[PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

On Tue, Jan 10, 2017 at 04:51:12PM +0100, Petr Mladek wrote:

On Thu 2016-12-08 22:46:55, Abel Vesa wrote:

quoted

On Thu, Dec 08, 2016 at 09:46:35PM +0000, Abel Vesa wrote:

quoted

From: Jean-Jacques Hiblot <redacted>

From: Jean-Jacques Hiblot <redacted>

quoted

From statement twice in the commit message. Will resend.

The DYNAMIC_FTRACE_WITH_REGS configuration makes it possible for a ftrace
operation to specify if registers need to saved/restored by the ftrace handler.
This is needed by kgraft and possibly other ftrace-based tools, and the ARM
architecture is currently lacking this feature. It would also be the first step
to support the "Kprobes-on-ftrace" optimization on ARM.

This patch introduces a new ftrace handler that stores the registers on the
stack before calling the next stage. The registers are restored from the stack
before going back to the instrumented function.

A side-effect of this patch is to activate the support for ftrace_modify_call()
as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture

Signed-off-by: Jean-Jacques Hiblot <redacted>
Signed-off-by: Abel Vesa <redacted>
---
 arch/arm/Kconfig               |  2 ++
 arch/arm/include/asm/ftrace.h  |  4 +++
 arch/arm/kernel/entry-ftrace.S | 78 ++++++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/ftrace.c       | 33 ++++++++++++++++++
 4 files changed, 117 insertions(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index b5d529f..87f1a9f 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig

@@ -50,6 +50,7 @@ config ARM
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
+	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
 	select HAVE_EXIT_THREAD
 	select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)

@@ -90,6 +91,7 @@ config ARM
 	select PERF_USE_VMALLOC
 	select RTC_LIB
 	select SYS_SUPPORTS_APM_EMULATION
+	select FRAME_POINTER if DYNAMIC_FTRACE_WITH_REGS && FUNCTION_GRAPH_TRACER

Hi Petr,

FRAME_POINTER is not for free. It takes space on the stack. Also there
is a performance penalty. Do we really need to depend on it? If so,
it might be worth a note in the commit message.

I was trying to create my own patch when I found this work done by 
Jean-Jacques, so I haven't looked specifically for the FRAME_POINTER 
part. I looked now at it and you seem to be right, FRAME_POINTER is 
not needed. 

I will get rid of the FRAME_POINTER part, change the authorship and
send it again in the following days.

I made only a quick look at the patch. It looks reasonable. But I do
not have enough knowledge about the arm architecture, assembly, and
ftrace-specifics. Also I cannot test it easily. So issues might
be hidden to my eyes.

Best Regards,
Petr

Thanks,
Abel