linux-kernel - Re: PAE and PSE ??

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Message-ID: <453C00B7.3040909@zytor.com>
Date:	Sun, 22 Oct 2006 16:37:27 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
CC:	Sandeep Kumar <sandeepksinha@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: PAE and PSE ??

Rafael J. Wysocki wrote:
> Hi,
> 
> On Sunday, 22 October 2006 08:29, Sandeep Kumar wrote:
>> Hi all,
>> I have read in UTLK by bovet that the linux kernel does not uses the
>> PSE bit on an x86
>> machine. Then how come we have the hugetlbfs, which provides support
>> for 4MB pages ?
> 
> AFAIK, PSE is only used when PAE is not set and then it enables the 4 MB
> pages.  If PAE is set, the 4 MB pages are impossible because there are only
> 512 entries per page table, but 2 MB pages can be used instead (and you don't
> need to set PSE to use them).
> 

You're wrong.

PSE refers to 4 MB pages when PAE is not used, and 2 MB pages when PAE 
is used.

In theory, you could have PAE without PSE, which would only support 4K 
pages.

Linux uses PSE; it may or may not use PAE depending on the configuration.

	-hpa
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