s390/ftrace: hotpatch support for function tracing

Make use of gcc's hotpatch support to generate better code for ftrace
function tracing.
The generated code now contains only a six byte nop in each function
prologue instead of a 24 byte code block which will be runtime patched to
support function tracing.
With the new code generation the runtime overhead for supporting function
tracing is close to zero, while the original code did show a significant
performance impact.

Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/ftrace.c

@@ -46,6 +46,13 @@
  *	lg	%r14,8(%r15)		# offset 18
  * The jg instruction branches to offset 24 to skip as many instructions
  * as possible.
+ * In case we use gcc's hotpatch feature the original and also the disabled
+ * function prologue contains only a single six byte instruction and looks
+ * like this:
+ * >	brcl	0,0			# offset 0
+ * To enable ftrace the code gets patched like above and afterwards looks
+ * like this:
+ * >	brasl	%r0,ftrace_caller	# offset 0
  */
 
 unsigned long ftrace_plt;
@@ -64,9 +71,15 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 	if (probe_kernel_read(&old, (void *) rec->ip, sizeof(old)))
 		return -EFAULT;
 	if (addr == MCOUNT_ADDR) {
-		/* Initial code replacement; we expect to see stg r14,8(r15) */
+		/* Initial code replacement */
+#ifdef CC_USING_HOTPATCH
+		/* We expect to see brcl 0,0 */
+		ftrace_generate_nop_insn(&orig);
+#else
+		/* We expect to see stg r14,8(r15) */
 		orig.opc = 0xe3e0;
 		orig.disp = 0xf0080024;
+#endif
 		ftrace_generate_nop_insn(&new);
 	} else if (old.opc == BREAKPOINT_INSTRUCTION) {
 		/*