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
