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Date:   Mon, 8 Oct 2018 09:21:34 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Matthew Helsley <mhelsley@vmware.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        ard.biesheuvel@linaro.org, Andy Lutomirski <luto@kernel.org>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature
 "jump_function"
Message-ID: <20181008072134.GB5663@hirez.programming.kicks-ass.net>
References: <20181006015110.653946300@goodmis.org>
 <20181006015720.634688468@goodmis.org>
 <20181006121211.GA5663@hirez.programming.kicks-ass.net>
 <20181006093905.46276505@vmware.local.home>
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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@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).

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