On Fri, Nov 09, 2018 at 08:28:11AM +0100, Ingo Molnar wrote: > > - I'm not sure about the objtool approach. Objtool is (currently) > > x86-64 only, which means we have to use the "unoptimized" version > > everywhere else. I may experiment with a GCC plugin instead. > > I'd prefer the objtool approach. It's a pretty reliable first-principles > approach while GCC plugin would have to be replicated for Clang and any > other compilers, etc.
The benefit of a plugin is that we'd only need two of them: GCC and Clang. And presumably, they'd share a lot of code. The prospect of porting objtool to all architectures is going to be much more of a daunting task (though we are at least already considering it for some arches). > > - Does this feature have much value without retpolines? If not, should > > we make it depend on retpolines somehow? > > Paravirt patching, as you mention in your later reply? > > > - Find some actual users of the interfaces (tracepoints? crypto?) > > I'd be very happy with a demonstrated paravirt optimization already - > i.e. seeing the before/after effect on the vmlinux with an x86 distro > config. > > All major Linux distributions enable CONFIG_PARAVIRT=y and > CONFIG_PARAVIRT_XXL=y on x86 at the moment, so optimizing it away as much > as possible in the 99.999% cases where it's not used is a primary > concern. For paravirt, I was thinking of it as more of a cleanup than an optimization. The paravirt patching code already replaces indirect branches with direct ones -- see paravirt_patch_default(). Though it *would* reduce the instruction footprint a bit, as the 7-byte indirect calls (later patched to 5-byte direct + 2-byte nop) would instead be 5-byte direct calls to begin with. > All other usecases are bonus, but it would certainly be interesting to > investigate the impact of using these APIs for tracing: that too is a > feature enabled everywhere but utilized only by a small fraction of Linux > users - so literally every single cycle or instruction saved or hot-path > shortened is a major win. With retpolines, and with tracepoints enabled, it's definitely a major win. Steve measured an 8.9% general slowdown on hackbench caused by retpolines. But with tracepoints disabled, I believe static jumps are used, which already minimizes the impact on hot paths. -- Josh
