anchor.diff:      # configuration, where the regular value for the `fileSystems'
       # attribute should be disregarded for the purpose of building a VM
       # test image (since those filesystems don't exist in the VM).
  -    fileSystems = mkVMOverride (
  -      cfg.fileSystems //
  -      { "/".device = cfg.bootDevice;
  -        ${if cfg.writableStore then "/nix/.ro-store" else "/nix/store"} =
  -          { device = "store";
  -            fsType = "9p";
  -            options = [ "trans=virtio" "version=9p2000.L" "cache=loose" ] ++ lib.optional (cfg.msize != null) "msize=${toString cfg.msize}";
  -            neededForBoot = true;
  -          };
  +    fileSystems =
  +    let
  +      mkSharedDir = tag: share:
  +        {
  +          name =
  +            if tag == "nix-store" && cfg.writableStore
  +              then "/nix/.ro-store"
  +              else share.target;
  +          value.device = tag;
  +          value.fsType = "9p";
  +          value.neededForBoot = true;
  +          value.options =
  +            [ "trans=virtio" "version=9p2000.L"  "msize=${toString cfg.msize}" ]
  +            ++ lib.optional (tag == "nix-store") "cache=loose";
  +        };
  +    in
  +      mkVMOverride (cfg.fileSystems //
  +      {
  +        "/".device = cfg.bootDevice;
  +
           "/tmp" = mkIf config.boot.tmpOnTmpfs
             { device = "tmpfs";
               fsType = "tmpfs";
  
positive.code:     virtualisation.bootDevice = mkDefault (driveDeviceName 1);
  
      virtualisation.pathsInNixDB = [ config.system.build.toplevel ];
  
      # FIXME: Consolidate this one day.
      virtualisation.qemu.options = mkMerge [
  
negative.code:           -smp ${toString config.virtualisation.cores} \
            -device virtio-rng-pci \
            ${concatStringsSep " " config.virtualisation.qemu.networkingOptions} \
            ${concatStringsSep " \\\n    "
              (mapAttrsToList
                (tag: share: "-virtfs local,path=${share.source},security_model=none,mount_tag=${tag}")
                config.virtualisation.sharedDirectories)} \
            ${drivesCmdLine config.virtualisation.qemu.drives} \
            ${concatStringsSep " \\\n    " config.virtualisation.qemu.options} \
            $QEMU_OPTS \
  