Adding --gdb argument for ukvm

lib/mirage.ml

@@ -1681,6 +1681,9 @@ let configure i =
   let target = Key.(get ctx target) in
   Log.info (fun m -> m "Configuring for target: %a" Key.pp_target target);
   let opam_name = unikernel_name target name in
+  let target_debug = Key.(get ctx target_debug) in
+  if target_debug && target <> `Ukvm then
+    Log.warn (fun m -> m "-g not supported for target: %a" Key.pp_target target);
   configure_myocamlbuild () >>= fun () ->
   configure_opam ~name:opam_name i >>= fun () ->
   configure_makefile ~opam_name >>= fun () ->
@@ -1797,7 +1800,7 @@ let ldflags pkg = pkg_config pkg ["--variable=ldflags"]
 
 let ldpostflags pkg = pkg_config pkg ["--variable=ldpostflags"]
 
-let link info name target =
+let link info name target target_debug =
   let libs = Info.libraries info in
   match target with
   | `Unix | `MacOSX ->
@@ -1861,7 +1864,7 @@ let link info name target =
         | "mirage-net-solo5" -> "net" :: acc
         | "mirage-block-solo5" -> "blk" :: acc
         | _ -> acc)
-        [] libs
+        [] libs @ (if target_debug then ["gdb"] else [])
     in
     pkg_config "solo5-kernel-ukvm" ["--variable=libdir"] >>= function
     | [ libdir ] ->
@@ -1878,9 +1881,10 @@ let build i =
   let warn_error = Key.(get ctx warn_error) in
   let target = Key.(get ctx target) in
   let libs = Info.libraries i in
+  let target_debug = Key.(get ctx target_debug) in
   check_entropy libs >>= fun () ->
   compile libs warn_error target >>= fun () ->
-  link i name target >>| fun out ->
+  link i name target target_debug >>| fun out ->
   Log.info (fun m -> m "Build succeeded: %s" out)
 
 let clean i =
@@ -1927,6 +1931,7 @@ module Project = struct
       method keys = [
         Key.(abstract target);
         Key.(abstract warn_error);
+        Key.(abstract target_debug);
       ]
       method packages =
         let common = [

lib/mirage_key.ml

@@ -156,6 +156,13 @@ let warn_error =
   let key = Arg.(opt ~stage:`Configure bool false doc) in
   Key.create "warn_error" key
 
+let target_debug =
+  let doc = "Enables target-specific support for debugging. Supported \
+             targets: ukvm (compiles ukvm-bin with GDB server support)." in
+  let doc = Arg.info ~docs:mirage_section ~docv:"DEBUG" ~doc ["g"] in
+  let key = Arg.flag ~stage:`Configure doc in
+  Key.create "target_debug" key
+
 (** {3 Tracing} *)
 
 let tracing_size default =

lib/mirage_key.mli

@@ -50,6 +50,9 @@ val warn_error: bool key
 (** [--warn-error]. Enable {i -warn-error} for OCaml sources. Set to [false] by
     default, but might might enabled by default in later releases. *)
 
+val target_debug: bool key
+(** Enables target-specific support for debugging. *)
+
 val tracing_size: int -> int key
 (** [--tracing-size]: Key setting the tracing ring buffer size. *)