nixos/undervolt: adding undervolt module

We want to be able to configure persistent undervolting
in the NixOS configuration

nixos/modules/module-list.nix

@@ -281,6 +281,7 @@
   ./services/hardware/upower.nix
   ./services/hardware/usbmuxd.nix
   ./services/hardware/thermald.nix
+  ./services/hardware/undervolt.nix
   ./services/logging/SystemdJournal2Gelf.nix
   ./services/logging/awstats.nix
   ./services/logging/fluentd.nix

nixos/modules/services/hardware/undervolt.nix

@@ -0,0 +1,134 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.services.undervolt;
+in {
+  options.services.undervolt = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to undervolt intel cpus.
+      '';
+    };
+
+    verbose = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to enable verbose logging.
+      '';
+    };
+
+    package = mkOption {
+      type = types.package;
+      default = pkgs.undervolt;
+      defaultText = "pkgs.undervolt";
+      description = ''
+        undervolt derivation to use.
+      '';
+    };
+
+    coreOffset = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        The amount of voltage to offset the CPU cores by. Accepts a floating point number.
+      '';
+    };
+
+    gpuOffset = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        The amount of voltage to offset the GPU by. Accepts a floating point number.
+      '';
+    };
+
+    uncoreOffset = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        The amount of voltage to offset uncore by. Accepts a floating point number.
+      '';
+    };
+
+    analogioOffset = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        The amount of voltage to offset analogio by. Accepts a floating point number.
+      '';
+    };
+
+    temp = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        The temperature target. Accepts a floating point number.
+      '';
+    };
+
+    tempAc = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        The temperature target on AC power. Accepts a floating point number.
+      '';
+    };
+
+    tempBat = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        The temperature target on battery power. Accepts a floating point number.
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    boot.kernelModules = [ "msr" ];
+
+    environment.systemPackages = [ cfg.package ];
+
+    systemd.services.undervolt = {
+      path = [ pkgs.undervolt ];
+
+      description = "Intel Undervolting Service";
+      serviceConfig = {
+        Type = "oneshot";
+        Restart = "no";
+
+        # `core` and `cache` are both intentionally set to `cfg.coreOffset` as according to the undervolt docs:
+        #
+        #     Core or Cache offsets have no effect. It is not possible to set different offsets for
+        #     CPU Core and Cache. The CPU will take the smaller of the two offsets, and apply that to
+        #     both CPU and Cache. A warning message will be displayed if you attempt to set different offsets.
+        ExecStart = ''
+          ${pkgs.undervolt}/bin/undervolt \
+            ${optionalString cfg.verbose "--verbose"} \
+            ${optionalString (cfg.coreOffset != null) "--core ${cfg.coreOffset}"} \
+            ${optionalString (cfg.coreOffset != null) "--cache ${cfg.coreOffset}"} \
+            ${optionalString (cfg.gpuOffset != null) "--gpu ${cfg.gpuOffset}"} \
+            ${optionalString (cfg.uncoreOffset != null) "--uncore ${cfg.uncoreOffset}"} \
+            ${optionalString (cfg.analogioOffset != null) "--analogio ${cfg.analogioOffset}"} \
+            ${optionalString (cfg.temp != null) "--temp ${cfg.temp}"} \
+            ${optionalString (cfg.tempAc != null) "--temp-ac ${cfg.tempAc}"} \
+            ${optionalString (cfg.tempBat != null) "--temp-bat ${cfg.tempBat}"}
+        '';
+      };
+    };
+
+    systemd.timers.undervolt = {
+      description = "Undervolt timer to ensure voltage settings are always applied";
+      partOf = [ "undervolt.service" ];
+      wantedBy = [ "multi-user.target" ];
+      timerConfig = {
+        OnBootSec = "2min";
+        OnUnitActiveSec = "30";
+      };
+    };
+  };
+}