nixos/systemd-boot: add windows option for easy dual-booting

When installing NixOS on a machine with Windows, the "easiest" solution
to dual-boot is re-using the existing EFI System Partition (ESP), which
allows systemd-boot to detect Windows automatically.

However, if there are multiple ESPs, maybe even on multiple disks,
systemd-boot is unable to detect the other OSes, and you either have to
use Grub and os-prober, or do a tedious manual configuration as
described in the wiki:
https://wiki.nixos.org/w/index.php?title=Dual_Booting_NixOS_and_Windows&redirect=no#EFI_with_multiple_disks

This commit automates and documents this properly so only a single line
like

    boot.loader.systemd-boot.windows."10".efiDeviceHandle = "HD0c2";

is required.

In the future, we might want to try automatically detecting this
during installation, but finding the correct device handle while the
kernel is running is tricky.

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -522,6 +522,8 @@
   The derivation now installs "impl" headers selectively instead of by a wildcard.
   Use `imgui.src` if you just want to access the unpacked sources.
 
+- The new `boot.loader.systemd-boot.windows` option makes setting up dual-booting with Windows on a different drive easier
+
 - Linux 4.19 has been removed because it will reach its end of life within the lifespan of 24.11
 
 - Unprivileged access to the kernel syslog via `dmesg` is now restricted by default. Users wanting to keep an

nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix

@@ -182,7 +182,7 @@ in
 
     sortKey = mkOption {
       default = "nixos";
-      type = lib.types.str;
+      type = types.str;
       description = ''
         The sort key used for the NixOS bootloader entries.
         This key determines sorting relative to non-NixOS entries.
@@ -438,6 +438,87 @@ in
         Windows can unseal the encryption key.
       '';
     };
+
+    windows = mkOption {
+      default = { };
+      description = ''
+        Make Windows bootable from systemd-boot. This option is not necessary when Windows and
+        NixOS use the same EFI System Partition (ESP). In that case, Windows will automatically be
+        detected by systemd-boot.
+
+        However, if Windows is installed on a separate drive or ESP, you can use this option to add
+        a menu entry for each installation manually.
+
+        The attribute name is used for the title of the menu entry and internal file names.
+      '';
+      example = literalExpression ''
+        {
+          "10".efiDeviceHandle = "HD0c3";
+          "11-ame" = {
+            title = "Windows 11 Ameliorated Edition";
+            efiDeviceHandle = "HD0b1";
+          };
+          "11-home" = {
+            title = "Windows 11 Home";
+            efiDeviceHandle = "FS1";
+            sortKey = "z_windows";
+          };
+        }
+      '';
+      type = types.attrsOf (
+        types.submodule (
+          { name, ... }:
+          {
+            options = {
+              efiDeviceHandle = mkOption {
+                type = types.str;
+                example = "HD1b3";
+                description = ''
+                  The device handle of the EFI System Partition (ESP) where the Windows bootloader is
+                  located. This is the device handle that the EDK2 UEFI Shell uses to load the
+                  bootloader.
+
+                  To find this handle, follow these steps:
+                  1. Set {option}`boot.loader.systemd-boot.edk2-uefi-shell.enable` to `true`
+                  2. Run `nixos-rebuild boot`
+                  3. Reboot and select "EDK2 UEFI Shell" from the systemd-boot menu
+                  4. Run `map -c` to list all consistent device handles
+                  5. For each device handle (for example, `HD0c1`), run `ls HD0c1:\EFI`
+                  6. If the output contains the directory `Microsoft`, you might have found the correct device handle
+                  7. Run `HD0c1:\EFI\Microsoft\Boot\Bootmgfw.efi` to check if Windows boots correctly
+                  8. If it does, this device handle is the one you need (in this example, `HD0c1`)
+
+                  This option is required, there is no useful default.
+                '';
+              };
+
+              title = mkOption {
+                type = types.str;
+                example = "Michaelsoft Binbows";
+                default = "Windows ${name}";
+                defaultText = ''attribute name of this entry, prefixed with "Windows "'';
+                description = ''
+                  The title of the boot menu entry.
+                '';
+              };
+
+              sortKey = mkOption {
+                type = types.str;
+                default = "o_windows_${name}";
+                defaultText = ''attribute name of this entry, prefixed with "o_windows_"'';
+                description = ''
+                  `systemd-boot` orders the menu entries by their sort keys,
+                  so if you want something to appear after all the NixOS entries,
+                  it should start with {file}`o` or onwards.
+
+                  See also {option}`boot.loader.systemd-boot.sortKey`..
+                '';
+              };
+            };
+          }
+        )
+      );
+    };
   };
 
   config = mkIf cfg.enable {
@@ -490,7 +571,13 @@ in
           assertion = !(hasInfix "nixos/.extra-files" (toLower filename));
           message = "boot.loader.systemd-boot.extraFiles.${lib.strings.escapeNixIdentifier filename} is invalid: files cannot be placed in the nixos/.extra-files directory";
         }
-      ]) (builtins.attrNames cfg.extraFiles);
+      ]) (builtins.attrNames cfg.extraFiles)
+      ++ concatMap (winVersion: [
+        {
+          assertion = lib.match "^[-_0-9A-Za-z]+$" winVersion != null;
+          message = "boot.loader.systemd-boot.windows.${winVersion} is invalid: key must only contain alphanumeric characters, hyphens, and underscores";
+        }
+      ]) (builtins.attrNames cfg.windows);
 
     boot.loader.grub.enable = mkDefault false;
 
@@ -503,34 +590,44 @@ in
       (mkIf cfg.netbootxyz.enable {
         "efi/netbootxyz/netboot.xyz.efi" = "${pkgs.netbootxyz-efi}";
       })
-      (mkIf cfg.edk2-uefi-shell.enable {
+      (mkIf (cfg.edk2-uefi-shell.enable || cfg.windows != { }) {
         ${edk2ShellEspPath} = "${pkgs.edk2-uefi-shell}/shell.efi";
       })
     ];
 
-    boot.loader.systemd-boot.extraEntries = mkMerge [
-      (mkIf cfg.memtest86.enable {
-        "memtest86.conf" = ''
-          title  Memtest86+
-          efi    /efi/memtest86/memtest.efi
-          sort-key ${cfg.memtest86.sortKey}
-        '';
-      })
-      (mkIf cfg.netbootxyz.enable {
-        "netbootxyz.conf" = ''
-          title  netboot.xyz
-          efi    /efi/netbootxyz/netboot.xyz.efi
-          sort-key ${cfg.netbootxyz.sortKey}
-        '';
-      })
-      (mkIf cfg.edk2-uefi-shell.enable {
-        "edk2-uefi-shell.conf" = ''
-          title  EDK2 UEFI Shell
-          efi    /${edk2ShellEspPath}
-          sort-key ${cfg.edk2-uefi-shell.sortKey}
+    boot.loader.systemd-boot.extraEntries = mkMerge (
+      [
+        (mkIf cfg.memtest86.enable {
+          "memtest86.conf" = ''
+            title  Memtest86+
+            efi    /efi/memtest86/memtest.efi
+            sort-key ${cfg.memtest86.sortKey}
+          '';
+        })
+        (mkIf cfg.netbootxyz.enable {
+          "netbootxyz.conf" = ''
+            title  netboot.xyz
+            efi    /efi/netbootxyz/netboot.xyz.efi
+            sort-key ${cfg.netbootxyz.sortKey}
+          '';
+        })
+        (mkIf cfg.edk2-uefi-shell.enable {
+          "edk2-uefi-shell.conf" = ''
+            title  EDK2 UEFI Shell
+            efi    /${edk2ShellEspPath}
+            sort-key ${cfg.edk2-uefi-shell.sortKey}
+          '';
+        })
+      ]
+      ++ (mapAttrsToList (winVersion: cfg: {
+        "windows_${winVersion}.conf" = ''
+          title ${cfg.title}
+          efi /${edk2ShellEspPath}
+          options -nointerrupt -nomap -noversion ${cfg.efiDeviceHandle}:EFI\Microsoft\Boot\Bootmgfw.efi
+          sort-key ${cfg.sortKey}
         '';
-      })
-    ];
+      }) cfg.windows)
+    );
 
     boot.bootspec.extensions."org.nixos.systemd-boot" = {
       inherit (config.boot.loader.systemd-boot) sortKey;

nixos/tests/systemd-boot.nix

@@ -354,6 +354,51 @@ in
     '';
   };
 
+  windows = makeTest {
+    name = "systemd-boot-windows";
+    meta.maintainers = with pkgs.lib.maintainers; [ iFreilicht ];
+
+    nodes.machine = { ... }: {
+      imports = [ common ];
+      boot.loader.systemd-boot.windows = {
+        "7" = {
+          efiDeviceHandle = "HD0c1";
+          sortKey = "before_all_others";
+        };
+        "Ten".efiDeviceHandle = "FS0";
+        "11" = {
+          title = "Title with-_-punctuation ...?!";
+          efiDeviceHandle = "HD0d4";
+          sortKey = "zzz";
+        };
+      };
+    };
+
+    testScript = ''
+      machine.succeed("test -e /boot/efi/edk2-uefi-shell/shell.efi")
+
+      machine.succeed("test -e /boot/loader/entries/windows_7.conf")
+      machine.succeed("test -e /boot/loader/entries/windows_Ten.conf")
+      machine.succeed("test -e /boot/loader/entries/windows_11.conf")
+
+      machine.succeed("grep 'efi /efi/edk2-uefi-shell/shell.efi' /boot/loader/entries/windows_7.conf")
+      machine.succeed("grep 'efi /efi/edk2-uefi-shell/shell.efi' /boot/loader/entries/windows_Ten.conf")
+      machine.succeed("grep 'efi /efi/edk2-uefi-shell/shell.efi' /boot/loader/entries/windows_11.conf")
+
+      machine.succeed("grep 'HD0c1:EFI\\\\Microsoft\\\\Boot\\\\Bootmgfw.efi' /boot/loader/entries/windows_7.conf")
+      machine.succeed("grep 'FS0:EFI\\\\Microsoft\\\\Boot\\\\Bootmgfw.efi' /boot/loader/entries/windows_Ten.conf")
+      machine.succeed("grep 'HD0d4:EFI\\\\Microsoft\\\\Boot\\\\Bootmgfw.efi' /boot/loader/entries/windows_11.conf")
+
+      machine.succeed("grep 'sort-key before_all_others' /boot/loader/entries/windows_7.conf")
+      machine.succeed("grep 'sort-key o_windows_Ten' /boot/loader/entries/windows_Ten.conf")
+      machine.succeed("grep 'sort-key zzz' /boot/loader/entries/windows_11.conf")
+
+      machine.succeed("grep 'title Windows 7' /boot/loader/entries/windows_7.conf")
+      machine.succeed("grep 'title Windows Ten' /boot/loader/entries/windows_Ten.conf")
+      machine.succeed('grep "title Title with-_-punctuation ...?!" /boot/loader/entries/windows_11.conf')
+    '';
+  };
+
   memtestSortKey = makeTest {
     name = "systemd-boot-memtest-sortkey";
     meta.maintainers = with pkgs.lib.maintainers; [ julienmalka ];