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

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Message-ID: <alpine.LSU.2.21.2001210922060.6036@pobox.suse.cz>
Date:   Tue, 21 Jan 2020 09:35:28 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, 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,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

On Mon, 20 Jan 2020, Josh Poimboeuf wrote:

> On Mon, Oct 21, 2019 at 10:05:49AM -0500, Josh Poimboeuf wrote:
> > 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.
> 
> I also just discovered that -flive-patching is responsible for all those
> "unreachable instruction" objtool warnings which Randy has been
> dutifully bugging me about over the last several months.  For some
> reason it subtly breaks GCC implicit noreturn detection for local
> functions.

Ugh, that is unfortunate. Have you reported it?
 
> At this point, I only see downsides of -flive-patching, at least until
> we actually have real upstream code which needs it.

Can you explain this? The option makes GCC to avoid optimizations which 
are difficult to detect and would make live patching unsafe. I consider it 
useful as it is, so if you shared the other downsides and what you meant 
by real upstream code, we could discuss it.

> If there aren't any objections I'll be posting a patch soon to revert.

I think it would be a setback.

Regards
Miroslav