From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Miroslav Benes <mbenes@suse.cz>
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: Tue, 21 Jan 2020 10:10:45 -0600 [thread overview]
Message-ID: <20200121161045.dhihqibnpyrk2lsu@treble> (raw)
In-Reply-To: <alpine.LSU.2.21.2001210922060.6036@pobox.suse.cz>
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).
> > 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.
Only SLES needs it right? Why inflict it on other livepatch users? By
"real upstream code" I mean there's no (documented) way to create live
patches using the method which relies on this flag. So I don't see any
upstream benefits for having it enabled.
--
Josh
next prev parent reply other threads:[~2020-01-21 16:11 UTC|newest]
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 ` [PATCH v3 5/6] x86/ftrace: Use text_poke() 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 [this message]
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
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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