Build a bare bones bootable nixos qcow2 image suitable for running with libvirt/qemu/kvm.

readme.md

Build a bare bones bootable nixos qcow2 image suitable for running with libvirt/qemu/kvm.

nix-build '<nixpkgs/nixos>' -A config.system.build.qcow2 --arg configuration "{ imports = [ ./build-qcow2.nix ]; }"

Should create a results directory that symlinks to a qcow2 image in the store.

I basically copied this from the openstack image in nixpkgs because I don't know a better way.

Update:

Thanks to @d-goldin I've just learnt I can start a VM using just:

nix-build '<nixpkgs/nixos>' -A vm -I nixos-config=./machine-config.nix build-vm
./result/bin/run-nixos-vm

Which is pretty neat, and I guess pretty common.

build-qcow2.nix

{ config, lib, pkgs, ... }:

with lib;

{
  imports =
    [ 
      <nixpkgs/nixos/modules/installer/cd-dvd/channel.nix>
      ./machine-config.nix
    ];

  system.build.qcow2 = import <nixpkgs/nixos/lib/make-disk-image.nix> {
    inherit lib config;
    pkgs = import <nixpkgs> { inherit (pkgs) system; }; # ensure we use the regular qemu-kvm package
    diskSize = 8192;
    format = "qcow2";
    configFile = pkgs.writeText "configuration.nix"
      ''
        {
          imports = [ <./machine-config.nix> ];
        }
      '';
  };
}

machine-config.nix

{ pkgs, lib, ... }:

with lib;

{
  imports = [
    <nixpkgs/nixos/modules/profiles/qemu-guest.nix>
  ];

  config = {
    fileSystems."/" = {
      device = "/dev/disk/by-label/nixos";
      fsType = "ext4";
      autoResize = true;
    };

    boot.growPartition = true;
    boot.kernelParams = [ "console=ttyS0" ];
    boot.loader.grub.device = "/dev/vda";
    boot.loader.timeout = 0;

    users.extraUsers.root.password = "";

  };
}

virt-install                                                \
    --connect qemu:///system                                \
    --name nixos                                            \
    --vcpus 1                                               \
    --memory 2048                                           \
    --autostart                                             \
    --memballoon virtio                                     \
    --network default                                       \
    --boot hd                                               \
    --disk "vol=vm/nixos.qcow2,format=qcow2,bus=virtio"     \
    --noautoconsole                                         \
    --graphics none                                         \
    --os-variant centos8                                    \
    --console pty,target_type=serial                        \
    --machine q35