nixos/iso-image: add an option to disable BIOS boot

This change adds an option to disable legacy BIOS boot support for ISO
images. The implementation uses syslinux package that currently does not
support non-x86 platforms and thus cannot be cross-compiled, e.g. from
AArch64 system.

2 files changed, 16 insertions, 6 deletions

diff --git a/nixos/modules/installer/cd-dvd/installation-cd-base.nix b/nixos/modules/installer/cd-dvd/installation-cd-base.nix
index 3f92b779d60..3c7c7e30a0b 100644
--- a/nixos/modules/installer/cd-dvd/installation-cd-base.nix
+++ b/nixos/modules/installer/cd-dvd/installation-cd-base.nix
@@ -21,6 +21,9 @@ with lib;
   # ISO naming.
   isoImage.isoName = "${config.isoImage.isoBaseName}-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}.iso";
 
+  # BIOS booting
+  isoImage.makeBiosBootable = true;
+
   # EFI booting
   isoImage.makeEfiBootable = true;
 
diff --git a/nixos/modules/installer/cd-dvd/iso-image.nix b/nixos/modules/installer/cd-dvd/iso-image.nix
index 86c2cd1a8c7..0245a522f7e 100644
--- a/nixos/modules/installer/cd-dvd/iso-image.nix
+++ b/nixos/modules/installer/cd-dvd/iso-image.nix
@@ -535,6 +535,13 @@ in
       '';
     };
 
+    isoImage.makeBiosBootable = mkOption {
+      default = false;
+      description = lib.mdDoc ''
+        Whether the ISO image should be a BIOS-bootable disk.
+      '';
+    };
+
     isoImage.makeEfiBootable = mkOption {
       default = false;
       description = lib.mdDoc ''
@@ -693,7 +700,7 @@ in
     boot.loader.grub.enable = false;
 
     environment.systemPackages =  [ grubPkgs.grub2 grubPkgs.grub2_efi ]
-      ++ optional canx86BiosBoot pkgs.syslinux
+      ++ optional (config.isoImage.makeBiosBootable && canx86BiosBoot) pkgs.syslinux
     ;
 
     # In stage 1 of the boot, mount the CD as the root FS by label so
@@ -744,7 +751,7 @@ in
         { source = pkgs.writeText "version" config.system.nixos.label;
           target = "/version.txt";
         }
-      ] ++ optionals canx86BiosBoot [
+      ] ++ optionals (config.isoImage.makeBiosBootable && canx86BiosBoot) [
         { source = config.isoImage.splashImage;
           target = "/isolinux/background.png";
         }
@@ -771,7 +778,7 @@ in
         { source = config.isoImage.efiSplashImage;
           target = "/EFI/boot/efi-background.png";
         }
-      ] ++ optionals (config.boot.loader.grub.memtest86.enable && canx86BiosBoot) [
+      ] ++ optionals (config.boot.loader.grub.memtest86.enable && config.isoImage.makeBiosBootable && canx86BiosBoot) [
         { source = "${pkgs.memtest86plus}/memtest.bin";
           target = "/boot/memtest.bin";
         }
@@ -786,10 +793,10 @@ in
     # Create the ISO image.
     system.build.isoImage = pkgs.callPackage ../../../lib/make-iso9660-image.nix ({
       inherit (config.isoImage) isoName compressImage volumeID contents;
-      bootable = canx86BiosBoot;
+      bootable = config.isoImage.makeBiosBootable && canx86BiosBoot;
       bootImage = "/isolinux/isolinux.bin";
-      syslinux = if canx86BiosBoot then pkgs.syslinux else null;
-    } // optionalAttrs (config.isoImage.makeUsbBootable && canx86BiosBoot) {
+      syslinux = if config.isoImage.makeBiosBootable && canx86BiosBoot then pkgs.syslinux else null;
+    } // optionalAttrs (config.isoImage.makeUsbBootable && config.isoImage.makeBiosBootable && canx86BiosBoot) {
       usbBootable = true;
       isohybridMbrImage = "${pkgs.syslinux}/share/syslinux/isohdpfx.bin";
     } // optionalAttrs config.isoImage.makeEfiBootable {