Add option to pass cpu-config to firecracker

Author: wtfaremyinitials

lib/runners/firecracker.nix

@@ -11,6 +11,7 @@ let
     interfaces volumes shares devices
     kernel initrdPath
     storeDisk;
+  inherit (microvmConfig.firecracker) cpu;
 
   kernelPath = {
     x86_64-linux = "${kernel.dev}/vmlinux";
@@ -60,6 +61,8 @@ let
       else throw "Network interface type ${type} not implemented for Firecracker"
     ) interfaces;
     vsock = null;
+  } // lib.optionalAttrs (cpu != null) {
+    cpu-config = pkgs.writeText "cpu-config.json" (builtins.toJSON cpu);
   };
 
   configFile = pkgs.writers.writeJSON "firecracker-${hostName}.json" config;

nixos-modules/microvm/options.nix

@@ -533,6 +533,12 @@ in
       description = "A Hypervisor's sandbox directory";
     };
 
+    firecracker.cpu = mkOption {
+      type = with types; nullOr attrs;
+      default = null;
+      description = "Custom CPU template passed to firecracker.";
+    };
+
     prettyProcnames = mkOption {
       type = types.bool;
       default = true;