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

[Cyril Chemparathy]

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@xxxxxx>
> > Signed-off-by: Vitaly Andrianov <vitalya@xxxxxx>
> > ---
> >   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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