Building a NixOS VM from a flake

nix.dev has documentation for building NixOS VMs using the handy qemu-vm.nix module. With a bit of digging into how this works it’s easy enough to adapt How to build a NixOS configuration from a flake with nix build to also build this script from a flake and run your NixOS configuration as a VM.

At the time of writing, the vm property is set here in nixpkgs/nixos/default.nix:

{
  ...
  inherit (eval.config.system.build) vm vmWithBootLoader;
}

and that property comes from qemu-vm.nix here:

system.build.vm =
  hostPkgs.runCommand "nixos-vm"
    ...

hostPkgs.runCommand creates a derivation which when built will run a shell script, in this case it’s the script which builds the run-nixos-vm script. So to use it in a flake, have a nixosConfiguration.<host> property and a configuration.nix like normal:

{
  description = "A very basic flake";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
  };

  outputs = { self, nixpkgs }: {

    nixosConfigurations.test = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ./configuration.nix
      ];
    };

  };
}

and then run (-L for logs)

nix build -L .#nixosConfigurations.test.config.system.build.vm

QEMU_KERNEL_PARAMS=console=ttyS0 ./result/bin/run-nixos-vm -nographic; reset

Addendum

Ok it turns out you have to explicitly import qemu-vm.nix for this to work properly and give you all the virtualisation options, so add:

  imports = [
    "${modulesPath}/virtualisation/qemu-vm.nix"
  ];

to configuration.nix.