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

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Message-ID: <CA+55aFxRwAEe1VH-E_HMPf+jpxAUWaUkwLbJLXG2U44qHga+Dg@mail.gmail.com>
Date:   Thu, 4 Jan 2018 10:25:35 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.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 4, 2018 at 10:17 AM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> Clearly Paul's approach to retpoline without lfence is faster.
> I'm guessing it wasn't shared with amazon/intel until now and
> this set of patches going to adopt it, right?
>
> Paul, could you share a link to a set of alternative gcc patches
> that do retpoline similar to llvm diff ?

What is the alternative approach? Is it literally just doing a

      call 1f
1:    mov real_target,(%rsp)
       ret

on the assumption that the "ret" will always just predict to that "1"
due to the call stack?

                Linus