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

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Date:   Thu, 13 Jul 2017 09:12:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andres Freund <andres@...razel.de>, 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>, Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

On Wed, Jul 12, 2017 at 05:32:25PM -0500, Josh Poimboeuf wrote:
> 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.

So the problem with userspace stuff is that the unwind data isn't
readily available from NMI context.

So the kernel unwinder will trigger a fault and abort.

The very best we can hope for is using the EH [*] stuff that all
binaries actually have _and_ map. The only problem is that most programs
don't actually use the EH stuff much so while its mapped, its not
actually paged in, so we're still stuck.

[*] C++ ABI requires EH bits for stack unwinding for exception handling
and the like, and because C++ can unwind through C code, C ABI also
mandates EH bits be present.

ORC doesn't much change this. What is currently an option is for perf to
simply copy out the top n-Kb of the stack for each sample (talk about
expensive) and then have userspace unwind it. And for userspace
unwinding in userspace, libunwind and the like are fine, I see absolutely
no reason to use ORC bits here.