anchor.diff:            -initrd ${config.system.build.toplevel}/initrd \
             ${qemuGraphics} \
             $QEMU_OPTS \
  -          -append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.bootStage2} systemConfig=${config.system.build.toplevel} ${kernelConsole} $QEMU_KERNEL_PARAMS"
  +          -append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.bootStage2} systemConfig=${config.system.build.toplevel} regInfo=${regInfo} ${kernelConsole} $QEMU_KERNEL_PARAMS"
  +    '';
  +
  +    
  +  regInfo = pkgs.runCommand "reginfo"
  +    { exportReferencesGraph =
  +        map (x: [("closure-" + baseNameOf x) x]) config.virtualisation.pathsInNixDB;
  +      buildInputs = [ pkgs.perl ];
  +    }
  +    ''
  +      printRegistration=1 perl ${pkgs.pathsFromGraph} closure-* > $out
       '';
   
   in
  
positive.code:         options = "bind";
          neededForBoot = true;
        }
        # Mount the host's Nix database.  This allows read-only Nix
        # operations in the guest to work properly.
        { mountPoint = "/nix/var/nix/db";
          device = "/hostfs/nix/var/nix/db";
          options = "bind";
        }
      ];
  
    # Starting DHCP brings down eth0, which kills the connection to the
  
negative.code:         default = true;
          description =
            ''
              Whether to run QEMU with a graphics window, or access
              the guest computer serial port through the host tty.
            '';
        };
  
      virtualisation.pathsInNixDB =
        mkOption {
          default = [];
          description =
            ''
              The list of paths whose closure is registered in the Nix
              database in the VM.  All other paths in the host Nix store
              appear in the guest Nix store as well, but are considered
              garbage (because they are not registered in the Nix
              database in the guest).
            '';
        };
        
    };
  
    cfg = config.virtualisation;
  