Re: [PATCH] xen: core dom0 support
From: H. Peter Anvin
Date: Sun Mar 01 2009 - 18:56:02 EST
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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