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: Tue, 21 Jan 2020 09:35:28 +0100 (CET) Message-ID: <alpine.LSU.2.21.2001210922060.6036@pobox.suse.cz> (raw) In-Reply-To: <20200120165039.6hohicj5o52gdghu@treble> 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
next prev 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 [this message] 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 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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