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    SubjectRe: [PATCH] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS
    2017-01-12 1:19 GMT+01:00 Abel Vesa <abelvesa@gmail.com>:
    > On Tue, Jan 10, 2017 at 04:51:12PM +0100, Petr Mladek wrote:
    >> 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@traphandler.com>
    >> > >
    >> > > From: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
    >> >
    >> > >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@traphandler.com>
    >> > > Signed-off-by: Abel Vesa <abelvesa@linux.com>
    >> > > ---
    >> > > 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.
    >

    FRAME_POINTER is not needed. the dependency is wrong and should be removed.
    The code must be modified to not use fp register:

    --- a/arch/arm/kernel/entry-ftrace.S
    +++ b/arch/arm/kernel/entry-ftrace.S
    @@ -130,7 +130,8 @@ ftrace_graph_regs_call:
    #ifdef CONFIG_FUNCTION_GRAPH_TRACER
    .macro __ftrace_graph_regs_caller

    - sub r0, fp, #4 @ lr of instrumented
    routine (parent)
    + add r0, sp, #64 @ r0 is now a pointer to lr of
    + @ instrumented routine

    @ called from __ftrace_regs_caller
    ldr r1, [sp, #56] @ instrumented routine (func)
    @@ -139,8 +140,9 @@ ftrace_graph_regs_call:
    mov r2, fp @ frame pointer
    bl prepare_ftrace_return

    - ldr lr, [fp, #-4] @ restore lr from the stack
    - ldmia sp, {r0-r11, ip, sp} @ restore r0 through sp
    + ldr lr, [sp, #64] @ get the previous LR value from stack
    + ldmia sp, {r0-r11, ip, sp} @ pop the saved registers INCLUDING
    + @ the stack pointer
    ret ip
    .endm
    #endif

    Jean-Jacques


    > 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

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