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

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Message-ID: <500E7E5E.1030005@ti.com>
Date:	Tue, 24 Jul 2012 06:52:14 -0400
From:	Cyril Chemparathy <cyril@...com>
To:	Catalin Marinas <catalin.marinas@....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 7/24/2012 6:05 AM, Catalin Marinas wrote:
> 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.
>

Sure, I will modify accordingly in the next version.

-- 
Thanks
- Cyril
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