linux-kernel - Re: [PATCH 2/3] objtool: Ignore retpoline alternatives

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Message-ID: <1515691163.22302.382.camel@infradead.org>
Date:   Thu, 11 Jan 2018 17:19:23 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jikos@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        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>,
        Kees Cook <keescook@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/3] objtool: Ignore retpoline alternatives

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.

But really, if %= is supported at least as far back as 4.4 (and maybe
further?) then I'm not going to care about it unless someone really
screams.

I think we're fairly much done, unless Andi you really want to make the
RSB stuffing code out-of-line?

Thomas, do you want to leave Josh's patches on top with a revert, as
they are at the moment in my tree, or rebase and fold them in as we go?
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