anchor.diff:            '';
         };
   
  +    virtualisation.useDefaultFilesystems =
  +      mkOption {
  +        type = types.bool;
  +        default = true;
  +        description =
  +          ''
  +            If enabled, the boot disk of the virtual machine will be
  +            formatted and mounted with the default filesystems for
  +            testing. Swap devices and LUKS will be disabled.
  +
  +            If disabled, a root filesystem has to be specified and
  +            formatted (for example in the initial ramdisk).
  +          '';
  +      };
  +
       virtualisation.efiVars =
         mkOption {
           type = types.str;
  
positive.code:     );
      boot.loader.grub.gfxmodeBios = with cfg.resolution; "${toString x}x${toString y}";
  
      boot.initrd.extraUtilsCommands = lib.mkIf (!config.boot.initrd.systemd.enable)
        ''
          # We need mke2fs in the initrd.
          copy_bin_and_libs ${pkgs.e2fsprogs}/bin/mke2fs
        '';
  
      boot.initrd.postDeviceCommands = lib.mkIf (!config.boot.initrd.systemd.enable)
        ''
          # If the disk image appears to be empty, run mke2fs to
          # initialise.
  
negative.code:         };
      in
        mkVMOverride (cfg.fileSystems //
        optionalAttrs cfg.useDefaultFilesystems {
          "/".device = cfg.bootDevice;
          "/".fsType = "ext4";
          "/".autoFormat = true;
        } //
        optionalAttrs config.boot.tmpOnTmpfs {
          "/tmp" = {
            device = "tmpfs";
            fsType = "tmpfs";
            neededForBoot = true;
            # Sync with systemd's tmp.mount;
            options = [ "mode=1777" "strictatime" "nosuid" "nodev" "size=${toString config.boot.tmpOnTmpfsSize}" ];
          };
        } //
        optionalAttrs cfg.useNixStoreImage {
          "/nix/${if cfg.writableStore then ".ro-store" else "store"}" = {
            device = "${lookupDriveDeviceName "nix-store" cfg.qemu.drives}";
            neededForBoot = true;
            options = [ "ro" ];
          };
        } //
        optionalAttrs (cfg.writableStore && cfg.writableStoreUseTmpfs) {
          "/nix/.rw-store" = {
            fsType = "tmpfs";
            options = [ "mode=0755" ];
            neededForBoot = true;
          };
        } //
        optionalAttrs cfg.useBootLoader {
          # see note [Disk layout with `useBootLoader`]
          "/boot" = {
            device = "${lookupDriveDeviceName "boot" cfg.qemu.drives}2"; # 2 for e.g. `vdb2`, as created in `bootDisk`
            fsType = "vfat";
            noCheck = true; # fsck fails on a r/o filesystem
          };
        } // lib.mapAttrs' mkSharedDir cfg.sharedDirectories);
  
      boot.initrd.systemd = lib.mkIf (config.boot.initrd.systemd.enable && cfg.writableStore) {
  