Re: [RFC PATCH 0/5] livepatch: add support on arm64

[Li Bin]

On 2015/6/2 10:15, AKASHI Takahiro wrote:
> On 05/30/2015 09:01 AM, Masami Hiramatsu wrote:
>> On 2015/05/28 14:51, Li Bin wrote:
>>> This patchset propose a method for gcc -mfentry feature(profile
>>> before prologue) implementation for arm64, and propose the livepatch
>>> implementation for arm64 based on this feature.
>>> The gcc implementation about this feature will be post to the gcc
>>> community soon.
>>>
>>> With this -mfentry feature, the entry of each function like:
>>>
>>> foo:
>>>      mov x9, x30
>>>      bl __fentry__
>>>      mov x30, x9
>>>      [prologue]
>>>      ...
>>>
>>> The x9 is a callee corruptible register, and the __fentry__ function
>>> is responsible to protect all registers, so it can be used to protect
>>> the x30. And the added two instructions which is register mov operation
>>> have ralatively small impact on performance.
>>
>> Hm, this implementation looks good to me :)
>> This also enables us to KPROBES_ON_FTRACE too.
> 
> Even if x9 is a callee-saved register, there is no way to restore its original
> value in setting up a pt_regs in ftrace_reg_caller.

Hi, Takahiro AKASHI

Firstly, x9 is not a callee-saved but a caller-saved register(or being called
corruptible register).
Secondly, I think x9 is already protected properly, please reference the patch:
[PATCH 1/5] livepatch: ftrace: arm64: Add support for DYNAMIC_FTRACE_WITH_REGS
[PATCH 3/5] livepatch: ftrace: arm64: Add support for -mfentry on arm64

> It's not the right thing for KPROBES_ON_FTRACE, is it?
> 
> Saving Link register in stack is not a big deal since the overhead of ftrace
> is much bigger.

Performance overhead is only one aspect of the problem, and more importantly,
even worse is that it would break the arm64 ABI rules.

Thanks,
	Li Bin
> 
> -Takahiro AKASHI
> 
> 
>> Thanks,
>>
>>>
>>> This patchset has been tested on arm64 platform.
>>>
>>> Li Bin (4):
>>>    livepatch: ftrace: arm64: Add support for DYNAMIC_FTRACE_WITH_REGS
>>>    livepatch: ftrace: add ftrace_function_stub_ip function
>>>    livepatch: ftrace: arm64: Add support for -mfentry on arm64
>>>    livepatch: arm64: add support for livepatch on arm64
>>>
>>> Xie XiuQi (1):
>>>    livepatch: arm64: support relocation in a module
>>>
>>>   arch/arm64/Kconfig                 |    5 +
>>>   arch/arm64/include/asm/ftrace.h    |    9 +
>>>   arch/arm64/include/asm/livepatch.h |   45 +++++
>>>   arch/arm64/kernel/Makefile         |    1 +
>>>   arch/arm64/kernel/arm64ksyms.c     |    4 +
>>>   arch/arm64/kernel/entry-ftrace.S   |  154 +++++++++++++++-
>>>   arch/arm64/kernel/ftrace.c         |   28 +++-
>>>   arch/arm64/kernel/livepatch.c      |   41 ++++
>>>   arch/arm64/kernel/module.c         |  355 ++++++++++++++++++------------------
>>>   include/linux/ftrace.h             |    1 +
>>>   kernel/livepatch/core.c            |   17 ++-
>>>   kernel/trace/ftrace.c              |   32 ++++
>>>   scripts/recordmcount.pl            |    2 +-
>>>   13 files changed, 508 insertions(+), 186 deletions(-)
>>>   create mode 100644 arch/arm64/include/asm/livepatch.h
>>>   create mode 100644 arch/arm64/kernel/livepatch.c
>>>
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>>
>>
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