Your first unikernel in OCaml

OCaml is better than C

C is a good language for low-level programming, which is why Solo5 is implemented in C, but we would like to switch to a much 'safer' and more expressive language, namely OCaml. To this end, we are developing and maintaining a modified version of the OCaml compiler, available here: ocaml-solo5. This compiler can be installed via:

$ opam install ocaml-solo5

Just like Solo5, it provides an OCaml toolchain for building an unikernel in OCaml. Here is an example of OCaml code:

let () = print_endline "Hello World!"

To do this, we need another C file, startup.c, which will define our solo5_app_main function and run the OCaml code:

#ifdef __ocaml_solo5__
#include "solo5.h"
#include <caml/callback.h>

static char *argv[] = { "unikernel", NULL };

void _nolibc_init(uintptr_t, size_t);

int solo5_app_main(const struct solo5_start_info *si) {

Since Solo5 offers very little, ocaml-solo5 provides certain building blocks (such as malloc()) required by OCaml. These functions are notably provided by our aptly named nolibc. Furthermore, we must initialise this library in order to specify the available "heap" to it.

  _nolibc_init(si->heap_start, si->heap_size);

caml_startup is the function that will execute the OCaml code we have linked to our unikernel.

  caml_startup(argv);
  return (0);
}
#endif

Compilation

Next, we'll use ocamlfind, a utility that orchestrates the OCaml compiler ocamlopt so that we can build our unikernel:

$ ocamlfind opt -toolchain solo5 -c startup.c -o startup.o

Note the manifest.o file that was built earlier, as well as the -cclib "-z solo5-abi=hvt" option. Without these options, the unikernel would be malformed.

$ ocamlfind opt -toolchain solo5 startup.o manifest.o main.ml \
  -cclib "-z solo5-abi=hvt" -o main.hvt

Execution

Just like our previous unikernel, all you need to do is run our unikernel using solo5-hvt:

$ solo5-hvt main.hvt
            |      ___|
  __|  _ \  |  _ \ __ \
\__ \ (   | | (   |  ) |
____/\___/ _|\___/____/
Solo5: Bindings version v0.12.0
Solo5: Memory map: 512 MB addressable:
Solo5:   reserved @ (0x0 - 0xfffff)
Solo5:       text @ (0x100000 - 0x150fff)
Solo5:     rodata @ (0x151000 - 0x169fff)
Solo5:       data @ (0x16a000 - 0x375fff)
Solo5:       heap >= 0x376000 < stack < 0x20000000
Hello World!
Solo5: solo5_exit(0) called

There you go – you've created your very first unikernel in OCaml!

Some notes

At this stage, it is important to describe two features of ocaml-solo5:

ocaml-solo5 is essentially used to provide a consistent toolchain for compiling C files alongside OCaml files. However, ocaml-solo5 does not compile OCaml files any differently. Better still, it is possible to use pure OCaml libraries compiled using your default compiler (and installed via OPAM) within your unikernel:

let () = Fmt.pr "Hello World!"

$ ocamlfind opt -toolchain solo5 -linkpkg -package fmt \
  startup.o manifest.o main.ml \
  -cclib "-z solo5-abi=hvt" -o main.hvt

The OCaml distribution contains libraries that allow interaction with the system, such as the Unix module. However, in the case of ocaml-solo5, it is impossible and undesirable for us to provide such a module. In other words, you must treat the Unix module as if it did not exist in the context of unikernels.