netdev - Re: [PATCH v2] x86: make DR*

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Message-ID: <20130426171526.GA30875@redhat.com>
Date:	Fri, 26 Apr 2013 19:15:26 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.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, H. Peter Anvin wrote:
>
> On 04/26/2013 09:38 AM, Oleg Nesterov wrote:
> >
> > 	- 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.

OK, I meant that it also clears the bits that are not specified in
DR6_RESERVED mask.

> 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.

Then we should do

	- #define DR6_RESERVED    0xFFFF0FF0
	+ #define DR6_RESERVED    0xFFFFFFFFFFFF0FF0

?

or what? (just in case, I will happily agree with "do nothing" ;)

> The comment should probably be corrected, though.

Which one?

	/* Define reserved bits in DR6 which are always set to 1 */
	#define DR6_RESERVED    (0xFFFF0FF0UL)

	/* Filter out all the reserved bits which are preset to 1 */
	dr6 &= ~DR6_RESERVED;

I guess both should be updated then. But if I read the doc correctly
the lower reserved bits are set to 1.

However do_debug() does set_debugreg(0, 6) and this looks correct, the
doc says "debug handlers should clear the register before returning to
the interrupted task".

Oleg.

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