linux-kernel - Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT

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Date:	Mon, 11 May 2009 13:10:58 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Keir Fraser <keir.fraser@...citrix.com>,
	Ian Campbell <Ian.Campbell@...citrix.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Linux@...or.com" <Linux@...or.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jan Beulich <jbeulich@...ell.com>, Lu <yinghai@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT


* H. Peter Anvin <hpa@...or.com> wrote:

> Alan Cox wrote:
> > 
> > There are quite a few different CPUs with PAT errata. I've no idea why
> > there are so many errata about that specific bit.
> > 
> 
> I suspect it is simply because:
> 
> a) it moves around between the various page table formats;
> b) the default configuration and existing OSes don't use it;
> c) it isn't even *present* in some formats.

I also suspect those bits is also on the TLB miss critical path so 
there's probably quite a bit of extra complexity around it on the 
hardware side to keep the latency down.

	Ingo
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