linux-kernel - Re: [PATCH UPDATED] x86: set PAE PHYSICAL_MASK

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Date:	Fri, 06 Jun 2008 15:15:37 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Jan Beulich <jbeulich@...ell.com>,
	Stable Kernel <stable@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH UPDATED] x86: set PAE PHYSICAL_MASK_SHIFT to 44 bits.

Jeremy Fitzhardinge <jeremy@...p.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
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