On Fri, Jan 05, 2018 at 10:55:38AM +0000, David Woodhouse wrote: > On Fri, 2018-01-05 at 02:28 -0800, Paul Turner wrote: > > On Thu, Jan 04, 2018 at 07:27:58PM +0000, David Woodhouse wrote: > > > On Thu, 2018-01-04 at 10:36 -0800, Alexei Starovoitov wrote: > > > > > > > > Pretty much. > > > > Paul's writeup: https://support.google.com/faqs/answer/7625886 > > > > tldr: jmp *%r11 gets converted to: > > > > call set_up_target; > > > > capture_spec: > > > > pause; > > > > jmp capture_spec; > > > > set_up_target: > > > > mov %r11, (%rsp); > > > > ret; > > > > where capture_spec part will be looping speculatively. > > > > > > That is almost identical to what's in my latest patch set, except that > > > the capture_spec loop has 'lfence' instead of 'pause'. > > > > When choosing this sequence I benchmarked several alternatives here, > > including > > (nothing, nops, fences, and other serializing instructions such as cpuid). > > > > The "pause; jmp" sequence proved minutely faster than "lfence;jmp" which is > > why > > it was chosen. > > > > "pause; jmp" 33.231 cycles/call 9.517 ns/call > > "lfence; jmp" 33.354 cycles/call 9.552 ns/call > > > > (Timings are for a complete retpolined indirect branch.) > > Yeah, I studiously ignored you here and went with only what Intel had > *assured* me was correct and put into the GCC patches, rather than > chasing those 35 picoseconds ;)
It's also notable here that while the difference is small in terms of absolute values, it's likely due to reduced variation: I would expect: - pause to be extremely consistent in its timings - pause and lfence to be close on their average timings, particularly in a micro-benchmark. Which suggests that the difference may be larger in the occasional cases that you are getting "unlucky" and seeing some other uarch interaction in the lfence path. > > The GCC patch set already had about four different variants over time, > with associated "oh shit, that one doesn't actually work; try this". > What we have in my patch set is precisely what GCC emits at the moment. > > I'm all for optimising it further, but maybe not this week. > > Other than that, is there any other development from your side that I > haven't captured in the latest (v4) series? > http://git.infradead.org/users/dwmw2/linux-retpoline.git/
