[PATCH v2 5/7] ftrace: Update ftrace_location() for powerpc -mprofile-kernel

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Now that we are patching the preceding 'mflr r0' instruction with
-mprofile-kernel, we need to update ftrace_location() to recognise that
as being part of ftrace.

To do this, we introduce FTRACE_IP_EXTENSION to denote the length (in
bytes) of the mcount caller. By default, this is set to 0. For powerpc
with CONFIG_MPROFILE_KERNEL, we set this to MCOUNT_INSN_SIZE so that
this works if ftrace_location() is called with the address of the actual
ftrace entry call, or with the address of the preceding 'mflr r0'
instruction.

Note that we do not check if the preceding instruction is indeed the
'mflr r0'. Earlier -mprofile-kernel ABI included a 'std r0,stack'
instruction between the 'mflr r0' and the 'bl _mcount'. This is harmless
as the 'std r0,stack' instruction is inconsequential and is not relied
upon.

Suggested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/ftrace.h | 8 ++++++++
 include/linux/ftrace.h            | 9 +++++++++
 kernel/trace/ftrace.c             | 2 +-
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index 3dfb80b86561..510a8ac8ac8d 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h
@@ -61,6 +61,14 @@ struct dyn_arch_ftrace {
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 #define ARCH_SUPPORTS_FTRACE_OPS 1
+
+#ifdef CONFIG_MPROFILE_KERNEL
+/*
+ * We consider two instructions -- 'mflr r0', 'bl _mcount' -- to be part
+ * of ftrace with -mprofile-kernel
+ */
+#define FTRACE_IP_EXTENSION	MCOUNT_INSN_SIZE
+#endif
 #endif
 #endif /* CONFIG_FUNCTION_TRACER */
 
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index fa653a561da5..310b514438cb 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -29,6 +29,15 @@
 #define ARCH_SUPPORTS_FTRACE_OPS 0
 #endif
 
+/*
+ * This denotes the number of instructions (in bytes) that is used by the
+ * arch mcount caller. All instructions in this range will be owned by
+ * ftrace.
+ */
+#ifndef FTRACE_IP_EXTENSION
+#define FTRACE_IP_EXTENSION 0
+#endif
+
 /*
  * If the arch's mcount caller does not support all of ftrace's
  * features, then it must call an indirect function that
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 21d8e201ee80..308555925b81 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1575,7 +1575,7 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
  */
 unsigned long ftrace_location(unsigned long ip)
 {
-	return ftrace_location_range(ip, ip);
+	return ftrace_location_range(ip, ip + FTRACE_IP_EXTENSION);
 }
 
 /**
-- 
2.22.0