linux-kernel - Re: Why preallocate pmd in x86 32-bit PAE?

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Message-ID: <473CCB66.2080601@zytor.com>
Date:	Thu, 15 Nov 2007 14:42:46 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	William Lee Irwin III <wli@...omorphy.com>,
	Andi Kleen <ak@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Why preallocate pmd in x86 32-bit PAE?

Linus Torvalds wrote:
> 
> IIRC, the present bit is ignored in the magic 4-entry PGD.  All entries 
> have to be present.
> 

This is true, although you could point a PGD to an all-zero page if you 
really wanted to.  You have to re-load CR3 after modifying the top-level 
entries.

> What earlier CPU's did was to basically load all four values into the CPU 
> when you loaded %cr3. There was no "three-level page table walker" at all: 
> it was still a two-level page table walker, there were just for magic 
> internal page tables that were indexed off the two high bits.

They still are.  Loading CR3 in PAE really loads four registers from 
memory.  x86-64 is different, of course.

	-hpa
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