* Added an option "nixpkgs.system" to specify the system type for

which NixOS should be built.  This is useful in NixOS network
  specifications, because it allows machines in the network to have
  different types, e.g.,

    {
      machine1 =
        { config, pkgs, ... }:
        { nixpkgs.system = "i686-linux";
          ... other config ...
        };

      machine2 =
        { config, pkgs, ... }:
        { nixpkgs.system = "x86_64-linux";
          ... other config ...
        };
    }

  It can also be useful in distributed NixOS tests.

svn path=/nixos/trunk/; revision=24823

lib/eval-config.nix

@@ -11,7 +11,7 @@
 , modules
 }:
 
-let extraArgs_ = extraArgs; pkgs_ = pkgs; in
+let extraArgs_ = extraArgs; pkgs_ = pkgs; system_ = system; in
 
 rec {
 
@@ -39,7 +39,8 @@ rec {
   # Import Nixpkgs, allowing the NixOS option nixpkgs.config to
   # specify the Nixpkgs configuration (e.g., to set package options
   # such as firefox.enableGeckoMediaPlayer, or to apply global
-  # overrides such as changing GCC throughout the system).  This is
+  # overrides such as changing GCC throughout the system), and the
+  # option nixpkgs.system to override the platform type.  This is
   # tricky, because we have to prevent an infinite recursion: "pkgs"
   # is passed as an argument to NixOS modules, but the value of "pkgs"
   # depends on config.nixpkgs.config, which we get from the modules.
@@ -50,12 +51,13 @@ rec {
     then pkgs_
     else import nixpkgs (
       let
+        system = if nixpkgsOptions.system != "" then nixpkgsOptions.system else system_;
         nixpkgsOptions = (import ./eval-config.nix {
           inherit system nixpkgs services extraArgs modules;
           # For efficiency, leave out most NixOS modules; they don't
           # define nixpkgs.config, so it's pointless to evaluate them.
           baseModules = [ ../modules/misc/nixpkgs.nix ];
-          pkgs = import nixpkgs { inherit system; config = {}; };
+          pkgs = import nixpkgs { system = system_; config = {}; };
         }).optionDefinitions.nixpkgs;
       in
       {

modules/misc/nixpkgs.nix

@@ -13,6 +13,17 @@
       '';
     };
 
+    nixpkgs.system = pkgs.lib.mkOption {
+      default = "";
+      description = ''
+        Specifies the Nix platform type for which NixOS should be built.
+        If unset, it defaults to the platform type of your host system
+        (<literal>${builtins.currentSystem}</literal>).
+        Specifying this option is useful when doing distributed
+        multi-platform deployment, or when building virtual machines.
+      '';
+    };
+
     nixpkgs.platform = pkgs.lib.mkOption {
       default = pkgs.platforms.pc;
       description = ''