nixos/memtest: Allow user to specify memtest86 boot parameters

nixos/modules/installer/cd-dvd/installation-cd-base.nix

@@ -36,7 +36,7 @@ with pkgs.lib;
   isoImage.makeEfiBootable = true;
 
   # Add Memtest86+ to the CD.
-  boot.loader.grub.memtest86 = true;
+  boot.loader.grub.memtest86.enable = true;
 
   # Get a console as soon as the initrd loads fbcon on EFI boot
   boot.initrd.kernelModules = [ "fbcon" ];

nixos/modules/system/boot/loader/grub/memtest.nix

@@ -6,28 +6,51 @@ with pkgs.lib;
 
 let
   memtest86 = pkgs.memtest86plus;
+  cfg = config.boot.loader.grub.memtest86;
+  params = concatStringsSep " " cfg.params;
 in
 
 {
   options = {
 
-    boot.loader.grub.memtest86 = mkOption {
-      default = false;
-      type = types.bool;
-      description = ''
-        Make Memtest86+, a memory testing program, available from the
-        GRUB boot menu.
-      '';
+    boot.loader.grub.memtest86 = {
+
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          Make Memtest86+, a memory testing program, available from the
+          GRUB boot menu.
+        '';
+      };
+
+      params = mkOption {
+        default = [];
+        example = [ "console=ttyS0,115200" ];
+        type = types.listOf types.str;
+        description = ''
+          Parameters added to the Memtest86+ command line. As of memtest86+ 5.01
+          the following list of (apparently undocumented) parameters are
+          accepted:
+            console=... -- set up a serial console.
+            btrace      -- enable boot trace.
+            maxcpus=... -- limit number of CPUs.
+            onepass     -- run one pass and exit if there are no errors.
+            tstlist=... -- list of tests to run.
+            cpumask=... -- set a CPU mask, to select CPUs to use for testing.
+        '';
+      };
+
     };
   };
 
-  config = mkIf config.boot.loader.grub.memtest86 {
+  config = mkIf cfg.enable {
 
     boot.loader.grub.extraEntries =
       if config.boot.loader.grub.version == 2 then
         ''
           menuentry "Memtest86+" {
-            linux16 @bootRoot@/memtest.bin
+            linux16 @bootRoot@/memtest.bin ${params}
           }
         ''
       else