linux-kernel - Re: [RFC] Fix 2.6.33 x86 regression to kgdb hw breakpoints

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Message-ID: <20091212132428.GB22389@elte.hu>
Date:	Sat, 12 Dec 2009 14:24:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jason Wessel <jason.wessel@...driver.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] Fix 2.6.33 x86 regression to kgdb hw breakpoints - due to
 perf API changes


* Jason Wessel <jason.wessel@...driver.com> wrote:

> I'll lead with a question.  Are the people making the Perf API changes 
> booting allyesconfig kernels?
>
> The regression tests built into the kernel for kgdb fail as a result 
> of the perf API changes and can result in a hard kernel hang.
>
> My hope would have been that someone would have reported the problem, 
> created a patch to disable the test that hangs the kernel, or to fix 
> kgdb such that it works with the API changes.  Likewise, if there are 
> tests I should run to regression test the changes to the perf API, I 
> would like to know since we all want to make use of the same hw 
> resource.
> 
> A patch has been included in this mail which allows kgdb to pass the 
> internal regression tests for hw breakpoints.  I would like to get 
> some comments or start a discussion as to how to solve this problem in 
> the 2.6.33 cycle, as it is a regression in functionality.

Hm, the kgdb hw-breakpoint changes freshly put into v2.6.33 look pretty 
broken: they access the raw hw registers and ignore the higher level 
abstraction. Your patch still uses way too low level primitives. Please 
use the highlevel abstraction: register/unregister_wide_hw_breakpoint() 
should do the trick. (if there's any changes/extensions needed to it 
then please let us know about it.)

If that's too much for v2.6.33 then i guess we need to revert or disable 
the kgdb hw-breakpoint changes for now.

Thanks,

	Ingo
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