linux-kernel - Re: [PATCH] x86/debug: Remove perpetually broken, unmaintainable dwarf annotations

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Message-ID: <20150529202716.GA24561@treble.redhat.com>
Date:	Fri, 29 May 2015 15:27:16 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>, Jan Beulich <JBeulich@...e.com>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Brian Gerst <brgerst@...il.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Dave Anderson <anderson@...hat.com>, x86@...nel.org
Subject: Re: [PATCH] x86/debug: Remove perpetually broken, unmaintainable
 dwarf annotations

On Fri, May 29, 2015 at 10:47:31AM -0700, Andy Lutomirski wrote:
> On Thu, May 28, 2015 at 6:17 AM, Ingo Molnar <mingo@...nel.org> wrote:
> > * Jan Beulich <JBeulich@...e.com> wrote:
> >> > and meanwhile you can keep a revert of this patch ported to SUSE kernels in
> >> > whatever fashion you prefer.
> >>
> >> Funny suggestion - I don't think that's reasonable for us to do. Or if we were
> >> to, we could as well invest in doing the re-work you're asking for; I don't
> >> think anyone will have the time to do either.
> >
> > That's fair enough: if there's not enough resources to keep a feature maintainable
> > upstream then it should not be upstream in that form.
> >
> > This isn't just some driver we can let bit-rot in peace until it finds a
> > maintainer (or not), without affecting anyone but users of that driver.
> >
> > This is hundreds of usage sites of ugly code intermixed with critical pieces of
> > assembly code that negatively affects the hackability of everything.
> >
> > Also, with the feature missing completely, maybe someone finds a method to
> > introduce it in a maintainable fashion, while with the feature included upstream
> > there's very little pressure to do that. As a bonus we'd also win a workable dwarf
> > unwinder.
> 
> Before doing something drastic like this, I think we should get Josh's
> opinion, since I think he's working on a new (?) unwinder.

I'd definitely like to replace all the asm DWARF CFI annotations with
something more automated and robust.  So it doesn't really affect me
whether they're ripped out now or replaced later.  

But it might be a few months before I have the code.  If they get ripped
out now, it might affect those tools which rely on debuginfo like perf,
kprobes, systemtap, gdb, crash, etc.

Then again, I'm not sure how useful or reliable the existing annotations
are anyway, so maybe it doesn't matter much.

CCing some possible consumers of kernel x86 DWARF data to see if they
have any opinions.

-- 
Josh
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