linux-kernel - Re: [PATCH v3 5/6] x86/ftrace: Use text

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Date:   Mon, 21 Oct 2019 10:05:49 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        x86@...nel.org, linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, live-patching@...r.kernel.org
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

On Wed, Oct 16, 2019 at 09:42:17AM +0200, Peter Zijlstra wrote:
> > which are not compatible with livepatching. GCC upstream now has
> > -flive-patching option, which disables all those interfering optimizations.
> 
> Which, IIRC, has a significant performance impact and should thus really
> not be used...
> 
> If distros ship that crap, I'm going to laugh at them the next time they
> want a single digit performance improvement because *important*.

I have a crazy plan to try to use objtool to detect function changes at
a binary level, which would hopefully allow us to drop this flag.

But regardless, I wonder if we enabled this flag prematurely.  We still
don't have a reasonable way to use it for creating source-based live
patches upstream, and it should really be optional for CONFIG_LIVEPATCH,
since kpatch-build doesn't need it.

-- 
Josh