Is it possible to build a nixos VM with a different architecture to the host?

Help
1

Hello!
I have an x86-64 laptop running nixos and I’m planning to use it to develop config for some of my other hosts (an x86-64 home server and a raspberry pi attached to my 3d printer).
For x86-64 configs, I can easily sanity check them with nixos-rebuild build-vm
but that doesn’t work for the aarch64 config:

       error: assertion '(system != (pkgs).stdenv.hostPlatform.system)' failed
       at /nix/store/myf9yi8lam7z19q8dydwzk8vyjxs360j-source/nixos/modules/system/boot/binfmt.nix:354:9:
          353|         system:
          354|         assert system != pkgs.stdenv.hostPlatform.system;
             |         ^
          355|         {

I assume I’m just missing some magic switch to allow my laptop to cross-build aarch64 packages, but i’m struggling to find any documentation about this.
Any help is hugely appreciated!

2

No, unless there has been some recent work on this, it’s not supported. The script that runs the VM uses qemu-kvm under the hood: as there is no emulation, the gust arch must equal the host arch.

It’s probably not to difficult to use the cross-compilation machinery of Nixpkgs to implement it, but it has never been done.

3

Ah, that’s a shame! Thanks for letting me know so I can stop wasting time on it! :pray:

4

You can consider boot.binfmt.emulatedSystems = ["aarch64-linux"]; in your host system to enable what you want

5

yeah, i have that set and it allows me to cross-build SD images for the raspberry pi, but not to build a VM. which came as a suprise…

6

If you just want to test the system configuration, a VM with the host architecture is probably good enough, just uncomment the device specific parts, or use virtualisatin.vmVariant. Even if you were able to build a generic aarch64 VM, it’s likely not going to be able to boot the raspberry pi SD image.

I tested this a few years ago, and while Qemu could nominally emulate the raspberry pi 1 hardware, I never managed to run u-boot on it.

7

Isn’t it possible to generate a UEFI VM image for the build architecture? Not that it matters too much unless you’re trying to test some architecture-specific stuff.

8

that’s a REALLY good point! i’ll give that a try, thanks!

9

supposedly, yes. although i couldn’t get it working properly on an RPI5. plus i believe you lose access to some features.