Minimal kexec image builder

flake.nix

@@ -32,13 +32,10 @@
     archivebox.inputs.flake-utils.follows = "flake-utils";
   };
 
-  outputs = inputs: {
+  outputs = { self, nixpkgs, nixpkgs-unstable, ... }@inputs: {
 
     nixosConfigurations =
     let
-      nixpkgs = inputs.nixpkgs;
-      nixpkgs-unstable = inputs.nixpkgs-unstable;
-
       modules = system: [
         ./common
         inputs.simple-nixos-mailserver.nixosModule
@@ -119,5 +116,16 @@
       "n6" = mkSystem "aarch64-linux" nixpkgs ./machines/compute/n6/configuration.nix;
       "n7" = mkSystem "aarch64-linux" nixpkgs ./machines/compute/n7/configuration.nix;
     };
+
+    packages = let
+      mkKexec = system:
+        (nixpkgs.lib.nixosSystem {
+          inherit system;
+          modules = [ ./machines/kexec.nix ];
+        }).config.system.build.kexec_tarball;
+    in {
+      "x86_64-linux"."kexec" = mkKexec "x86_64-linux";
+      "aarch64-linux"."kexec" = mkKexec "aarch64-linux";
+    };
   };
 }

machines/kexec.nix

@@ -0,0 +1,72 @@
+# From https://mdleom.com/blog/2021/03/09/nixos-oracle/#Build-a-kexec-tarball
+# Builds a kexec img
+
+{ config, pkgs, modulesPath, ... }:
+{
+  imports = [
+    (modulesPath + "/installer/netboot/netboot.nix")
+    (modulesPath + "/profiles/qemu-guest.nix")
+  ];
+
+  # stripped down version of https://github.com/cleverca22/nix-tests/tree/master/kexec
+  system.build = rec {
+    image = pkgs.runCommand "image" { buildInputs = [ pkgs.nukeReferences ]; } ''
+      mkdir $out
+      if [ -f ${config.system.build.kernel}/bzImage ]; then
+        cp ${config.system.build.kernel}/bzImage $out/kernel
+      else
+        cp ${config.system.build.kernel}/Image $out/kernel
+      fi
+      cp ${config.system.build.netbootRamdisk}/initrd $out/initrd
+      nuke-refs $out/kernel
+    '';
+    kexec_script = pkgs.writeTextFile {
+      executable = true;
+      name = "kexec-nixos";
+      text = ''
+        #!${pkgs.stdenv.shell}
+        set -e
+        ${pkgs.kexectools}/bin/kexec -l ${image}/kernel --initrd=${image}/initrd --append="init=${builtins.unsafeDiscardStringContext config.system.build.toplevel}/init ${toString config.boot.kernelParams}"
+        sync
+        echo "executing kernel, filesystems will be improperly umounted"
+        ${pkgs.kexectools}/bin/kexec -e
+      '';
+    };
+    kexec_tarball = pkgs.callPackage (modulesPath + "/../lib/make-system-tarball.nix") {
+      storeContents = [
+        {
+          object = config.system.build.kexec_script;
+          symlink = "/kexec_nixos";
+        }
+      ];
+      contents = [ ];
+    };
+  };
+
+  boot.initrd.availableKernelModules = [ "ata_piix" "uhci_hcd" "e1000" "e1000e" "virtio_pci" "r8169" ];
+  boot.kernelParams = [
+    "panic=30" "boot.panic_on_fail" # reboot the machine upon fatal boot issues
+    "console=ttyS0" # enable serial console
+    "console=tty1"
+  ];
+  boot.kernel.sysctl."vm.overcommit_memory" = "1";
+
+  environment.systemPackages = with pkgs; [
+    cryptsetup
+    btrfs-progs
+  ];
+  environment.variables.GC_INITIAL_HEAP_SIZE = "1M";
+
+  networking.useDHCP = true;
+
+  networking.hostName = "kexec";
+
+  services.openssh = {
+    enable = true;
+    challengeResponseAuthentication = false;
+    passwordAuthentication = false;
+  };
+
+  services.getty.autologinUser = "root";
+  users.users.root.openssh.authorizedKeys.keys = (import ../common/ssh.nix).users;
+}

machines/ray/configuration.nix

@@ -17,6 +17,8 @@
     allowDiscards = true;
   };
 
+  boot.binfmt.emulatedSystems = [ "aarch64-linux" ];
+
   networking.hostName = "ray";
 
   hardware.enableAllFirmware = true;