linux-kernel - Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

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Date:   Wed, 12 Jul 2017 17:32:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andres Freund <andres@...razel.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

On Wed, Jul 12, 2017 at 02:49:20PM -0700, Andres Freund wrote:
> Hi,
> 
> On 2017-07-11 10:33:37 -0500, Josh Poimboeuf wrote:
> > The simpler debuginfo format also enables the unwinder to be much faster
> > than DWARF, which is important for perf and lockdep.
> 
> Is this going to be usable for userland call-graphs as well? If one
> converts dwarf to that, I mean? Because right now with perf dwarf is
> often the only thing that works properly through libc, as libc isn't
> compiled with fps and has hardcoded asm not preserving fp. lbr isn't
> available for many events, and often not at all available in VMs etc.

Just to clarify, these patches are completely separate from DWARF and
shouldn't break any existing DWARF-based functionality for user space
tooling.  So perf can still use DWARF for user space binaries just fine.

(Also, tools which rely on CONFIG_DEBUG_INFO for kernel debugging, like
gdb and crash, will continue to work.)

If you want perf to be able to use ORC instead of DWARF for user space
binaries, that's not currently possible, though I don't see any
technical blockers for doing so.  Perf would need to be taught to read
ORC data.

And I think it should be possible to convert DWARF to ORC, assuming the
DWARF data is trusted.  We could probably add an objtool subcommand for
that.

-- 
Josh