Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit

Jeremy Fitzhardinge
2008-06-06T08:09:10Z

Arjan van de Ven wrote:
> On Thu, 05 Jun 2008 16:21:14 +0100
> Jeremy Fitzhardinge <jeremy@goop.org> 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