linux-kernel - Support for i386 PATs

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Message-ID: <45BB1952.8000903@tungstengraphics.com>
Date:	Sat, 27 Jan 2007 10:20:18 +0100
From:	Thomas Hellström <thomas@...gstengraphics.com>
To:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Support for i386 PATs

Hi!

Does anybody have a strong opinion against adding support for
i386 Page Attribute Tables?

The main benefit would be that one can have write-combining memory
regions without setting up MTRRs. This will come in handy for a
device we're working with where the device driver needs to allocate the
display memory directly from system memory, and it may be difficult to fit
the mtrr alignment constraints.

Outline:
The PAT may be set up at boot time with fixed backwards-compatible
memory types for the different PAT entries + defines like the following:

#define _PAGE_PAT_WB   xxx
#define _PAGE_PAT_WT   xxx
#define _PAGE_PAT_UC0 xxx
#define _PAGE_PAT_UC1 xxx
#define _PAGE_PAT_WC   xxx

which can be used in parallel with the old _PAGE_PWT and _PAGE_PCD bits.

The idea is that new memory types, WC for example, will use the pat entries
7 downto 4, whereas 0-3 are left to boot setting to maintain backwards 
compatibility.

Issues:
1) The _PAGE_BIT_PAT will be the same as _PAGE_PSE, and _PAGE_PROTNONE.
As I understand it, _PAGE_PROTNONE is not used when the page is present, 
so this might not be an issue.
What about _PAGE_PSE?

2) The PATs need to be setup for each processor just after system boot.
Where is the best place to do this?

/Thomas Hellström

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