Date: Thu, 6 Mar 2008 12:34:13 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Roland McGrath <roland@redhat.com>
Cc: Jan Beulich <jbeulich@novell.com>, tglx@linutronix.de, hpa@zytor.com,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86: fix typo(?) in step.c
Message-ID: <20080306113413.GD17906@elte.hu>
References: <47CE69B0.76E4.0078.0@novell.com> <20080305134150.GA16477@elte.hu> <20080306075223.BA28B2700FD@magilla.localdomain>
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* Roland McGrath <roland@redhat.com> wrote:

> The block-step test only tested that PTRACE_SINGLEBLOCK worked right. 
> I just souped it up to also test that PTRACE_SINGLESTEP still works 
> immediately afterwards.  This still does not show any problem from 
> this bug.  The case that would be broken by it is rather more arcane.  
> I haven't worked out the test case that fails with the bogon.

my interpretation of the bug would be that we fail to mask out the 
block-step MSR bit [because we mask out bit 25 instead of bit 1], and 
hence the bug would cause that MSR bit to stay enabled in other tasks 
too.

So in theory the bug should manifest itself as block-step mode never 
clearing itself, once activated. (but this would never leak into other 
tasks because we've got the thread.debugctlmsr abstraction that protects 
them)

	Ingo
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