linux-kernel - Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump

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Date:   Mon, 8 Oct 2018 09:21:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Matthew Helsley <mhelsley@...are.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        ard.biesheuvel@...aro.org, Andy Lutomirski <luto@...nel.org>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature
 "jump_function"

On Sat, Oct 06, 2018 at 09:39:05AM -0400, Steven Rostedt wrote:
> On Sat, 6 Oct 2018 14:12:11 +0200
> Peter Zijlstra <peterz@...radead.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).

Can't we hijack the relocation records for these functions before they
get thrown out in the (final) link pass or something?