On Tue, Feb 16, 2016 at 09:09:16PM +1100, Michael Ellerman wrote: > On Mon, 2016-02-15 at 15:04 +0100, Torsten Duwe wrote: > > If you use "-pg -mprofile-kernel", gcc seems to forget that, and omits the TOC > > load, for a similar assembler calling sequence. > > That's by design. Ah, ok. > mprofile-kernel is supposed to create as little overhead as possible in the > non-traced case. All of the burden is shifted to the trace function (_mcount). ... or its helpers, see below. > The reason to do that is because modern distros always build with tracing, but > most of the time tracing will not actually be active. So we want the cost of > tracing-built-in-but-disabled to be ~zero. Ok, that's a design goal. > > That was the alternative I asked about; but given that the _mcount / ftrace_caller > > trampoline hardly differs from a normal trampoline (so far), loading R2 would be the > > general case, or an excessive special case handling would result. > > I'm not sure I follow what you mean there at the end. This suggests you have not yet actively debugged this problem ;-) > Requiring ftrace_caller() to load the kernel TOC is not a problem IMHO. The problem is, you don't get to ftrace_caller in the first place :) > I think I have an easier way to do it, I'll reply to the patch with that (if it > works). I doubt so. Either it works, _or_ it is easier ;) To save you some work: by the design of minimal overhead you try to follow, SQUASH_TOC_SAVE_INSN from my patches isn't sufficient. You'll need to load the _current_ TOC _on_ the trampoline, and in turn it will be different from the regular trampolines; and that needs to be recognised, or the normal module linker logic won't work. OTOH my proposed GCC change only affects a very limited number of functions... Looking forward to your patch! Torsten -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
