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

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Date:   Fri, 5 Jan 2018 04:20:01 -0800
From:   Paul Turner <pjt@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        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 Fri, Jan 5, 2018 at 3:26 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> On 05/01/2018 11:28, Paul Turner wrote:
>>
>> 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
>
> Do you have timings for a non-retpolined indirect branch with the
> predictor suppressed via IBRS=1?  So at least we can compute the break
> even point.

The data I collected here previously had the run-time cost as a wash.
On Skylake, an IBRS=1 and a retpolined indirect branch had cost within
a few cycles.

The costs to consider when making a choice here are:

- The transition overheads.  This is how frequently will you be
switching in and out of protected code (as IBRS needs to be enabled
and disabled at these boundaries).
- The frequency at which you will be executing protected code on one
sibling, and unprotected code on another (enabling IBRS may affect
sibling execution, depending on SKU)
- The implementation cost (retpoline requires auditing/rebuilding your
target, while IBRS can be used out of the box).


>
> Paolo