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Date: Mon, 11 May 2009 13:10:58 +0200
From: Ingo Molnar <mingo@elte.hu>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Keir Fraser <keir.fraser@eu.citrix.com>,
       Ian Campbell <Ian.Campbell@eu.citrix.com>,
       Jeremy Fitzhardinge <jeremy@goop.org>,
       the arch/x86 maintainers <x86@kernel.org>,
       Suresh Siddha <suresh.b.siddha@intel.com>,
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       Thomas Gleixner <tglx@linutronix.de>,
       Arjan van de Ven <arjan@infradead.org>
Subject: Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT
Message-ID: <20090511111058.GJ4648@elte.hu>
References: <4A05CD29.6050505@zytor.com> <C62C5B3C.AB1A%keir.fraser@eu.citrix.com> <20090510142928.66bb57bb@lxorguk.ukuu.org.uk> <4A071518.8000307@zytor.com>
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* H. Peter Anvin <hpa@zytor.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