1 files changed, 91 insertions, 0 deletions
diff --git a/nixos/modules/hardware/cpu/x86-msr.nix b/nixos/modules/hardware/cpu/x86-msr.nix
new file mode 100644
index 000000000000..554bec1b7db1
--- /dev/null
+++ b/nixos/modules/hardware/cpu/x86-msr.nix
@@ -0,0 +1,91 @@
+{ lib
+, config
+, options
+, ...
+}:
+let
+  inherit (builtins) hasAttr;
+  inherit (lib) mkIf mdDoc;
+  cfg = config.hardware.cpu.x86.msr;
+  opt = options.hardware.cpu.x86.msr;
+  defaultGroup = "msr";
+  isDefaultGroup = cfg.group == defaultGroup;
+  set = "to set for devices of the `msr` kernel subsystem.";
+
+  # Generates `foo=bar` parameters to pass to the kernel.
+  # If `module = baz` is passed, generates `baz.foo=bar`.
+  # Adds double quotes on demand to handle `foo="bar baz"`.
+  kernelParam = { module ? null }: name: value:
+    assert lib.asserts.assertMsg (!lib.strings.hasInfix "=" name) "kernel parameter cannot have '=' in name";
+    let
+      key = (if module == null then "" else module + ".") + name;
+      valueString = lib.generators.mkValueStringDefault {} value;
+      quotedValueString = if lib.strings.hasInfix " " valueString
+        then lib.strings.escape ["\""] valueString
+        else valueString;
+    in "${key}=${quotedValueString}";
+  msrKernelParam = kernelParam { module = "msr"; };
+in
+{
+  options.hardware.cpu.x86.msr = with lib.options; with lib.types; {
+    enable = mkEnableOption (mdDoc "the `msr` (Model-Specific Registers) kernel module and configure `udev` rules for its devices (usually `/dev/cpu/*/msr`)");
+    owner = mkOption {
+      type = str;
+      default = "root";
+      example = "nobody";
+      description = mdDoc "Owner ${set}";
+    };
+    group = mkOption {
+      type = str;
+      default = defaultGroup;
+      example = "nobody";
+      description = mdDoc "Group ${set}";
+    };
+    mode = mkOption {
+      type = str;
+      default = "0640";
+      example = "0660";
+      description = mdDoc "Mode ${set}";
+    };
+    settings = mkOption {
+      type = submodule {
+        freeformType = attrsOf (oneOf [ bool int str ]);
+        options.allow-writes = mkOption {
+          type = nullOr (enum ["on" "off"]);
+          default = null;
+          description = "Whether to allow writes to MSRs (`\"on\"`) or not (`\"off\"`).";
+        };
+      };
+      default = {};
+      description = "Parameters for the `msr` kernel module.";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = hasAttr cfg.owner config.users.users;
+        message = "Owner '${cfg.owner}' set in `${opt.owner}` is not configured via `${options.users.users}.\"${cfg.owner}\"`.";
+      }
+      {
+        assertion = isDefaultGroup || (hasAttr cfg.group config.users.groups);
+        message = "Group '${cfg.group}' set in `${opt.group}` is not configured via `${options.users.groups}.\"${cfg.group}\"`.";
+      }
+    ];
+
+    boot = {
+      kernelModules = [ "msr" ];
+      kernelParams = lib.attrsets.mapAttrsToList msrKernelParam (lib.attrsets.filterAttrs (_: value: value != null) cfg.settings);
+    };
+
+    users.groups.${cfg.group} = mkIf isDefaultGroup { };
+
+    services.udev.extraRules = ''
+      SUBSYSTEM=="msr", OWNER="${cfg.owner}", GROUP="${cfg.group}", MODE="${cfg.mode}"
+    '';
+  };
+
+  meta = with lib; {
+    maintainers = with maintainers; [ lorenzleutgeb ];
+  };
+}