Add old kernel specialisation in fox

Allow booting an old LTS kernel to compare performance.

m/fox/configuration.nix

@@ -19,6 +19,9 @@
   # Select the this using the ID to avoid mismatches
   boot.loader.grub.device = "/dev/disk/by-id/wwn-0x500a07514b0c1103";
 
+  # Increase time so we can boot other entries
+  boot.loader.timeout = 60;
+
   # No swap, there is plenty of RAM
   swapDevices = lib.mkForce [];
 
@@ -31,7 +34,10 @@
   # Use performance for benchmarks
   powerManagement.cpuFreqGovernor = "performance";
 
-  services.amd-uprof.enable = true;
+  # Enable amd-uprof in >= 6.15 kernels only
+  services.amd-uprof.enable =
+    let ver = config.boot.kernelPackages.kernel.version;
+    in (lib.strings.compareVersions ver "6.15") >= 0;
 
   # Disable NUMA balancing
   boot.kernel.sysctl."kernel.numa_balancing" = 0;
@@ -42,6 +48,11 @@
   # Disable NMI watchdog to save one hw counter (for AMD uProf)
   boot.kernel.sysctl."kernel.nmi_watchdog" = 0;
 
+  specialisation.oldKernel.configuration = {
+    system.nixos.tags = [ "old-kernel" ];
+    boot.kernelPackages = lib.mkForce pkgs.linuxPackages_6_12;
+  };
+
   services.openssh.settings.X11Forwarding = true;
 
   services.fail2ban.enable = true;