on 2015/12/15 23:43, Petr Mladek wrote:
On Mon 2015-12-14 20:49:35, Li Bin wrote:
quoted
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.
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.
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 teach scripts/recordmcount.c
to find the new location. Also you might need to update
arch/arm/kernel/ftrace.c. Also please make sure that ftrace
supports DYNAMIC_FTRACE_WITH_REGS on 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
It is harder but the right way to go. Your current patch set
looks like a hack.
Best Regards,
Petr
.