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

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Message-Id: <55671D63020000780007E885@mail.emea.novell.com>
Date:	Thu, 28 May 2015 12:51:31 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Ingo Molnar" <mingo@...nel.org>
Cc:	"Borislav Petkov" <bp@...en8.de>,
	"Andy Lutomirski" <luto@...capital.net>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>, <mingo@...e.hu>,
	"Brian Gerst" <brgerst@...il.com>, <fweisbec@...il.com>,
	<tglx@...utronix.de>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Denys Vlasenko" <dvlasenk@...hat.com>,
	"Josh Poimboeuf" <jpoimboe@...hat.com>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86/debug: Remove perpetually broken,
 unmaintainable dwarf annotations

>>> On 28.05.15 at 13:20, <mingo@...nel.org> wrote:
> * Jan Beulich <JBeulich@...e.com> wrote:
>> Not sure why assembly code should look like C code. It's a matter of taste 
>> perhaps, and I can see your point, but I'm also not really eager to do 
> changes 
>> just to match other people's taste. And just like above - certainly not 
>> something for this patch I would think.
> 
> Yeah, no, so this isn't going to work that way.
> 
> On one hand you want dwarf annotations mostly for the out of tree 
> dwarf-unwinding 
> stack backtraces patch on SUSE kernels, while for the upstream kernel it's 
> mostly 
> just unreadable gunk in some of the most security sensitive code paths of 
> the 
> kernel, which only gets in the way of readability.
> 
> But on the other hand you are unwilling to (or don't have the time to) do a 
> proper 
> job of making this palatable for upstream.
> 
> That's unacceptable from the upstream kernel's POV, so instead of limping 
> forward 
> I'll do the attached patch: it gets rid of the unmaintainable dwarf mess 
> from low 
> level x86 assembly code. This isn't a new concern, a couple of years ago we 
> almost 
> did this.

I can understand your motivation, yet I still view it as rather sad that
you move this way. Indeed I don't have the time to do major rework
in this area, but I don't think you can blame me for not having tried to
at least investigate and eliminate breakage when I found such (which,
as you say, happens every now and then). Yet I do recall people
indicating that the unwind data can be useful for other than the
out-of-tree live stack unwinder. I.e. those will be broken along with
that code which we're _forced_ to maintain out-of-tree.

> 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.

Jan

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