Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751771AbdGLT14 (ORCPT <rfc822;w@1wt.eu>);
        Wed, 12 Jul 2017 15:27:56 -0400
Received: from mail-wm0-f67.google.com ([74.125.82.67]:33826 "EHLO
        mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750738AbdGLT1y (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 12 Jul 2017 15:27:54 -0400
Date: Wed, 12 Jul 2017 21:27:50 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>, Jiri Slaby <jslaby@suse.cz>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>, Mike Galbraith <efault@gmx.de>
Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)
Message-ID: <20170712192750.p4wwz6ptjrub7bav@gmail.com>
References: <cover.1499786555.git.jpoimboe@redhat.com>
 <20170712082710.g2syanmhtwqeus4o@gmail.com>
 <20170712144254.tihj43mvdj2so74d@treble>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170712144254.tihj43mvdj2so74d@treble>
User-Agent: NeoMutt/20170113 (1.7.2)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2548
Lines: 71


* Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> On Wed, Jul 12, 2017 at 10:27:10AM +0200, Ingo Molnar wrote:
> > > Create a new "ORC" unwinder, enabled by CONFIG_ORC_UNWINDER, and plug it
> > > into the x86 unwinder framework.  Objtool is used to generate the ORC
> > > debuginfo.  The ORC debuginfo format is basically a simplified version
> > > of DWARF CFI.  More details below.
> > 
> > BTW., we should perhaps consolidate our unwinder related Kconfig space, 
> > hierarchically:
> > 
> > 	CONFIG_UNWINDER
> > 	CONFIG_UNWINDER_ORC
> > 	CONFIG_UNWINDER_FRAME_POINTERS
> > 
> > Note that as a side effect it would be a valid small systems build option to have 
> > no unwinder at all, if CONFIG_EXPERT=y is set and such: !CONFIG_UNWINDER=n would 
> > be a sibling to !CONFIG_BUG.
> 
> So is the idea that CONFIG_UNWINDER=n means "use the 'guess' unwinder"?
> Or should it mean that the unwind API isn't available?
> 
> Without frame pointers and orc, it defaults to the 'guess' unwinder, for
> which the only overhead is a tiny amount of code.  It's still
> technically considered an unwinder because it plugs into the unwind
> interfaces (unwind_start(), unwind_next_frame(), etc) and is used for
> things like /proc/<pid>/stack.
> 
> So I'm not really sure CONFIG_UNWINDER=n would make sense.  Maybe there
> should just be a multiple-choice where you have to choose one of
> CONFIG_UNWINDER_{ORC,FRAME_POINTER,GUESS}.

Ok, you are right.

Maybe we could offer a menu of unwinders - i.e. make the whole Kconfig interface a 
bit nicer:

  CONFIG_UNWINDER_FRAME_POINTER
  CONFIG_UNWINDER_ORC
  CONFIG_UNWINDER_GUESS

... or so?

Default would be the historic FRAME_POINTER, at least initially, I think.

I wouldn't mind making CONFIG_UNWINDER_ORC the new default either, due to the 
non-trivial speedup it offers - but maybe folks would object?

> > CONFIG_FRAME_POINTERS et al would be left for architectures where it has a meaning 
> > beyond backtrace generation. (Not sure whether there's any such architectures.)
> 
> Well, on x86, hardened usercopy relies on frame pointers, but not the
> unwinder.  It does the frame pointer walk manually to avoid the full
> unwinder overhead.  See arch_within_stack_frames().

Oh well...

> Ok, how about:
> 
>   "Orc unwind tables take up ~50% more RAM (+1.3MB on an x86 defconfig
>   kernel) than DWARF eh_frame tables."
> 
> (My previous 1MB number was from my distro-based config, and it also
> forgot to take into account the fast lookup table (".orc_lookup")).

Sounds good to me!

Thanks,

	Ingo