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

To: "H. Peter Anvin" <hpa@xxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT
From: Ingo Molnar <mingo@xxxxxxx>
Date: Mon, 11 May 2009 13:10:58 +0200
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Jeremy Fitzhardinge <Jeremy.Fitzhardinge@xxxxxxxxxx>, Suresh Siddha <suresh.b.siddha@xxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Pallipadi, Venkatesh" <venkatesh.pallipadi@xxxxxxxxx>, "Linux@xxxxxxxxx" <Linux@xxxxxxxxx>, Lu <yinghai@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>, Arjan van de Ven <arjan@xxxxxxxxxxxxx>
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* H. Peter Anvin <hpa@xxxxxxxxx> 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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