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>

4 files changed, 20 insertions, 4 deletions

diff --git a/arch/s390/kernel/Makefile b/arch/s390/kernel/Makefile
index 204c43a4c245..31fab2676fe9 100644
--- a/arch/s390/kernel/Makefile
+++ b/arch/s390/kernel/Makefile
@@ -4,8 +4,8 @@
 
 ifdef CONFIG_FUNCTION_TRACER
 # Don't trace early setup code and tracing code
-CFLAGS_REMOVE_early.o = -pg
-CFLAGS_REMOVE_ftrace.o = -pg
+CFLAGS_REMOVE_early.o = $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_ftrace.o = $(CC_FLAGS_FTRACE)
 endif
 
 #
diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
index 3dabcae40e04..82c19899574f 100644
--- a/arch/s390/kernel/ftrace.c
+++ b/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) {
                 /*
diff --git a/arch/s390/kernel/kprobes.c b/arch/s390/kernel/kprobes.c
index 1e4c710dfb92..f516edc1fbe3 100644
--- a/arch/s390/kernel/kprobes.c
+++ b/arch/s390/kernel/kprobes.c
@@ -69,7 +69,8 @@ static void copy_instruction(struct kprobe *p)
                 /*
                  * If kprobes patches the instruction that is morphed by
                  * ftrace make sure that kprobes always sees the branch
-                 * "jg .+24" that skips the mcount block
+                 * "jg .+24" that skips the mcount block or the "brcl 0,0"
+                 * in case of hotpatch.
                  */
                 ftrace_generate_nop_insn((struct ftrace_insn *)p->ainsn.insn);
                 p->ainsn.is_ftrace_insn = 1;
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)