Live-Patching Archive on lore.kernel.org
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From: Miroslav Benes <mbenes@suse.cz>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Joe Lawrence <joe.lawrence@redhat.com>,
	Jessica Yu <jeyu@kernel.org>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	mhiramat@kernel.org, bristot@redhat.com, jbaron@akamai.com,
	torvalds@linux-foundation.org, tglx@linutronix.de,
	mingo@kernel.org, namit@vmware.com, hpa@zytor.com,
	luto@kernel.org, ard.biesheuvel@linaro.org,
	live-patching@vger.kernel.org,
	Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()
Date: Wed, 22 Jan 2020 13:15:49 +0100 (CET)
Message-ID: <alpine.LSU.2.21.2001221312030.15957@pobox.suse.cz> (raw)
In-Reply-To: <20200121161045.dhihqibnpyrk2lsu@treble>

On Tue, 21 Jan 2020, Josh Poimboeuf wrote:

> On Tue, Jan 21, 2020 at 09:35:28AM +0100, Miroslav Benes wrote:
> > 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?
> 
> Not yet (but I plan to).

My findings so far...

I bisected through GCC options which -flive-patching disables and 
-fno-ipa-pure-const is the culprit. I got no warnings without the option 
with my config.

Then I found out allmodconfig was ok even with -flive-patching. 
CONFIG_GCOV is the difference. CONFIG_GCOV=y seems to make the warnings go 
away here.

/me goes staring

  parent reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20191007081945.10951536.8@infradead.org>
     [not found] ` <20191008104335.6fcd78c9@gandalf.local.home>
     [not found]   ` <20191009224135.2dcf7767@oasis.local.home>
     [not found]     ` <20191010092054.GR2311@hirez.programming.kicks-ass.net>
     [not found]       ` <20191010091956.48fbcf42@gandalf.local.home>
     [not found]         ` <20191010140513.GT2311@hirez.programming.kicks-ass.net>
     [not found]           ` <20191010115449.22044b53@gandalf.local.home>
     [not found]             ` <20191010172819.GS2328@hirez.programming.kicks-ass.net>
     [not found]               ` <20191011125903.GN2359@hirez.programming.kicks-ass.net>
     [not found]                 ` <20191015130739.GA23565@linux-8ccs>
     [not found]                   ` <20191015135634.GK2328@hirez.programming.kicks-ass.net>
2019-10-15 14:13                     ` Miroslav Benes
2019-10-15 15:06                       ` Joe Lawrence
2019-10-15 15:31                         ` Jessica Yu
2019-10-15 22:17                           ` Joe Lawrence
2019-10-15 22:27                             ` Steven Rostedt
2019-10-16  7:42                               ` Peter Zijlstra
2019-10-16 10:15                                 ` Miroslav Benes
2019-10-21 15:05                                 ` Josh Poimboeuf
2020-01-20 16:50                                   ` Josh Poimboeuf
2020-01-21  8:35                                     ` Miroslav Benes
2020-01-21 16:10                                       ` Josh Poimboeuf
2020-01-22 10:09                                         ` Miroslav Benes
2020-01-22 21:42                                           ` Josh Poimboeuf
2020-01-28  9:28                                             ` Miroslav Benes
2020-01-28 15:00                                               ` Josh Poimboeuf
2020-01-28 15:40                                                 ` Petr Mladek
2020-01-28 17:02                                                   ` Josh Poimboeuf
2020-01-29  0:46                                                     ` Jiri Kosina
2020-01-29  2:17                                                       ` Josh Poimboeuf
2020-01-29  3:14                                                         ` Jiri Kosina
2020-01-29 12:28                                                     ` Miroslav Benes
2020-01-29 15:59                                                       ` Josh Poimboeuf
2020-01-30  9:53                                                         ` Petr Mladek
2020-01-30 14:17                                                           ` Josh Poimboeuf
2020-01-31  7:17                                                             ` Petr Mladek
2020-01-22 12:15                                         ` Miroslav Benes [this message]
2020-01-22 15:05                                           ` Miroslav Benes
2020-01-22 22:03                                             ` Josh Poimboeuf
2020-01-23 10:19                                               ` Martin Jambor
2019-10-16  7:49                               ` Peter Zijlstra
2019-10-16 10:20                                 ` Miroslav Benes
2019-10-16 13:29                                   ` Miroslav Benes
2019-10-18 13:03                                     ` Jessica Yu
2019-10-18 13:40                                       ` Petr Mladek
2019-10-21 14:14                                         ` Jessica Yu
2019-10-21 15:31                                         ` Josh Poimboeuf
2019-10-22  8:27                                       ` Miroslav Benes
2019-10-22 14:31                                         ` Josh Poimboeuf
2019-10-23  9:04                                           ` Miroslav Benes
2019-10-16  6:51                         ` Miroslav Benes
2019-10-16  9:23                           ` Peter Zijlstra
2019-10-16  9:36                             ` Jessica Yu
2019-10-16  9:51                               ` Peter Zijlstra
2019-10-16 12:39                           ` Peter Zijlstra
2019-10-22  8:45                             ` Miroslav Benes

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