_##name = (payload_size),                       \
        __KHO_SER_NR_REQUIRED_##name = (nr_required),                 \
        __KHO_SER_NR_OPTIONAL_##name = (nr_optional),                 \
    }

  DECLARE_KHO_SER(memfd_luo_ser, 16, MEMFD_LUO_NR_REQUIRED, MEMFD_LUO_NR_OPTIONAL);

The incoming path can also use __KHO_SER_NR_OPTIONAL_##name to validate
the incoming header only contains required features that it knows how to
handle:

#define kho_ser_validate(hdr, name) \
    __kho_ser_validate(hdr, __KHO_SER_NR_REQUIRED_##name)

> > Plus helpers that allocate and initialize the header and return the
> > payload as a typed pointer. Both the outgoing and incoming paths find
> > the payload the same way, by computing its offset from the header,
> > rather than through a struct with a fixed payload offset:
> > 
> >   /* outgoing */
> >   hdr = kho_ser_alloc(luo_memfd_ser, LUO_MEMFD_NR_FEATS);
> >   ser = kho_ser_payload(hdr, luo_memfd_ser);
> > 
> >   /* incoming */
> >   hdr = phys_to_virt(args->serialized_data);
> >   ser = kho_ser_payload(hdr, luo_memfd_ser);
> 
> These would become, where without enum, the init would fail:
> 
> #define kho_ser_init(buf, name)                                     \
>       __kho_ser_init(buf, __KHO_SER_SIZE_##name,                    \
>                           __KHO_SER_NR_FEATS_##name)
> 
> And payload can also check the existance of DECLARE_KHO_SER, using 
> (expr1, expr2):
> 
> #define kho_ser_payload(hdr, name)                                  \
>       ((void)__KHO_SER_SIZE_##name,                                 \
>        (struct name *)__kho_ser_payload(hdr))
> 
> We can also add an optional kho_ser_alloc(name) to allocate, init, and 
> preserve.

+1