Re: [PATCH UPDATED] x86: set PAE PHYSICAL_MASK_SHIFT to 44 bits.

Andi Kleen
2008-06-06T13:16:07Z

Jeremy Fitzhardinge <jeremy@goop.org> writes:

> When a 64-bit x86 processor runs in 32-bit PAE mode, a pte can
> potentially have the same number of physical address bits as the
> 64-bit host ("Enhanced Legacy PAE Paging"). This means, in theory,
> we could have up to 52 bits of physical address in a pte.
>
> The 32-bit kernel uses a 32-bit unsigned long to represent a pfn.
> This means that it can only represent physical addresses up to 32+12=44
> bits wide. Rather than widening pfns everywhere, just set 2^44 as the
> Linux x86_32-PAE architectural limit for physical address size.

43bits might be actally safer because of potential sign bugs.

But of course it won't work anyways likely.

-Andi