tools/ocaml/xb: Drop Xs_ring.write

This function is unusued (only Xs_ring.write_substring is used), and the
bytes/string conversion here is backwards: the C stub implements the bytes
version and then we use a Bytes.unsafe_of_string to convert a string into
bytes.

However the operation here really is read-only: we read from the string and
write it to the ring, so the C stub should implement the read-only string
version, and if needed we could use Bytes.unsafe_to_string to be able to send
'bytes'. However that is not necessary as the 'bytes' version is dropped above.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>

tools/ocaml/libs/xb/xs_ring.ml

@@ -25,14 +25,11 @@ module Server_features = Set.Make(struct
 end)
 
 external read: Xenmmap.mmap_interface -> bytes -> int -> int = "ml_interface_read"
-external write: Xenmmap.mmap_interface -> bytes -> int -> int = "ml_interface_write"
+external write_substring: Xenmmap.mmap_interface -> string -> int -> int = "ml_interface_write"
 
 external _internal_set_server_features: Xenmmap.mmap_interface -> int -> unit = "ml_interface_set_server_features" [@@noalloc]
 external _internal_get_server_features: Xenmmap.mmap_interface -> int = "ml_interface_get_server_features" [@@noalloc]
 
-let write_substring mmap buff len =
-	write mmap (Bytes.unsafe_of_string buff) len
-
 let get_server_features mmap =
 	(* NB only one feature currently defined above *)
 	let x = _internal_get_server_features mmap in

tools/ocaml/libs/xb/xs_ring_stubs.c

@@ -112,7 +112,7 @@ CAMLprim value ml_interface_write(value ml_interface,
 	CAMLlocal1(ml_result);
 
 	struct mmap_interface *interface = GET_C_STRUCT(ml_interface);
-	const unsigned char *buffer = Bytes_val(ml_buffer);
+	const char *buffer = String_val(ml_buffer);
 	int len = Int_val(ml_len);
 	int result;