linux-kernel - Re: [PATCH 12/12] xen/mtrr: Add mtrr

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Message-ID: <4CA232EF.3080906@goop.org>
Date:	Tue, 28 Sep 2010 11:24:47 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>,
	"sct@...hat.com" <sct@...hat.com>
Subject: Re: [PATCH 12/12] xen/mtrr: Add mtrr_if support for Xen mtrr

 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?

    J
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