[MirageOS-devel] Cross-compiling OCaml, Mirage OS for rumprun, OPAM integration

[Martin Lucina <martin@xxxxxxxxxx>]

[Re-sending with correct opam-devel address]

Hi all,

I'm working on getting OCaml and Mirage OS running on top of the rumprun
unikernel stack[1].

Rumprun provides a unikernel stack with a POSIXy (NetBSD-HEAD) userspace
and can run on Xen, QEMU/KVM, bare metal and POSIX userspace.  The
resulting Mirage + rumprun unikernel will thus be able to run on all of the
platforms rumprun supports.

The first step to get this working is building an OCaml cross compiler that
uses the rumprun cross compiler as its backend for building C code and
linking.

I've researched the various cross-compilation options for OCaml and for now
have based my code off the approach used by Peter Zotov in his
`opam-android' repository[2].

My work in progress OPAM repository is here:

https://github.com/mato/opam-rumprun

This is enough to get an OCaml "Hello, World" running on rumprun, and I've
also tested that the Unix package works as expected. So far so good.

However, in order to get Mirage to compile, I need to cross-compile Lwt and
that does not work as expected:

    $ OCAMLFIND_TOOLCHAIN=rumprun ocaml setup.ml -configure --prefix 
/home/mato/.opam/system/x86_64-rumprun-netbsd/ --disable-libev

Results in:

    not checking for pkg-config
    not checking for libev
    testing for pthread: ........................... unavailable
    not checking for glib

    The following recquired C libraries are missing: pthread.

The reason this happens is that discover.ml is invoking the following
compile command:

/home/mato/projects/rumpkernel/rumprun/app-tools/rumprun-xen-cc -fno-defer-pop 
-Wall -D_FILE_OFFSET_BITS=64 -D_REENTRANT -c -I/usr/include -o 
/tmp/lwt_stubf12ac7.o -I/home/mato/.opam/system/x86_64-rumprun-netbsd/lib/ocaml 
/tmp/lwt_stubf12ac7.c

Note the inclusion of -I/usr/include -- this is presumably there so that
the system OCaml compiler's headers are available, however it results in a
conflict between the NetBSD headers provided by rumprun and the system
headers.

What can be done to fix the above? Should I be using a host compiler built
and installed by OPAM rather than the system compiler (4.02.1-1ppa3~precise
on Debian wheezy). [I'll try this and see if it helps...]

I tried to hack around the problem by using --enable-android-target, that
causes discover.ml to succeed and the build then proceeds, failing on a
different problem:

E: Failure("Expected built file '_build/src/unix/dlllwt-unix_stubs.so' doesn't 
exist.")

This is expected; the rumprun toolchain does not support dynamic linking
and I have configured the ocaml-rumprun compiler with -no-shared-libs. Is
there some way to tell OASIS to not expect any shared libraries to be
built?

General questions:

1) Is this the right strategy for building an OCaml cross-compiler and
integrating with OPAM?

AFAICS there are multiple approaches being used in the wild and OCaml
upstream *claims* to include support for cross-compiling via -host and
-target, however that support is not actually functional?

For the rumprun support to be as user-friendly as possible, we need an easy
way of switching the backend rumprun cross-compiler and linker used by
ocaml when the user wants to switch platforms or architectures. The
approach used by the android repository I based my work off implies either:

a) the user would have to *reinstall* the cross compiler (specifying eg.
RUMPRUN_CC in the environment) and all packages using native code whenever
they want to switch the backend compiler.

b) we would have to provide different opam packages of the compiler and
native libraries (Lwt) for each platform/arch combination supported by
rumprun. This is unmanageable.

2) Is there an easy way to switch ocamlc and the various other parts of the
build system(s) involved into a verbose mode so that I can see what
compilers are being invoked and why they fail? At the moment I'm using
`strace' for this :-/

All ideas and help much appreciated,

Cheers,

Martin

[1] http://wiki.rumpkernel.org/Repo%3A-rumprun
[2] https://github.com/whitequark/opam-android

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