linux-kernel - Re: [PATCH] x86: prevent unconditional writes to DebugCtl MSR

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Message-Id: <480CDE2B.76E4.0078.0@novell.com>
Date:	Mon, 21 Apr 2008 17:34:19 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Ingo Molnar" <mingo@...e.hu>, "Roland McGrath" <roland@...hat.com>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: prevent unconditional writes to DebugCtl MSR

>>> Ingo Molnar <mingo@...e.hu> 21.04.08 17:55 >>>
>
>* Roland McGrath <roland@...hat.com> wrote:
>
>> If TIF_DEBUGCTLMSR is ever set on a machine without the support, that 
>> is a bug we should diagnose earlier.  If you want some paranoia, then 
>> keep update_debugctlmsr but make it do:
>> 
>> 	BUG_ON(boot_cpu_data.x86 < 6);
>> 
>> instead.
>
>agreed. Jan, wanna send a fix for this?

Hmm, without knowing the context here I'd say the way it's done in
.25 is quite reasonable; a BUG_ON() here would seem quite rude to
me. I thought it really should be the call sites ensuring this doesn't
get called, and the silent ignoring is just to prevent bringing the
system down. A WARN_ON() (or perhaps even WARN_ON_ONCE()) would
be the most I'd be inclined to add there.

Jan

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