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[Xen-devel] Re: [PATCH 12/12] xen/mtrr: Add mtrr_if support for Xen mtrr

To: "H. Peter Anvin" <hpa@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 12/12] xen/mtrr: Add mtrr_if support for Xen mtrr
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 28 Sep 2010 11:24:47 -0700
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, "sct@xxxxxxxxxx" <sct@xxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Jeremy Fitzhardinge <Jeremy.Fitzhardinge@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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 On 09/28/2010 11:19 AM, H. Peter Anvin wrote:
> On 09/28/2010 11:13 AM, Jeremy Fitzhardinge wrote:
>>  On 09/28/2010 10:56 AM, H. Peter Anvin wrote:
>>> On 09/28/2010 10:13 AM, Jeremy Fitzhardinge wrote:
>>>> Yes, we could just mask out the MTRR CPU feature and rely entirely on PAT.
>>>> The alternative would be to use the wrmsr hooks to emulate the Intel
>>>> MTRR registers by mapping them to hypercalls, but that seems needlessly
>>>> complex.
>>> Indeed.  Relying on pure PAT is the Right Thing[TM].
>> Is there a plan to formally deprecate /proc/mtrr and the kernel
>> infrastructure behind it?
> No, and we really can't do it for a couple of reasons:
> a) Pre-PAT hardware;
> b) MTRRs and PAT interact on hardware;
> c) MTRRs, but not PAT, interact with SMM.

What about pre-PAT software (ie, X servers which still use /proc/mtrr)?

> However, since a virtual machine like Xen doesn't have these issues, it
> doesn't apply

Well, we're specifically talking about a virtual machine which has
direct access to hardware, so it is concerned about the real physical
memory properties of real physical pages.  If we can assume that
BIOS/Xen will always set up MTRR correctly then there shouldn't be any
need for the kernel to modify the MTRR itself.  How true is that in
general?  I don't know, but if we could rely on BIOS then there'd never
be a need to touch MTRR, would there?


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