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Date:	Sun, 01 Mar 2009 15:52:24 -0800
From:	"H. Peter Anvin" <>
To:	Jeremy Fitzhardinge <>
CC:	Andi Kleen <>,
	Andrew Morton <>,
	Xen-devel <>,
	the arch/x86 maintainers <>,
	Linux Kernel Mailing List <>
Subject: Re: [PATCH] xen: core dom0 support

Jeremy Fitzhardinge wrote:
> Unless we're planning on dropping support for processes with no or 
> broken PAT support, we're always going to have to deal with the non-PAT 
> case.  Xen just falls into the "processor with no PAT" case.  And 
> if/when we work out how to paravirtualize PAT, it will no longer be in 
> that case.

In this particular case, this is actually false.  "No PAT" in the 
processor is *not* the same thing as "no cacheability controls in the 
page tables".  Every processor since the 386 has had UC, WT, and WB 
controls in the page tables; PAT only added the ability to do WC (and 
WP, which we don't use).  Since the number of processors which can do WC 
at all but don't have PAT is a small set of increasingly obsolete 
processors, we may very well choose to simply ignore the WC capabilities 
of these particular processors.


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