RFC livepatch: arm64: add support for livepatch on arm64
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Cover letter [RFC PATCH 0/3] livepatch: arm64: add support for livepatch on arm64
Quoted 21 lines · Petr Mladek
Quoted 11 lines · Li Bin
This patchset depends on the on-going gcc feature "-fprolog-pad=N", which will generate a pad of N nops at the beginning of each function.
Livepatch on arm64 can using the feature (that always placing one nop at the beginning of the function). And when enable/disable func patching, just modify the pad code to nop or branch. And that NOP and B instruction are both safe instructions on arm64 which called "concurrent modification and execution of instructions", that can be executed by one thread of execution as they are being modified by another thread of execution without requiring explicit synchronization.
Quoted 3 lines
And this method will improve performance significantly compared with the method based on ftrace, especially for the critical function being frequently called.
It sounds like a great feature for ftrace. If the new prologue is usable for LivePatching, it should be usable to call the ftrace handler as well. If you teach ftrace to use the new prologue, you will not need all these crazy arch-specific hacks for LivePatching. Then both ftrace and livepatch will benefit from the feature.
Quoted 6 lines
I suggest to read the ftrace documentation in
Documentation/trace/, especially ftrace-design.txt. I have never ported ftrace to a new architecture. I guess that you need to teachscripts/recordmcount.c
to find the new location. Also you might need to updatearch/arm/kernel/ftrace.c. Also please make sure that ftrace supportsDYNAMIC_FTRACE_WITH_REGSon Arm.
Hi Petr,
I had posted one method for livepatch implementaion on arm64 based on ftrace with DYNAMIC_FTRACE_WITH_REGS.
https://lwn.net/Articles/646317/
This requires GCC changes to support it, and I propose a method that implement -mfentry feature which following some other arch such as x86/mips/s390 etc. This method is architecture-specific, and Maxim Kuvyrkov propose a new method for gcc that implement a target-independent option -fprolog-pad=N, which will generate a pad of N nops at the beginning of each function.
https://gcc.gnu.org/ml/gcc/2015-05/msg00267.html
And based on this, DYNAMIC_FTRACE_WITH_REGS feature can be implemented as mentioned in:
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-November/386815.html
And in this patchset, I only concern the performance for livepatch on arm64, and I hope this also can be resolved by improving ftrace, and thanks to Steve that he pointed that he is working on a way to make ftrace a bit better for livepatch.
Thanks,
Li Bin
Quoted 8 lines
It is harder but the right way to go. Your current patch set looks like a hack.
Best Regards,
Petr.