linux-kernel - Re: bisected boot regression post 2.6.25-rc3.. please revert

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Date:	Tue, 4 Mar 2008 07:58:53 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>, hans.rosenfeld@....com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: bisected boot regression post 2.6.25-rc3.. please revert


* Segher Boessenkool <segher@...nel.crashing.org> wrote:

> so (on 32-bit) ~_PAGE_PSE is ~0x80L is 0xffffff7f, which when cast to 
> 64-bit is 0x00000000ffffff7f, so in
>
> 	(~PAGE_MASK & ~_PAGE_USER & ~_PAGE_PSE & ~_PAGE_NX)
>
> all the high bits are lost, while the original
>
> 	~(PAGE_MASK | _PAGE_USER | _PAGE_PSE | _PAGE_NX)
>
> works as intended, since the bit inversion is done on a 64-bit number.

but we really are interested in the low bits here (lets ignore the NX 
bit for now) and the patch has been in the queue for a long time (more 
than a month), so if there was a trivial mask mixup problem it would 
have shown on the first day. So i suspect some gcc bug instead - and 
certainly the colorful mixture of types and signs in this expression 
might have surprised a new version of gcc somewhere.

	Ingo
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