* Josh Poimboeuf <jpoim...@redhat.com> wrote:
> 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).
Which architectures would benefit from ORC support the most?
I really think that hard reliance on GCC plugins is foolish - but maybe
Clang's plugin infrastructure is a guarantee that it remains a sane and
usable interface.
> > 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.
Yes.
> > 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.
How much of that slowdown is reversed?
> But with tracepoints disabled, I believe static jumps are used, which
> already minimizes the impact on hot paths.
Yeah.
Thanks,
Ing
