linux-kernel - Re: [RFC 02/23] ARM: LPAE: use signed arithmetic for mask definitions

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Date:	Tue, 24 Jul 2012 11:05:11 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Cyril Chemparathy <cyril@...com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nico@...aro.org" <nico@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	Vitaly Andrianov <vitalya@...com>
Subject: Re: [RFC 02/23] ARM: LPAE: use signed arithmetic for mask
 definitions

On Tue, Jul 24, 2012 at 02:09:04AM +0100, Cyril Chemparathy wrote:
> This patch applies to PAGE_MASK, PMD_MASK, and PGDIR_MASK, where forcing
> unsigned long math truncates the mask at the 32-bits.  This clearly does bad
> things on PAE systems.
> 
> This patch fixes this problem by defining these masks as signed quantities.
> We then rely on sign extension to do the right thing.
> 
> Signed-off-by: Cyril Chemparathy <cyril@...com>
> Signed-off-by: Vitaly Andrianov <vitalya@...com>
> ---
>  arch/arm/include/asm/page.h           |    7 ++++++-
>  arch/arm/include/asm/pgtable-3level.h |    6 +++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/include/asm/page.h b/arch/arm/include/asm/page.h
> index ecf9019..1c810d2 100644
> --- a/arch/arm/include/asm/page.h
> +++ b/arch/arm/include/asm/page.h
> @@ -13,7 +13,12 @@
>  /* PAGE_SHIFT determines the page size */
>  #define PAGE_SHIFT		12
>  #define PAGE_SIZE		(_AC(1,UL) << PAGE_SHIFT)
> -#define PAGE_MASK		(~(PAGE_SIZE-1))
> +
> +/*
> + * We do not use PAGE_SIZE in the following because we rely on sign
> + * extension to appropriately extend upper bits for PAE systems
> + */
> +#define PAGE_MASK		(~((1 << PAGE_SHIFT) - 1))

Would it work if we use a 1ULL here and avoid the sign trick? I'm
worried about some context where this would lose the sign. We could have
an #ifdef here while the PGDIR/PMD masks are in a separate file already.

-- 
Catalin
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