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, 15 insertions, 0 deletions

diff --git a/arch/s390/include/asm/ftrace.h b/arch/s390/include/asm/ftrace.h
index abb618f1ead2..836c56290499 100644
--- a/arch/s390/include/asm/ftrace.h
+++ b/arch/s390/include/asm/ftrace.h
@@ -3,8 +3,12 @@
 
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 
+#ifdef CC_USING_HOTPATCH
+#define MCOUNT_INSN_SIZE        6
+#else
 #define MCOUNT_INSN_SIZE        24
 #define MCOUNT_RETURN_FIXUP     18
+#endif
 
 #ifndef __ASSEMBLY__
 
@@ -37,18 +41,29 @@ struct ftrace_insn {
 static inline void ftrace_generate_nop_insn(struct ftrace_insn *insn)
 {
 #ifdef CONFIG_FUNCTION_TRACER
+#ifdef CC_USING_HOTPATCH
+        /* brcl 0,0 */
+        insn->opc = 0xc004;
+        insn->disp = 0;
+#else
         /* jg .+24 */
         insn->opc = 0xc0f4;
         insn->disp = MCOUNT_INSN_SIZE / 2;
 #endif
+#endif
 }
 
 static inline int is_ftrace_nop(struct ftrace_insn *insn)
 {
 #ifdef CONFIG_FUNCTION_TRACER
+#ifdef CC_USING_HOTPATCH
+        if (insn->disp == 0)
+                return 1;
+#else
         if (insn->disp == MCOUNT_INSN_SIZE / 2)
                 return 1;
 #endif
+#endif
         return 0;
 }