linux-kernel - Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

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Date:   Fri, 5 Jan 2018 02:28:24 -0800
From:   Paul Turner <pjt@...gle.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

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.)
> 
> As Andi says, I'd want to see explicit approval from the CPU architects
> for making that change.

Beyond guaranteeing that speculative execution is constrained, the choice of
sequence here is a performance detail and not one of correctness.

> 
> We've already had false starts there — for a long time, Intel thought
> that a much simpler option with an lfence after the register load was
> sufficient, and then eventually worked out that in some rare cases it
> wasn't. While AMD still seem to think it *is* sufficient for them,
> apparently.

As an interesting aside, that speculation proceeds beyond lfence can be
trivially proven using the timings above.  In fact, if we substitute only:
  "lfence" (with no jmp)

We see:
  29.573 cycles/call 8.469 ns/call

Now, the only way for this timing to be different, is if speculation beyond the
lfence was executed differently.

That said, this is a negative result, it does suggest that the jmp is
contributing a larger than realized cost to our speculative loop.  We can likely
shave off some additional time with some unrolling.  I did try this previously,
but did not see results above the noise floor; it seems worth trying this again;
will take a look tomorrow.