diff --git a/tools/ocaml/libs/xb/xs_ring_stubs.c b/tools/ocaml/libs/xb/xs_ring_stubs.c
index fd561a2..5c771d6 100644
--- a/tools/ocaml/libs/xb/xs_ring_stubs.c
+++ b/tools/ocaml/libs/xb/xs_ring_stubs.c
@@ -99,12 +99,12 @@ CAMLprim value ml_interface_write(value ml_interface,
 	int result;
 
 	struct xenstore_domain_interface *intf = interface->addr;
-	XENSTORE_RING_IDX cons, prod;
+	XENSTORE_RING_IDX cons, old_prod, prod;
 	int can_write;
 	uint32_t connection;
 
 	cons = *(volatile uint32_t*)&intf->rsp_cons;
-	prod = *(volatile uint32_t*)&intf->rsp_prod;
+	old_prod = prod = *(volatile uint32_t*)&intf->rsp_prod;
 	connection = *(volatile uint32_t*)&intf->connection;
 
 	if (connection != XENSTORE_CONNECTED)
@@ -116,15 +116,30 @@ CAMLprim value ml_interface_write(value ml_interface,
 		goto exit;
 	}
 	if (MASK_XENSTORE_IDX(prod) >= MASK_XENSTORE_IDX(cons))
+	{
 		can_write = XENSTORE_RING_SIZE - MASK_XENSTORE_IDX(prod);
-	else 
+		if (can_write < len)
+		{
+			/* Not enough room at end of ring.  Copy what we can first.  */
+			memcpy(intf->rsp + MASK_XENSTORE_IDX(prod), buffer, can_write);
+			prod += can_write;
+			len -= can_write;
+			buffer += can_write;
+
+			/* We are now back at beginning of ring.  */
+			can_write = MASK_XENSTORE_IDX(cons) - MASK_XENSTORE_IDX(prod);
+		}
+	}
+	else
 		can_write = MASK_XENSTORE_IDX(cons) - MASK_XENSTORE_IDX(prod);
 	if (can_write < len)
+		/* Real short write: not enough room.  */
 		len = can_write;
 	memcpy(intf->rsp + MASK_XENSTORE_IDX(prod), buffer, len);
+	prod += len;
 	xen_mb();
-	intf->rsp_prod += len;
-	result = len;
+	intf->rsp_prod = prod;
+	result = prod - old_prod;
 exit:
 	ml_result = Val_int(result);
 	CAMLreturn(ml_result);