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Message-ID: <49AB1FB8.607@zytor.com>
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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