anchor.diff:  
       boot.kernelModules = [ "tun" ];
   
  -    systemd.services.libvirtd =
  -      { description = "Libvirt Virtual Machine Management Daemon";
  +    users.extraGroups.libvirtd.gid = config.ids.gids.libvirtd;
   
  -        wantedBy = [ "multi-user.target" ];
  -        after = [ "systemd-udev-settle.service" ]
  -                ++ optional vswitch.enable "vswitchd.service";
  +    systemd.services.libvirtd = {
  +      description = "Libvirt Virtual Machine Management Daemon";
   
  -        path = [ 
  -            pkgs.bridge-utils 
  -            pkgs.dmidecode 
  -            pkgs.dnsmasq
  -            pkgs.ebtables
  -          ] 
  -          ++ optional cfg.enableKVM pkgs.qemu_kvm
  -          ++ optional vswitch.enable vswitch.package;
  +      wantedBy = [ "multi-user.target" ];
  +      after = [ "systemd-udev-settle.service" ]
  +              ++ optional vswitch.enable "vswitchd.service";
   
  -        preStart =
  -          ''
  -            mkdir -p /var/log/libvirt/qemu -m 755
  -            rm -f /var/run/libvirtd.pid
  +      path = [
  +          pkgs.bridge-utils
  +          pkgs.dmidecode
  +          pkgs.dnsmasq
  +          pkgs.ebtables
  +        ]
  +        ++ optional cfg.enableKVM pkgs.qemu_kvm
  +        ++ optional vswitch.enable vswitch.package;
   
  -            mkdir -p /var/lib/libvirt
  -            mkdir -p /var/lib/libvirt/dnsmasq
  +      preStart = ''
  +        mkdir -p /var/log/libvirt/qemu -m 755
  +        rm -f /var/run/libvirtd.pid
   
  -            chmod 755 /var/lib/libvirt
  -            chmod 755 /var/lib/libvirt/dnsmasq
  +        mkdir -p /var/lib/libvirt
  +        mkdir -p /var/lib/libvirt/dnsmasq
   
  -            # Copy default libvirt network config .xml files to /var/lib
  -            # Files modified by the user will not be overwritten
  -            for i in $(cd ${pkgs.libvirt}/var/lib && echo \
  -                libvirt/qemu/networks/*.xml libvirt/qemu/networks/autostart/*.xml \
  -                libvirt/nwfilter/*.xml );
  -            do
  -                mkdir -p /var/lib/$(dirname $i) -m 755
  -                cp -npd ${pkgs.libvirt}/var/lib/$i /var/lib/$i
  -            done
  +        chmod 755 /var/lib/libvirt
  +        chmod 755 /var/lib/libvirt/dnsmasq
   
  -            # libvirtd puts the full path of the emulator binary in the machine
  -            # config file. But this path can unfortunately be garbage collected
  -            # while still being used by the virtual machine. So update the
  -            # emulator path on each startup to something valid (re-scan $PATH).
  -            for file in /etc/libvirt/qemu/*.xml /etc/libvirt/lxc/*.xml; do
  -                test -f "$file" || continue
  -                # get (old) emulator path from config file
  -                emulator=$(grep "^[[:space:]]*<emulator>" "$file" | sed 's,^[[:space:]]*<emulator>\(.*\)</emulator>.*,\1,')
  -                # get a (definitely) working emulator path by re-scanning $PATH
  -                new_emulator=$(PATH=${pkgs.libvirt}/libexec:$PATH command -v $(basename "$emulator"))
  -                # write back
  -                sed -i "s,^[[:space:]]*<emulator>.*,    <emulator>$new_emulator</emulator> <!-- WARNING: emulator dirname is auto-updated by the nixos libvirtd module -->," "$file"
  -            done
  -          ''; # */
  +        # Copy default libvirt network config .xml files to /var/lib
  +        # Files modified by the user will not be overwritten
  +        for i in $(cd ${pkgs.libvirt}/var/lib && echo \
  +            libvirt/qemu/networks/*.xml libvirt/qemu/networks/autostart/*.xml \
  +            libvirt/nwfilter/*.xml );
  +        do
  +            mkdir -p /var/lib/$(dirname $i) -m 755
  +            cp -npd ${pkgs.libvirt}/var/lib/$i /var/lib/$i
  +        done
   
  -        serviceConfig = {
  -          ExecStart = ''@${pkgs.libvirt}/sbin/libvirtd libvirtd --config "${configFile}" --daemon ${concatStringsSep " " cfg.extraOptions}'';
  -          Type = "notify";
  -          KillMode = "process"; # when stopping, leave the VMs alone
  -          Restart = "on-failure";
  -        };
  +        # libvirtd puts the full path of the emulator binary in the machine
  +        # config file. But this path can unfortunately be garbage collected
  +        # while still being used by the virtual machine. So update the
  +        # emulator path on each startup to something valid (re-scan $PATH).
  +        for file in /etc/libvirt/qemu/*.xml /etc/libvirt/lxc/*.xml; do
  +            test -f "$file" || continue
  +            # get (old) emulator path from config file
  +            emulator=$(grep "^[[:space:]]*<emulator>" "$file" | sed 's,^[[:space:]]*<emulator>\(.*\)</emulator>.*,\1,')
  +            # get a (definitely) working emulator path by re-scanning $PATH
  +            new_emulator=$(PATH=${pkgs.libvirt}/libexec:$PATH command -v $(basename "$emulator"))
  +            # write back
  +            sed -i "s,^[[:space:]]*<emulator>.*,    <emulator>$new_emulator</emulator> <!-- WARNING: emulator dirname is auto-updated by the nixos libvirtd module -->," "$file"
  +        done
  +      ''; # */
  +
  +      serviceConfig = {
  +        ExecStart = ''@${pkgs.libvirt}/sbin/libvirtd libvirtd --config "${configFile}" --daemon ${concatStringsSep " " cfg.extraOptions}'';
  +        Type = "notify";
  +        KillMode = "process"; # when stopping, leave the VMs alone
  +        Restart = "on-failure";
         };
  +    };
   
       systemd.sockets.virtlogd = {
         description = "Virtual machine log manager socket";
  
positive.code:     ${cfg.extraConfig}
    '';
  
  in
  
  {
    ###### interface
  
    options = {
  
      virtualisation.libvirtd.enable =
        mkOption {
          type = types.bool;
          default = false;
          description =
            ''
              This option enables libvirtd, a daemon that manages
              virtual machines. Users in the "libvirtd" group can interact with
              the daemon (e.g. to start or stop VMs) using the
              <command>virsh</command> command line tool, among others.
            '';
        };
  
      virtualisation.libvirtd.enableKVM =
        mkOption {
          type = types.bool;
          default = true;
          description =
            ''
              This option enables support for QEMU/KVM in libvirtd.
            '';
        };
  
      virtualisation.libvirtd.extraConfig =
        mkOption {
          type = types.lines;
          default = "";
          description =
            ''
              Extra contents appended to the libvirtd configuration file,
              libvirtd.conf.
            '';
        };
  
      virtualisation.libvirtd.extraOptions =
        mkOption {
          type = types.listOf types.str;
          default = [ ];
          example = [ "--verbose" ];
          description =
            ''
              Extra command line arguments passed to libvirtd on startup.
            '';
        };
  
      virtualisation.libvirtd.onShutdown =
        mkOption {
          type = types.enum ["shutdown" "suspend" ];
          default = "suspend";
          description =
            ''
              When shutting down / restarting the host what method should
              be used to gracefully halt the guests. Setting to "shutdown"
              will cause an ACPI shutdown of each guest. "suspend" will
              attempt to save the state of the guests ready to restore on boot.
            '';
        };
  
  
    };
  
  
negative.code:       description = "Virtual machine lock manager";
        serviceConfig.ExecStart = "@${pkgs.libvirt}/sbin/virtlockd virtlockd";
      };
    };
  }
  