From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S935323AbeAKRXM (ORCPT <rfc822;ralf@linux-mips.org> + 1 other);
        Thu, 11 Jan 2018 12:23:12 -0500
Received: from bombadil.infradead.org ([65.50.211.133]:44255 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S933061AbeAKRXL (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 11 Jan 2018 12:23:11 -0500
Date: Thu, 11 Jan 2018 18:23:03 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Jiri Kosina <jikos@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Andi Kleen <ak@linux.intel.com>, Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Kees Cook <keescook@google.com>,
        Andy Lutomirski <luto@amacapital.net>,
        gnomes@lxorguk.ukuu.org.uk
Subject: Re: [PATCH 2/3] objtool: Ignore retpoline alternatives
Message-ID: <20180111172303.GI6176@hirez.programming.kicks-ass.net>
References: <cover.1515635197.git.jpoimboe@redhat.com>
 <a181ea89cc196187b85867cb5eb13707279a887a.1515635197.git.jpoimboe@redhat.com>
 <1515688058.22302.366.camel@infradead.org>
 <20180111163326.deqtbqt72avfweup@treble>
 <1515688778.22302.372.camel@infradead.org>
 <20180111164826.v26m2vqfqw667zx2@treble>
 <nycvar.YFH.7.76.1801111757410.11852@cbobk.fhfr.pm>
 <20180111170524.GH6176@hirez.programming.kicks-ass.net>
 <1515691163.22302.382.camel@infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <1515691163.22302.382.camel@infradead.org>
User-Agent: Mutt/1.9.2 (2017-12-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Return-Path: <linux-kernel-owner@vger.kernel.org>

On Thu, Jan 11, 2018 at 05:19:23PM +0000, David Woodhouse wrote:
> On Thu, 2018-01-11 at 18:05 +0100, Peter Zijlstra wrote:
> > On Thu, Jan 11, 2018 at 06:01:23PM +0100, Jiri Kosina wrote:
> > > On Thu, 11 Jan 2018, Josh Poimboeuf wrote:
> > > 
> > > > I think I heard that retpolines won't be ported to anything older than 
> > > > GCC 4.9, so maybe it's safe to use '%='.  I don't remember when it was 
> > > > introduced into GCC though.
> > > 
> > > I'm afraid we'll have to backport retpolines in some form to 4.3.x at 
> > > least. I'd be surprised if we'd be the only ones on this planet :)
> > 
> > So upstream code is going to require 4.5 at least, and 4.4 has %=.
> > Backport effort will just have to cope or backport more GCC bits, that
> > is, if you're backporting retpoline to 4.3 also backport asm-goto.
> 
> Again, the RSB thing is for more than just retpoline; it's needed for
> IBRS support too and that *doesn't* necessarily require a newer
> compiler at all.

I think the asm-goto was exactly for IBRS, because without that you get
to sprinkle LFENCE all over the place.