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

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Message-ID: <dfbc88a7-8417-af05-4500-5c78b0f28b6a@redhat.com>
Date:   Fri, 5 Jan 2018 12:26:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Paul Turner <pjt@...gle.com>, 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 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.

Paolo