[RFC][PATCH 3/7] ftrace/x86: Rename MCOUNT_SAVE_FRAME and add more detailed comments

Steven Rostedt
2014-11-25T00:44:17Z

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From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

The name MCOUNT_SAVE_FRAME is rather confusing as it really isn't a
function frame that is saved, but just the required mcount registers
that are needed to be saved before C code may be called. The word
"frame" confuses it as being a function frame which it is not.

Rename MCOUNT_SAVE_FRAME and MCOUNT_RESTORE_FRAME to save_mcount_regs
and restore_mcount_regs respectively. Noticed the lower case, which
keeps it from screaming at the reviewers.

Link: http://lkml.kernel.org/r/CA+55aFwF+qCGSKdGaEgW4p6N65GZ5_XTV=1NbtWDvxnd5yYLiw@mail.gmail.com

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
arch/x86/kernel/mcount_64.S | 32 ++++++++++++++++++++++++--------
1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/mcount_64.S b/arch/x86/kernel/mcount_64.S
index 94fe46725fe0..0a693d011980 100644
--- a/arch/x86/kernel/mcount_64.S
+++ b/arch/x86/kernel/mcount_64.S
@@ -21,8 +21,24 @@
# define function_hook mcount
#endif

+/*
+ * gcc -pg option adds a call to 'mcount' in most functions.
+ * When -mfentry is used, the call is to 'fentry' and not 'mcount'
+ * and is done before the function's stack frame is set up.
+ * They both require a set of regs to be saved before calling
+ * any C code and restored before returning back to the function.
+ *
+ * On boot up, all these calls are converted into nops. When tracing
+ * is enabled, the call can jump to either ftrace_caller or
+ * ftrace_regs_caller. Callbacks (tracing functions) that require
+ * ftrace_regs_caller (like kprobes) need to have pt_regs passed to
+ * it. For this reason, the size of the pt_regs structure will be
+ * allocated on the stack and the required mcount registers will
+ * be saved in the locations that pt_regs has them in.
+ */
+
/* skip is set if the stack was already partially adjusted */
-.macro MCOUNT_SAVE_FRAME skip=0
+.macro save_mcount_regs skip=0
/*
* We add enough stack to save all regs.
*/
@@ -39,7 +55,7 @@
movq %rdx, RIP(%rsp)
.endm

-.macro MCOUNT_RESTORE_FRAME skip=0
+.macro restore_mcount_regs skip=0
movq R9(%rsp), %r9
movq R8(%rsp), %r8
movq RDI(%rsp), %rdi
@@ -52,7 +68,7 @@

/* skip is set if stack has been adjusted */
.macro ftrace_caller_setup trace_label skip=0
- MCOUNT_SAVE_FRAME \skip
+ save_mcount_regs \skip

/* Save this location */
GLOBAL(\trace_label)
@@ -121,7 +137,7 @@ GLOBAL(ftrace_call)

restore_frame

- MCOUNT_RESTORE_FRAME
+ restore_mcount_regs

/*
* The copied trampoline must call ftrace_return as it
@@ -196,7 +212,7 @@ GLOBAL(ftrace_regs_call)
movq RBX(%rsp), %rbx

/* skip=8 to skip flags saved in SS */
- MCOUNT_RESTORE_FRAME 8
+ restore_mcount_regs 8

/* Restore flags */
popfq
@@ -240,7 +256,7 @@ trace:

call *ftrace_trace_function

- MCOUNT_RESTORE_FRAME
+ restore_mcount_regs

jmp fgraph_trace
END(function_hook)
@@ -249,7 +265,7 @@ END(function_hook)

#ifdef CONFIG_FUNCTION_GRAPH_TRACER
ENTRY(ftrace_graph_caller)
- MCOUNT_SAVE_FRAME
+ save_mcount_regs

#ifdef CC_USING_FENTRY
leaq SS+16(%rsp), %rdi
@@ -263,7 +279,7 @@ ENTRY(ftrace_graph_caller)

call prepare_ftrace_return

- MCOUNT_RESTORE_FRAME
+ restore_mcount_regs

retq
END(ftrace_graph_caller)
--
2.1.1