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SubjectRe: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
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> On Oct 6, 2018, at 6:39 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Sat, 6 Oct 2018 14:12:11 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
>
>>> On Fri, Oct 05, 2018 at 09:51:11PM -0400, Steven Rostedt wrote:
>>> +#define arch_dynfunc_trampoline(name, def) \
>>> + asm volatile ( \
>>> + ".globl dynfunc_" #name "; \n\t" \
>>> + "dynfunc_" #name ": \n\t" \
>>> + "jmp " #def " \n\t" \
>>> + ".balign 8 \n \t" \
>>> + : : : "memory" )
>>
>> Bah, what is it with you people and trampolines. Why can't we, just like
>> jump_label, patch the call directly?
>>
>> The whole call+jmp thing is silly, don't do that. It just wrecks I$ and
>> is slower for no real reason afaict.
>
> My first attempt was to do just that. But to add a label at the
> call site required handling all the parameters too. See my branch:
> ftrace/jump_function-v1 for how ugly it got (and it didn't work).
>
>>
>> Steve, also see:
>>
>> https://lkml.kernel.org/r/20181005081333.15018-1-ard.biesheuvel@linaro.org
>
> Interesting. I don't have time to look at it at the moment to see what
> was done, but will do so in the near future.
>
> Remember, this was a proof of concept and even with the trampolines, it
> showed a great level of improvement. One thought was to do a
> "recordmcount.c" type of action to find where the calls were and patch
> them directly at boot up. I tried to keep the API the same where this
> could actually be done as an improvement later.
>
> Perhaps a gcc plugin might work too.
>

My suggestion was to have objtool do the dirty work. Josh said something suspiciously like “sounds fun” on IRC :)

> I'll have to see what Ard did to handle the function parameters.
>
> -- Steve
>

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