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
