linux-kernel - Re: [PATCH 2/2] x86/unwind: Make CONFIG_UNWINDER

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Date:   Mon, 19 Mar 2018 13:31:30 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, peterz@...radead.org, hpa@...or.com,
        tglx@...utronix.de
Subject: Re: [PATCH 2/2] x86/unwind: Make CONFIG_UNWINDER_ORC=y the default
 in kconfig for 64-bit

El Mon, Mar 19, 2018 at 02:29:10PM -0500 Josh Poimboeuf ha dit:

> On Mon, Mar 19, 2018 at 11:57:32AM -0700, Matthias Kaehlcke wrote:
> > Hi Josh,
> > 
> > El Fri, Oct 13, 2017 at 03:02:01PM -0500 Josh Poimboeuf ha dit:
> > 
> > > The ORC unwinder has been stable in testing so far.  Give it much wider
> > > testing by making it the default in kconfig for x86_64.  It's not yet
> > > supported for 32-bit, so leave frame pointers as the default there.
> > > 
> > > Suggested-by: Ingo Molnar <mingo@...nel.org>
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > > ---
> > 
> > Building an upstream kernel with clang results in plenty of objtool
> > warnings like these:
> > 
> > drivers/gpu/drm/i915/dvo_ch7017.o: warning: objtool: ch7017_get_hw_state()+0x80: return with modified stack frame
> >   CC      drivers/gpu/drm/i915/i915_oa_cflgt2.o
> > ...
> >   CC      drivers/gpu/drm/i915/intel_lpe_audio.o
> > drivers/gpu/drm/i915/i915_gpu_error.o: warning: objtool: i915_error_printf()+0x6c: return with modified stack frame
> > drivers/gpu/drm/i915/intel_display.o: warning: objtool: pipe_config_err()+0xa6: return with modified stack frame
> > 
> > Bisecting shows that the warnings are generated since the ORC unwinder
> > was made the default. Not sure if the issue is on the unwinder side
> > or clang.
> > 
> > Any ideas on what could be the problems?
> 
> Hi Matthias,
> 
> The ORC unwinder relies on objtool, which reverse engineers the compiled
> code.  This is objtool's first exposure to clang, so I'm not at all
> surprised if it's getting confused.
> 
> Send me one of the .o files and I can take a quick look to see how bad
> it is, but I'm guessing it's going to be a lot of work to make objtool
> compatible with clang (and unfortunately I won't have the bandwidth to
> work on that in the near term.)
> 
> In the meantime I'd recommend that you use frame pointers (and
> CONFIG_STACK_VALIDATION=n) for clang-compiled kernels.

Thanks for your assessment!

dvo_ch7017.o is attached.

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