netdev - Re: [PATCH] x86: make DR*_RESERVED unsigned long

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Message-ID: <517884E1.5060404@zytor.com>
Date:	Wed, 24 Apr 2013 18:20:33 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Oleg Nesterov <oleg@...hat.com>, 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>
Subject: Re: [PATCH] x86: make DR*_RESERVED unsigned long

On 04/24/2013 04:31 PM, Frederic Weisbecker wrote:
>>
>> Now, DR6 is a bit special in that a bunch of the reserved bits are
>> hardwired to 1, not 0; I don't know offhand if that is true for bits
>> [63:32].
> 
> Hmm, good point, could it be a problem given that we clear the
> reserved dr6 bits on do_trap() and write that 'cleaned up" value back
> to "tsk->thread.debugreg6"? Probably not if those hardwired reserved
> bits are set to "1" on dr6 physical write whether those bits are
> cleared or not in their storage in thread struct before resuming the
> task?
> 

OK, the SDM states that DR6[63:32] are reserved and must be written as
zero (not one).

So the quiescent 64-bit value of DR6 is 0x0000_0000_FFFF_0FF0.

	-hpa

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