linux-kernel - Re: [x86] only print out DR registers if they are not power-on defaults.

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Message-ID: <51C225D9.8070100@zytor.com>
Date:	Wed, 19 Jun 2013 14:42:49 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: [x86] only print out DR registers if they are not power-on defaults.

On 06/18/2013 08:59 AM, Borislav Petkov wrote:
> On Tue, Jun 18, 2013 at 10:07:30AM -0400, Dave Jones wrote:
>> My intent here was to ignore cases where the reserved bits haven't
>> been set. I occasionally see DR6: 00000000fffe0ff0 for eg.
> 
> That's bit 16 which, according to the docs is read-as-1:
> 
> "All remaining bits in the DR6 register are reserved. Reserved bits
> 31:16 and 11:4 must all be set to 1, while reserved bit 12 must be
> cleared to 0. In 64-bit mode, the upper 32 bits of DR6 are reserved and
> must be written with zeros. Writing a 1 to any of the upper 32 bits
> results in a general-protection exception, #GP(0)."
> 
> This above if from AMD APM and Intel's SDM has a graphic showing the
> exact same thing:
> 
> [31:16] = set to 1; [12] = 0b, [11:4] = 1b
> 
> So if you see bit 16 cleared, then some BIOS or even hardware is doing
> funky things. I wouldn't wonder at all if BIOS dudes used reserved bits
> in registers as scratch space.
> 
>> But maybe you're right, and that is a clue and is worth printing ? I
>> can't personally recall ever diagnosing a bug using those register
>> dumps in the last 15 years.
> 
> Right, I don't know whether it would always help but if you have an
> oops and see, say bit 0 in DR6 set, i.e. a debug exception was caused
> by address breakpoint condition in DR0, then that could be useful info,
> methinks.
> 

There is serious confusion with regards to DR6 about the bits which are
*fixed* (forced to 1) and the ones which are *reserved* (should always
have a fixed value.)

There are some bits in DR6 which are used by hardware probes.

	-hpa


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