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

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Date:   Thu, 4 Jan 2018 10:40:23 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     David Woodhouse <dwmw@...zon.co.uk>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, 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>, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

> 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 ?

I don't think it's a good idea to use any sequence not signed
off by CPU designers and extensively tested. 

While another one may work for most tests, it could always fail in
some corner case.

Right now we have the more heavy weight one and I would
suggest to stay with that one for now. Then worry
about more optimizations later.

Correctness first.

-Andi