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

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Date:	Mon, 03 Mar 2008 14:56:17 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Segher Boessenkool <segher@...nel.crashing.org>
CC:	hans.rosenfeld@....com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"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 wrote:
>>> so (on 32-bit) ~_PAGE_PSE is ~0x80L is 0xffffff7f, which when cast to
>>> 64-bit is 0x00000000ffffff7f,
>>
>> Actually, it is signed, so this isn't true.  Comments about unsafeness
>> still apply.
>
> It turns out that PAGE_SIZE is unsigned.  So this gives us for
>
>     (~PAGE_MASK & ~_PAGE_USER & ~_PAGE_PSE & ~_PAGE_NX)
>
> the types UL, L, L, ULL resp.
>
> The associativity of & is left-to-right, so this in turn becomes
>
>     UL, L, ULL
>
>     UL, ULL
>
>     ULL
>
> and that cast from UL to ULL doesn't sign-extend. 

I went through and made a bunch of these flags signed so that they would 
sign-extend properly in 32 vs 64 bit.  We could make them 
unconditionally 64-bit, but I'm worried that will have unforeseen 
consequences too.

In this case, PAGE_MASK should probably be signed too, so the sign 
extension happens properly.

Alternatively, they could be cast to pteval_t and/or phys_addr_t so that 
they will have an inherent size which matches the pagetable format 
(using _AT() rather than _AC()).

    J
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