[Nix-dev] Trying to build L4Linux kernel on NixOS: how to cross-compile?

[Mateusz Czaplinski]

I'm testing my luck trying to get L4Linux kernel running on NixOS. I tried
to get the L4Linux kernel to build (it uses the regular kernel
infrastructure) with an expression like below (full contents at:
https://github.com/akavel/l4.nix/blob/9fb900a88619eb4830710e531cd3f076d74b584d/l4linux.nix
):

[...]
  kernel =
    pkgs.callPackage "${nixpkgs}/pkgs/os-specific/linux/kernel/generic.nix"
(rec {
      version = "4.7.0-l4-2016082114";  # TODO(akavel): ok or not?
      modDirVersion = "4.7.0-l4";  # see: nixpkgs issue #17801 and
linux-mptcp.nix
      src = l4re;
      kernelPatches = [];
    });
in
  kernel

But then I found out [1], that this actually builds a *vanilla* (non-L4)
kernel, because of nonempty argument ARCH passed to make by
<nixpkgs>/.../kernel/generic.nix. To fix this, I believe I need to use the
cross-compilation infrastructure of Nix, so that generic.nix would see
stdenv.cross.platform.kernelArch == "l4", and thus pass it as ARCH="l4" to
make. Now, based on some rudimentary info in The Nixpkgs  Manual + linked
wiki, I tried changing the final line to read "kernel.crossDrv", then
putting various variants of:

  crossSystem = { platform = { kernelArch = "l4"; }; };

in various places (including the {} just after `import <nixpkgs>`, or the
{} just after pkgs.callPackage), also supplemented with more fields stolen
from <nixpkgs>/.../platforms.nix's pcBase or from some random snippets
found on The Internets. But whatever I tried to do, nix-build always seemed
to reject my courting with a blunt:

  error: attribute `crossDrv' missing, at <path>/l4linux.nix:<line>:<column>

What should I do to make it happy?

Also, as a somewhat related question: what exactly triggers appearance of a
.crossDrv attribute in a derivation? (Is there some place in nixpkgs where
I can find the code for that?)

Thanks & Best Regards,
/Mateusz.

  [1]: http://os.inf.tu-dresden.de/pipermail/l4-hackers/2017/007995.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.science.uu.nl/pipermail/nix-dev/attachments/20170403/be81b7f1/attachment.html>