Set qemu gui backend to cocoa on darwin

lib/runners/qemu.nix

@@ -218,17 +218,23 @@ lib.warnIf (mem == 2048) ''
       "-drive" "id=store,format=raw,read-only=on,file=${storeDisk},if=none,aio=${aioEngine}"
       "-device" "virtio-blk-${devType},drive=store${lib.optionalString (devType == "pci") ",disable-legacy=on"}"
     ] ++
-    (if graphics.enable
-     then [
-      "-display" "gtk,gl=on"
-      "-device" "virtio-vga-gl"
-      "-device" "qemu-xhci"
-      "-device" "usb-tablet"
-      "-device" "usb-kbd"
-     ]
-     else [
-      "-nographic"
-     ]) ++
+       (if graphics.enable then (
+       let
+         displayArgs = {
+           cocoa = [
+             "-display" "cocoa" "-device" "virtio-gpu"
+           ];
+           gtk = [
+             "-display" "gtk,gl=on" "-device" "virtio-vga-gl"
+           ];
+         }.${graphics.backend};
+       in
+         displayArgs ++ [
+           "-device" "qemu-xhci"
+           "-device" "usb-tablet"
+           "-device" "usb-kbd"
+         ]
+       ) else [ "-nographic" ]) ++
     lib.optionals canSandbox [
       "-sandbox" "on"
     ] ++

nixos-modules/microvm/options.nix

@@ -475,21 +475,31 @@ in
       '';
     };
 
-    graphics.enable = mkOption {
-      type = types.bool;
-      default = false;
-      description = ''
-        Enable GUI support.
-
-        MicroVMs with graphics are intended for the interactive
-        use-case. They cannot be started through systemd jobs.
-
-        Support in Hypervisors:
-        - `qemu` starts a Gtk window with the framebuffer of the virtio-gpu
-      '';
-    };
-
-    graphics.socket = mkOption {
+     graphics.enable = mkOption {
+       type = types.bool;
+       default = false;
+       description = ''
+         Enable GUI support.
+
+         MicroVMs with graphics are intended for the interactive
+         use-case. They cannot be started through systemd jobs.
+
+         The display backend is chosen by `microvm.graphics.backend`.
+       '';
+     };
+
+     graphics.backend = mkOption {
+       type = types.enum [ "gtk" "cocoa" ];
+       default = if pkgs.stdenv.hostPlatform.isDarwin then "cocoa" else "gtk";
+       defaultText = lib.literalExpression ``if pkgs.stdenv.hostPlatform.isDarwin then "cocoa" else "gtk"``;
+       description = ''
+         QEMU display backend to use when `graphics.enable` is true.
+
+         Defaults to `cocoa` on Darwin hosts and `gtk` otherwise.
+       '';
+     };
+
+     graphics.socket = mkOption {
       type = types.str;
       default = "${hostName}-gpu.sock";
       description = ''