From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v2] x86: make DR*_RESERVED unsigned long
Date: Fri, 26 Apr 2013 19:15:26 +0200
Message-ID: <20130426171526.GA30875@redhat.com>
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To: "H. Peter Anvin" <hpa@zytor.com>
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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.