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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