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https://github.com/mirage/ocaml-solo5

Freestanding OCaml runtime
https://github.com/mirage/ocaml-solo5

mirageos ocaml

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Freestanding OCaml runtime

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# ocaml-solo5 -- OCaml compiler with Solo5 backend



This package provides a OCaml compiler suitable for linking with a

Solo5 base layer:



- package versions ≥ 1.0 and the `main` branch are compatible with OCaml 5+

  compilers, see the “[Supported compiler versions]” section for details,

- package versions 0.8.x and the `4.14` branch are compatible with OCaml 4.14

  compilers.



[Supported compiler versions]: #supported-compiler-versions



## License and contributions



All original contributions to this package are licensed under the standard MIT

license.



This package incorporates components derived or copied from musl libc, OpenBSD,

OpenLibm and other third parties. For full details of the licenses of these

third party components refer to the included LICENSE file.



The OCaml runtime ("OCaml Core System") built by this package is distributed

under the terms of the GNU LGPL version 2.1 with a special exception for static

or dynamic linking to produce an executable file. For details refer to the

LICENSE file included in the version of the `ocaml-src` OPAM package installed

on your system as a dependency when you build this package.



## Components



The following components are built and installed, where `$prefix` and `$sysroot`

are the values given to the corresponding `configure` arguments (ie the value of

`opam var prefix` and `opam var :lib` when installed via OPAM).



In `$prefix/bin`:



- the toolchain to build binaries, using the `-solo5-ocaml-` prefix.



`$sysroot` will contain the installation of the OCaml compiler and the `nolibc`

and OpenLibm support libraries.



In `$sysroot/bin`:

- `ocamlopt.{opt,byte}`: a native OCaml compiler configured for the chosen

  target.

- Some other standard tools such as the `ocaml` interpreter and

  `ocamlc.{byte,opt}` a bytecode OCaml compiler configured for the chosen

  target. Please note that the bytecode runtime is not supported.



In `$sysroot/lib/ocaml`:

- `libasmrun.a`: the OCaml native code runtime for the Solo5 target.

- The standard library.

- In `caml/`: Header files for the OCaml runtime.



In `$prefix/lib`:



- `libnolibc.a`: libc interfaces required by the OCaml runtime.

- `libopenlibm.a`: libm required by the OCaml runtime.



In `$sysroot/include`:



- Header files for `nolibc` and OpenLibm.



In `$prefix/lib/findlib.conf.d`:



- `solo5.conf`: ocamlfind definition of the cross-compilation toolchain.



### Usage



The installed compiler is able to build Solo5 executables. The Solo5 bindings

(xen, hvt, spt, ...) are chosen at link time, using the Solo5-specific

`-z solo5-abi=XXX` compiler/linker option. Linking an executable with no

bindings results in a _dummy_ executable.



To build with the Solo5 compiler toolchain, it has to be selected using

ocamlfind or dune:

- ocamlfind: `ocamlfind -toolchain solo5 ...`

- dune: `dune build -x solo5`, or add the toolchain in a build context

  in the dune workspace file.



#### Example



The `example` describes the minimal structure needed to build an ocaml-solo5

executable with dune, linked with the hvt bindings by default. It requires an

application manifest and a startup file to initialize the libc.



- Build: `dune build -x solo5`

- Run: `solo5-hvt _build/solo5/main.exe`



## Supported compiler versions



Tested against OCaml 5.2.1. Other versions would require specific patches (see

the `patches` directory).



## Porting to a different (uni)kernel base layer



Assuming your unikernel base layer is packaged for OPAM in a similar

fashion to Solo5 this should be as simple as:



1. Adding the appropriate clauses to determine the OPAM packages required

   and `MAKECONF_CFLAGS` for compilation to `configure.sh`.

2. Implementing a `nolibc/sysdeps_yourkernel.c`.



Note that the nolibc code is intentionally strict about namespacing of APIs

and header files. If your base layer exports symbols or defines types which

conflict with nolibc then the recommended course of action is to fix your

base layer to not export anything defined by "POSIX" or "standard C".



## Updating the vendored copy of OpenLibm



OpenLibm is "vendored" into this repository using `git subtree`:



    git subtree add --prefix openlibm https://github.com/JuliaLang/openlibm.git v0.5.4 --squash



To update the vendored copy of OpenLibm to the newer upstream version `TAG`,

use the following command _on a branch_ and then file a PR:



    git subtree pull --prefix openlibm https://github.com/JuliaLang/openlibm.git TAG --squash