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
