Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit
From: Jeremy Fitzhardinge
Date: Fri Jun 06 2008 - 04:09:27 EST
Arjan van de Ven wrote:
On Thu, 05 Jun 2008 16:21:14 +0100
Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
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").
the problem on 32 bit is that if you have that much ram, you run out of
lowmem FAST.... so you have bigger problems.
Sure, you'd have to be barking mad to give a 32-bit system 2^40 bytes of
RAM. But under Xen the host's physical addresses are used in guest
pagetables, so you could have a reasonably sized 32-bit PAE Xen guest be
exposed to huge host physical addresses.
But the basic point is that, given that Enhanced Legacy PAE Paging
exists, 36-bits is not correct, so we should fix it. And if the
platform allows addressable hardware to be physically discontigious -
either memory or devices - then you may end up using large numbers of
physical bits without having a stupid amount of memory actually present.
J
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