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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 4 Jan 2018 10:25:35 -0800
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Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>,
        Andi Kleen <ak@linux.intel.com>, Paul Turner <pjt@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Kees Cook <keescook@google.com>,
        Rik van Riel <riel@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
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On Thu, Jan 4, 2018 at 10:17 AM, Alexei Starovoitov
<alexei.starovoitov@gmail.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