nixos/hardware: Add option to enable KSM.

This is essentially what's been done for the official NixOS build slaves
and I'm using it as well for a few of my machines and my own Hydra
slaves.

Here's the same implementation from the Delft server configurations:

f47c2fc7f8/delft/common.nix (L91-L101)

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/hardware/ksm.nix

@@ -0,0 +1,18 @@
+{ config, lib, ... }:
+
+{
+  options.hardware.enableKSM = lib.mkEnableOption "Kernel Same-Page Merging";
+
+  config = lib.mkIf config.hardware.enableKSM {
+    systemd.services.enable-ksm = {
+      description = "Enable Kernel Same-Page Merging";
+      wantedBy = [ "multi-user.target" ];
+      after = [ "systemd-udev-settle.service" ];
+      script = ''
+        if [ -e /sys/kernel/mm/ksm ]; then
+          echo 1 > /sys/kernel/mm/ksm/run
+        fi
+      '';
+    };
+  };
+}

nixos/modules/module-list.nix

@@ -29,6 +29,7 @@
   ./hardware/all-firmware.nix
   ./hardware/cpu/amd-microcode.nix
   ./hardware/cpu/intel-microcode.nix
+  ./hardware/ksm.nix
   ./hardware/network/b43.nix
   ./hardware/network/intel-2100bg.nix
   ./hardware/network/intel-2200bg.nix