Fix 4.08 support wrt changes beta2

[anmonteiro]

Following up on ocaml/ocaml#2263 (comment), this PR fixes OCaml 4.08 support via https://github.com/mirage/mmap

[anmonteiro]

@diml I think in mirage-platform we only need the stubs (mmap.o and mmap_ba.o). Is this the right way to do it, or is libmmap_stubs.a a name private to Dune?

[hannesm]

if you need these C functions in a xen-MirageOS-kernel, you'll need to cross-compile them with appropriate CFLAGS and includes. see https://github.com/mirage/checkseum / https://github.com/inhabitedtype/bigstringaf on how to do this.

[hannesm]

uhm, why do we need mmap / Unix.map_file at all in MirageOS? at least in ocaml-freestanding we got rid of libotherlibs.a since 4.07.0 (when Bigarray was moved to stdlib). (NOTE: I've not yet investigated what needs to be changed in ocaml-freestanding for 4.08.0, mainly because the whole ppx (ocaml-migrate-parsetree) is still unsupported (and I neither have time nor energy to look into that) -- but I hope it'll become easier (i.e. remove more things) than harder (introduce more dependencies)).

The changes/modifications for supporting 4.06.0 and 4.07.0 in ocaml-freestanding are different than the changes here, which are copying the pre-4.06.0 behaviour (see mirage/ocaml-solo5#27 and mirage/ocaml-solo5#39 if you're interested). There's as well @yomimono's work on rebasing mirage-xen support onto ocaml-freestanding (and using unikraft, see https://github.com/yomimono/ocaml-freestanding/tree/xen-on-freestanding and #199) -- so maybe it is also not worth to investigate this further.

[anmonteiro]

uhm, why do we need mmap / Unix.map_file at all in MirageOS? at least in ocaml-freestanding we got rid of libotherlibs.a since 4.07.0 (when Bigarray was moved to stdlib).

Interesting. Perhaps this is not needed at all! I don't know what I'm doing most of the time 🙃

I've not yet investigated what needs to be changed in ocaml-freestanding for 4.08.0, mainly because the whole ppx (ocaml-migrate-parsetree) is still unsupported

That's also why I haven't been able to test this much. Lwt depends on some PPX for building which depends on OMP

There's as well @yomimono's work on rebasing mirage-xen support onto ocaml-freestanding (and using unikraft, see https://github.com/yomimono/ocaml-freestanding/tree/xen-on-freestanding and #199) -- so maybe it is also not worth to investigate this further.

Indeed. Happy to drop this PR if there's other ways of getting OCaml 4.08 to run on Xen.

[avsm]

Closed due to #211