{ pkgs
, name ? "sandbox"
, user ? "dummy"
, config ? { }
, restrictNetwork ? true  # to be replaced with virtualisation.restrictNetwork
, patchQemu9p ? false
}:

let
  # Patched QEMU to fix slow 9p file share.
  # https://linus.schreibt.jetzt/posts/qemu-9p-performance.html
  qemu_kvm_patched_9p =
  assert !(pkgs.lib.versionAtLeast pkgs.qemu_kvm.version "7.2.0");
  pkgs.qemu_kvm.overrideAttrs (o: {
    patches = o.patches ++ [ (pkgs.fetchpatch {
      name = "qemu-9p-performance-fix.patch";
      url = "https://github.com/qemu/qemu/commit/f5265c8.patch";
      sha256 = "sha256-PSOv0dhiEq9g6B1uIbs6vbhGr7BQWCtAoLHnk4vnvVg=";
    }) ];
  });

in rec {

  nixosConfigurations.${name} = pkgs.nixos
  ({ modulesPath, lib, pkgs, ... }: with lib; {
    imports = [
      (modulesPath + "/profiles/minimal.nix")
      { environment.noXlibs = false; }  # avoid mass rebuild

      (modulesPath + "/profiles/qemu-guest.nix")
      (modulesPath + "/virtualisation/qemu-vm.nix")

      config
    ];

    system.stateVersion = mkDefault trivial.release;

    networking = {
      hostName = name;
      firewall.enable = mkDefault false;
    };

    users.users.${user} = {
      isNormalUser = mkDefault true;
      password = mkDefault "";
      extraGroups = mkDefault [ "wheel" ];
    };

    security.sudo.wheelNeedsPassword = mkDefault false;

    services.getty = {
      autologinUser = mkDefault user;
      helpLine = mkDefault ''
        Press <CTRL-a> <x> to terminate the virtual machine.
        The working directory on the host is mounted to /mnt.
      '';
    };

    virtualisation = {
      graphics = mkDefault false;
      diskImage = mkDefault "$(mktemp).qcow2";

      sharedDirectories.host = {
        source = "$SHARED_CWD";
        target = "/mnt";
      };

      # Uncomment when this is merged:
      # https://github.com/NixOS/nixpkgs/pull/200225
      #restrictNetwork = mkDefault true;

      qemu.package = mkDefault
        (if patchQemu9p then qemu_kvm_patched_9p else pkgs.qemu_kvm);
    };
  });

  packages.${name} = nixosConfigurations.${name}.config.system.build.vm;

  apps.${name} = {
    type = "app";
    program = toString (pkgs.writeShellScript "sandbox-vm" (
    (pkgs.lib.optionalString restrictNetwork ''
      # Isolate from network
      # Stopgap solution until this is merged:
      # https://github.com/NixOS/nixpkgs/pull/200225
      QEMU_NET_OPTS="restrict=yes,''${QEMU_NET_OPTS:+,$QEMU_NET_OPTS}"
      export QEMU_NET_OPTS
    '') + ''
      # Save current directory for mounting in VM
      SHARED_CWD=$PWD
      export SHARED_CWD

      ${packages.${name}}/bin/run-${name}-vm

      reset
      echo "Exited virtual machine."
    ''));
  };

}