The commands that support impersonating a specific system have the --list-systems and --system options.
The --list-systems option lists all the supported systems that can be passed as an argument to --system.
$ guix build --list-systems The available systems are: - x86_64-linux [current] - aarch64-linux - armhf-linux - i586-gnu - i686-linux - mips64el-linux - powerpc-linux - powerpc64le-linux - riscv64-linux $ guix build --system=i686-linux hello /gnu/store/cc0km35s8x2z4pmwkrqqjx46i8b1i3gm-hello-2.12 $ file /gnu/store/cc0km35s8x2z4pmwkrqqjx46i8b1i3gm-hello-2.12/bin/hello /gnu/store/cc0km35s8x2z4pmwkrqqjx46i8b1i3gm-hello-2.12/bin/hello: ELF 32-bit LSB executable, Intel 80386 …
In the above example, the current system is x86_64-linux. The hello package is however built for the i686-linux system.
This is possible because the i686 CPU instruction set is a subset of the x86_64, hence i686 targeting binaries can be run on x86_64.
Still in the context of the previous example, if picking the
aarch64-linux system and the
--system=aarch64-linux hello has to build some derivations, an extra
step might be needed.
The aarch64-linux targeting binaries cannot directly be run on a x86_64-linux system. An emulation layer is requested. The GNU Guix daemon can take advantage of the Linux kernel binfmt_misc mechanism for that. In short, the Linux kernel can defer the execution of a binary targeting a foreign platform, here aarch64-linux, to a userspace program, usually an emulator.
There is a service that registers QEMU as a backend for the
binfmt_misc mechanism (see
qemu-binfmt-service-type). On Debian based foreign
distributions, the alternative would be the
binfmt_misc mechanism is not setup correctly, the building
will fail this way:
$ guix build --system=armhf-linux hello --check … unsupported-platform /gnu/store/jjn969pijv7hff62025yxpfmc8zy0aq0-hello-2.12.drv aarch64-linux while setting up the build environment: a `aarch64-linux' is required to build `/gnu/store/jjn969pijv7hff62025yxpfmc8zy0aq0-hello-2.12.drv', but I am a `x86_64-linux'…
whereas, with the
binfmt_misc mechanism correctly linked with
QEMU, one can expect to see:
$ guix build --system=armhf-linux hello --check /gnu/store/13xz4nghg39wpymivlwghy08yzj97hlj-hello-2.12
The main advantage of native building compared to cross-compiling, is that more packages are likely to build correctly. However it comes at a price: compilation backed by QEMU is way slower than cross-compilation, because every instruction needs to be emulated.
The availability of substitutes for the architecture targeted by the
--system option can mitigate this problem. An other way to work
around it is to install GNU Guix on a machine whose CPU supports
the targeted instruction set, and set it up as an offload machine
(see Daemon Offload Setup).