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Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181006093905.46276505@vmware.local.home>
Date:   Sat, 6 Oct 2018 08:13:18 -0700
Cc:     Peter Zijlstra <peterz@infradead.org>,
        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>
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References: <20181006015110.653946300@goodmis.org> <20181006015720.634688468@goodmis.org> <20181006121211.GA5663@hirez.programming.kicks-ass.net> <20181006093905.46276505@vmware.local.home>
To:     Steven Rostedt <rostedt@goodmis.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Oct 6, 2018, at 6:39 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>=20
> On Sat, 6 Oct 2018 14:12:11 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
>=20
>>> 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" ) =20
>>=20
>> Bah, what is it with you people and trampolines. Why can't we, just like
>> jump_label, patch the call directly?
>>=20
>> The whole call+jmp thing is silly, don't do that. It just wrecks I$ and
>> is slower for no real reason afaict.
>=20
> 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).
>=20
>>=20
>> Steve, also see:
>>=20
>>  https://lkml.kernel.org/r/20181005081333.15018-1-ard.biesheuvel@linaro.o=
rg
>=20
> 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.
>=20
> 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.
>=20
> Perhaps a gcc plugin might work too.
>=20

My suggestion was to have objtool do the dirty work. Josh said something sus=
piciously like =E2=80=9Csounds fun=E2=80=9D on IRC :)

> I'll have to see what Ard did to handle the function parameters.
>=20
> -- Steve
>=20