linux-kernel - Re: [PATCH] xen: core dom0 support

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Date:	Sun, 01 Mar 2009 15:52:24 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
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.

	-hpa

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