Re: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

[Petr Mladek]

On Thu 2016-12-08 22:46:55, Abel Vesa wrote:
> On Thu, Dec 08, 2016 at 09:46:35PM +0000, Abel Vesa wrote:
> > From: Jean-Jacques Hiblot <jjhiblot@xxxxxxxxxxxxxxx>
> > 
> > From: Jean-Jacques Hiblot <jjhiblot@xxxxxxxxxxxxxxx>
> 
> >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 <jjhiblot@xxxxxxxxxxxxxxx>
> > Signed-off-by: Abel Vesa <abelvesa@xxxxxxxxx>
> > ---
> >  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

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 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