linux-kernel - Re: [PATCH v2] x86: make DR*

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Message-ID: <517AAED5.7040400@zytor.com>
Date:	Fri, 26 Apr 2013 09:44:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	"Theodore Ts'o" <tytso@....edu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v2] x86: make DR*_RESERVED unsigned long

On 04/26/2013 09:38 AM, Oleg Nesterov wrote:
> DR6_RESERVED and DR_CONTROL_RESERVED are used to clear the set
> bits in the "unsigned long" data, make them long to ensure that
> "&~" doesn't clear the upper bits.
> 
> This is only cleanup, the usage of ~DR*_RESERVED is safe but
> doesn't look clean and the pattern is error prone.
> 
> 	- do_debug:
> 
> 		dr6 &= ~DR6_RESERVED;
> 
> 	  this also wrongly clears 32-63 bits. Fortunately these
> 	  bits are reserved and must be zero.
> 

I don't think this is wrongly at all.  The whole point is to mask out
the bits that the handler doesn't want to deal with, so masking out the
reserved bits [63:32] seems reasonable to me.

The comment should probably be corrected, though.

	-hpa


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