From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2] x86: make DR*_RESERVED unsigned long
Date: Fri, 26 Apr 2013 09:44:05 -0700
Message-ID: <517AAED5.7040400@zytor.com>
References: <CA+55aFwpLu0qNms=hkQr43yqD0K9DgESNm91OEWKT1ZuT8MU6Q@mail.gmail.com> <20130424072630.GB1780@gmail.com> <20130424170702.GA1867@redhat.com> <5178282D.9030902@zytor.com> <20130425144818.GA25921@redhat.com> <20130426163802.GA30351@redhat.com>
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Cc: Ingo Molnar <mingo@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Thomas Gleixner <tglx@linutronix.de>,
	David Miller <davem@davemloft.net>,
	"Theodore Ts'o" <tytso@mit.edu>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	"linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>,
	Fr??d??ric Weisbecker <fweisbec@gmail.com>
To: Oleg Nesterov <oleg@redhat.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20130426163802.GA30351@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-ext4.vger.kernel.org

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