linux-kernel - Re: [PATCH v2 11/12] retpoline/objtool: Disable some objtool warnings

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Date:   Thu, 4 Jan 2018 08:13:08 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andi Kleen <ak@...ux.intel.com>, Andi Kleen <andi@...stfloor.org>,
        tglx@...utronix.de, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com
Subject: Re: [PATCH v2 11/12] retpoline/objtool: Disable some objtool warnings

On Thu, Jan 04, 2018 at 10:06:01AM -0600, Josh Poimboeuf wrote:
> On Thu, Jan 04, 2018 at 07:59:14AM -0800, Andi Kleen wrote:
> > > NAK.  We can't blindly disable objtool warnings, that will break
> > > livepatch and the ORC unwinder.  If you share a .o file (or the GCC
> > > code) I can look at adding retpoline support.
> > 
> > I don't think we can wait for that. We can disable livepatch and the
> > unwinder for now. They are not essential. Frame pointers should work
> > well enough for unwinding
> 
> If you want to make this feature conflict with livepatch and ORC,
> silencing objtool warnings is not the way to do it.

I don't see why it would conflict with the unwinder anyways?

It doesn't change the long term stack state, so it should be invisible to the 
unwinder (unless you crash in the thunk, which is very unlikely)

I actually got some unwinder backtraces during development and they seemed
to work.

> 
> > and afaik nobody can use livepatch in mainline anyways.
> 
> Why not?  The patch creation tooling is still out-of-tree, but livepatch
> itself is fully supported in mainline.

Ok.

Still doesn't seem critical at this point if it's some out of tree
thing.

-Andi