firecracker: Set --enable-pci option by default when available

This enables the PCI virtio transport which is supposed to be faster than the MMIO one.

Support for this flag is currently only availble in Nixpkgs unstable.

I haven't been able to measure these perf improvements.

However this is still what the Firecracker docs say to do, so I guess let's make it the default?

Author: bjackman

lib/runners/firecracker.nix

@@ -86,15 +86,15 @@ in {
     then throw "hotpluggedMem not implemented for Firecracker"
     else if credentialFiles != {}
     then throw "credentialFiles are not implemented for Firecracker"
-    else lib.escapeShellArgs [
+    else lib.escapeShellArgs ([
       "${pkgs.firecracker}/bin/firecracker"
       "--config-file" configFile
       "--api-sock" (
         if socket != null
         then socket
         else throw "Firecracker must be configured with an API socket (option microvm.socket)!"
       )
-    ];
+    ] ++ lib.optional microvmConfig.firecracker.enablePci "--enable-pci");
 
   preStart = ''
     ${preStart}

nixos-modules/microvm/options.nix

@@ -590,6 +590,12 @@ in
       description = "Custom CPU template passed to firecracker.";
     };
 
+    firecracker.enablePci = mkOption {
+      type = types.bool;
+      default = lib.versionAtLeast pkgs.firecracker.version "1.13.0";
+      description = "Use PCI Virtio transport, improves performance.";
+    };
+
     prettyProcnames = mkOption {
       type = types.bool;
       default = true;