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

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Date:	Sun, 01 Mar 2009 16:14:24 -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:
> H. Peter Anvin wrote:
>> 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. 
> 
> I'm not quite sure what you're referring to with "this is actually 
> false".  Certainly we support cachability control in ptes under Xen.  We 
> just don't support full PAT because Xen uses PAT for itself.
> 

What do you define as "full PAT"?  If what you mean is that Xen lays 
claims to the PAT MSR and only allows a certain mapping that's hardly a 
problem... other than that it's not an exhaustible resource so I guess I 
really don't understand what you're trying to say here.

	-hpa
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