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

[Peter Zijlstra]

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.