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>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/s390/kernel/mcount.S b/arch/s390/kernel/mcount.S
index b6dfc5bfcb89..e499370fbccb 100644
--- a/arch/s390/kernel/mcount.S
+++ b/arch/s390/kernel/mcount.S
@@ -27,7 +27,9 @@ ENTRY(ftrace_caller)
         .globl  ftrace_regs_caller
         .set    ftrace_regs_caller,ftrace_caller
         lgr     %r1,%r15
+#ifndef CC_USING_HOTPATCH
         aghi    %r0,MCOUNT_RETURN_FIXUP
+#endif
         aghi    %r15,-STACK_FRAME_SIZE
         stg     %r1,__SF_BACKCHAIN(%r15)
         stg     %r1,(STACK_PTREGS_GPRS+15*8)(%r15)