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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT
From: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Date: Sun, 10 May 2009 14:29:28 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Jeremy Fitzhardinge <Jeremy.Fitzhardinge@xxxxxxxxxx>, Suresh Siddha <suresh.b.siddha@xxxxxxxxx>, "Pallipadi, Venkatesh" <venkatesh.pallipadi@xxxxxxxxx>, arch/x86 maintainers <x86@xxxxxxxxxx>, Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Molnar <mingo@xxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Ingo, the, Lu <yinghai@xxxxxxxxxx>, "Linux@xxxxxxxxx" <Linux@xxxxxxxxx>, Arjan van de Ven <arjan@xxxxxxxxxxxxx>
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> > There exists at least one processor erratum where the CPU will use
> > PAT[4-7] when the user requested PAT[0-3].  For those CPUs, it is unsafe
> > for *any* OS to have PAT[4-7] != PAT[0-3].
> 
> Would it be unreasonable for us to disable PAT on such processors?

That would be a regression as far as Linux is concerned. They work
perfectly well providing you stick to four PAT entries (which is enough
for any Linux system). I'm not sure how Windows handles this but that
might also matter for Xen I guess.

> have matching PAT configuration. No elfnote would mean use Xen's existing
> PAT setup (or if that's very dangerous then disable PAT altogether, perhaps
> dependent on CPU model/stepping?).

Hiding it on errata hit processors if the guest cannot support PAT
safely on such processors sounds a good policy and its one being a
hypervisor you can do neatly.

There are quite a few different CPUs with PAT errata. I've no idea why
there are so many errata about that specific bit.

Alan

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