Cross-platform Compilation

On any Linux platform, there are two ways to do cross-platform compilation. For example, to build an aarch64-linux program on an x86_64-linux host, you can use the following methods:

1. Use the cross-compilation toolchain to compile the aarch64 program.
   • The disadvantage is that you cannot use the NixOS binary cache, and you need to compile everything yourself (cross-compilation also has a cache, but there is basically nothing in it).
   • The advantages are that you don't need to emulate the instruction set, and the performance is high.
2. Use QEMU to emulate the aarch64 architecture and then compile the program in the emulator.
   • The disadvantage is that the instruction set is emulated, and the performance is poor.
   • The advantage is that you can use the NixOS binary cache, and you don't need to compile everything yourself.

If you use method one, you don't need to enable binfmt_misc, but you need to execute the compilation through the cross-compilation toolchain.

If you use method two, you need to enable the binfmt_misc of the aarch64 architecture in the NixOS configuration of the building machine.

Cross Compilation

nixpkgs provides a set of predefined host platforms for cross-compilation called pkgsCross. You can explore them in nix repl.

› nix repl '<nixpkgs>'
warning: future versions of Nix will require using `--file` to load a file
Welcome to Nix 2.13.3. Type :? for help.

Loading installable ''...
Added 19273 variables.
nix-repl> pkgsCross.<TAB>
pkgsCross.aarch64-android             pkgsCross.msp430
pkgsCross.aarch64-android-prebuilt    pkgsCross.musl-power
pkgsCross.aarch64-darwin              pkgsCross.musl32
pkgsCross.aarch64-embedded            pkgsCross.musl64
pkgsCross.aarch64-multiplatform       pkgsCross.muslpi
pkgsCross.aarch64-multiplatform-musl  pkgsCross.or1k
pkgsCross.aarch64be-embedded          pkgsCross.pogoplug4
pkgsCross.arm-embedded                pkgsCross.powernv
pkgsCross.armhf-embedded              pkgsCross.ppc-embedded
pkgsCross.armv7a-android-prebuilt     pkgsCross.ppc64
pkgsCross.armv7l-hf-multiplatform     pkgsCross.ppc64-musl
pkgsCross.avr                         pkgsCross.ppcle-embedded
pkgsCross.ben-nanonote                pkgsCross.raspberryPi
pkgsCross.fuloongminipc               pkgsCross.remarkable1
pkgsCross.ghcjs                       pkgsCross.remarkable2
pkgsCross.gnu32                       pkgsCross.riscv32
pkgsCross.gnu64                       pkgsCross.riscv32-embedded
pkgsCross.i686-embedded               pkgsCross.riscv64
pkgsCross.iphone32                    pkgsCross.riscv64-embedded
pkgsCross.iphone32-simulator          pkgsCross.rx-embedded
pkgsCross.iphone64                    pkgsCross.s390
pkgsCross.iphone64-simulator          pkgsCross.s390x
pkgsCross.loongarch64-linux           pkgsCross.sheevaplug
pkgsCross.m68k                        pkgsCross.vc4
pkgsCross.mingw32                     pkgsCross.wasi32
pkgsCross.mingwW64                    pkgsCross.x86_64-darwin
pkgsCross.mips-linux-gnu              pkgsCross.x86_64-embedded
pkgsCross.mips64-linux-gnuabi64       pkgsCross.x86_64-freebsd
pkgsCross.mips64-linux-gnuabin32      pkgsCross.x86_64-netbsd
pkgsCross.mips64el-linux-gnuabi64     pkgsCross.x86_64-netbsd-llvm
pkgsCross.mips64el-linux-gnuabin32    pkgsCross.x86_64-unknown-redox
pkgsCross.mipsel-linux-gnu
pkgsCross.mmix

If you want to set pkgs to a cross-compilation toolchain globally in a flake, you only need to add a Module in flake.nix, as shown below:

{
  description = "NixOS running on LicheePi 4A";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-24.11";
  };

  outputs = inputs@{ self, nixpkgs, ... }: {
    nixosConfigurations.lp4a = nixpkgs.lib.nixosSystem {
      # native platform
      system = "x86_64-linux";
      modules = [

        # add this module, to enable cross-compilation.
        {
          nixpkgs.crossSystem = {
            # target platform
            system = "riscv64-linux";
          };
        }

        # ...... other modules
      ];
    };
  };
}

The nixpkgs.crossSystem option is used to set pkgs to a cross-compilation toolchain, so that all the contents built will be riscv64-linux architecture.

Compile through emulated system

The second method is to cross-compile through the emulated system. This method does not require a cross-compilation toolchain.

To use this method, first your building machine needs to enable the binfmt_misc module in the configuration. If your building machine is NixOS, add the following configuration to your NixOS Module to enable the simulated build system of aarch64-linux and riscv64-linux architectures:

{ ... }:
{
  # ......

  # Enable binfmt emulation.
  boot.binfmt.emulatedSystems = [ "aarch64-linux" "riscv64-linux" ];

  # ......
}

As for flake.nix, its setting method is very simple, even simpler than the setting of cross-compilation, as shown below:

{
  description = "NixOS running on LicheePi 4A";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-24.11";
  };

  outputs = inputs@{ self, nixpkgs, ... }: {
    nixosConfigurations.lp4a = nixpkgs.lib.nixosSystem {
      # native platform
      system = "riscv64-linux";
      modules = [
        # ...... other modules
      ];
    };
  };
}

You do not need to add any additional modules, just specify system as riscv64-linux. Nix will automatically detect whether the current system is riscv64-linux during the build. If not, it will automatically build through the emulated system(QEMU). For users, these underlying operations are completely transparent.

Linux binfmt_misc

The previous section only provided an introduction on how to use Nix's emulated system, but if you want to understand the underlying details, here's a brief introduction.

binfmt_misc is a feature of the Linux kernel, which stands for Kernel Support for miscellaneous Binary Formats. It enables Linux to run programs for almost any CPU architecture, including X86_64, ARM64, RISCV64, and more.

To enable binfmt_misc to run programs in various formats, two things are required: a specific identification method for the binary format and the location of the corresponding interpreter. Although binfmt_misc sounds powerful, its implementation is surprisingly easy to understand. It works similarly to how the Bash interpreter determines the interpreter to use by reading the first line of a script file (e.g., #!/usr/bin/env python3). binfmt_misc defines a set of rules, such as reading the magic number at a specific location in the binary file or determining the executable file format based on the file extension (e.g., .exe, .py). It then invokes the corresponding interpreter to execute the program. The default executable file format in Linux is ELF, but binfmt_misc expands the execution possibilities by allowing a wide range of binary files to be executed using their respective interpreters.

To register a binary program format, you need to write a line in the format :name:type:offset:magic:mask:interpreter:flags to the /proc/sys/fs/binfmt_misc/register file. The detailed explanation of the format is beyond the scope of this discussion.

Since manually writing the registration information for binfmt_misc can be cumbersome, the community provides a container to assist with automatic registration. This container is called binfmt and running it will install various binfmt_misc emulators. Here's an example:

# Register all architectures
podman run --privileged --rm tonistiigi/binfmt:latest --install all

# Register only common arm/riscv architectures
docker run --privileged --rm tonistiigi/binfmt --install arm64,riscv64,arm

The binfmt_misc module was introduced in Linux version 2.6.12-rc2 and has undergone several minor changes in functionality since then. In Linux 4.8, the "F" (fix binary) flag was added, allowing the interpreter to be invoked correctly in mount namespaces and chroot environments. To work properly in containers where multiple architectures need to be built, the "F" flag is necessary. Therefore, the kernel version needs to be 4.8 or above.

In summary, binfmt_misc provides transparency compared to explicitly calling an interpreter to execute non-native architecture programs. With binfmt_misc, users no longer need to worry about which interpreter to use when running a program. It allows programs of any architecture to be executed directly. The configurable "F" flag is an added benefit, as it loads the interpreter program into memory during installation and remains unaffected by subsequent environment changes.

Custom build toolchain

Sometimes we may need to use a custom toolchain for building, such as using our own gcc, or using our own musl libc, etc. This modification can be achieved through overlays.

For example, let's try to use a different version of gcc, and test it through nix repl:

```shell
› nix repl -f '<nixpkgs>'
Welcome to Nix 2.13.3. Type :? for help.

Loading installable ''...
Added 17755 variables.

# replace gcc through overlays, this will create a new instance of nixpkgs
nix-repl> a = import <nixpkgs> { crossSystem = { config = "riscv64-unknown-linux-gnu"; }; overlays = [ (self: super: { gcc = self.gcc12; }) ]; }

# check the gcc version, it is indeed changed to 12.2
nix-repl> a.pkgsCross.riscv64.stdenv.cc
«derivation /nix/store/jjvvwnf3hzk71p65x1n8bah3hrs08bpf-riscv64-unknown-linux-gnu-stage-final-gcc-wrapper-12.2.0.drv»

# take a look at the default pkgs, it is still 11.3
nix-repl> pkgs.pkgsCross.riscv64.stdenv.cc
«derivation /nix/store/pq3g0wq3yfc4hqrikr03ixmhqxbh35q7-riscv64-unknown-linux-gnu-stage-final-gcc-wrapper-11.3.0.drv»

So how to use this method in Flakes? The example flake.nix is as follows:

{
  description = "NixOS running on LicheePi 4A";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-24.11-small";
  };

  outputs = { self, nixpkgs, ... }:
  {
    nixosConfigurations.lp4a = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        {
          nixpkgs.crossSystem = {
            config = "riscv64-unknown-linux-gnu";
          };

          # replace gcc with gcc12 through overlays
          nixpkgs.overlays = [ (self: super: { gcc = self.gcc12; }) ];
        }

        # other modules ......
      ];
    };
  };
}

nixpkgs.overlays is used to modify the pkgs instance globally, and the modified pkgs instance will take effect to the whole flake. It will likely cause a large number of cache missing, and thus require building a large number of Nix packages locally.

To avoid this problem, a better way is to create a new pkgs instance, and only use this instance when building the packages we want to modify. The example flake.nix is as follows:

{
  description = "NixOS running on LicheePi 4A";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-24.11-small";
  };

  outputs = { self, nixpkgs, ... }: let
    # create a new pkgs instance with overlays
    pkgs-gcc12 = import nixpkgs {
      localSystem = "x86_64-linux";
      crossSystem = {
        config = "riscv64-unknown-linux-gnu";
      };

      overlays = [
        (self: super: { gcc = self.gcc12; })
      ];
    };
  in {
    nixosConfigurations.lp4a = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      specialArgs = {
        # pass the new pkgs instance to the module
        inherit pkgs-gcc12;
      };
      modules = [
        {
          nixpkgs.crossSystem = {
            config = "riscv64-unknown-linux-gnu";
          };
        }

        ({pkgs-gcc12, ...}: {
          # use the custom pkgs instance to build the package hello
          environment.systemPackages = [ pkgs-gcc12.hello ];
        })

        # other modules ......
      ];
    };
  };
}

Through the above method, we can easily customize the build toolchain of some packages without affecting the build of other packages.

References

• Cross compilation - nix.dev