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

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Message-ID: <4CA23ABD.1010208@goop.org>
Date:	Tue, 28 Sep 2010 11:58:05 -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 11:46 AM, H. Peter Anvin wrote:
> 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?
> Well, in the past MTRRs were abused for device properties mainly by the
> X server, but other than that, no, not really.  The other thing we do is
> the MTRR cleanup (which doesn't involve /proc/mtrr) to deal with
> brokenness in the BIOS setup, but that really belongs in the hypervisor
> in your case since it fundamentally affects how memory is handled.

Yeah, the hypervisor should definitely deal with that.  I have no
problem in principle with leaving MTRRs entirely to Xen, but I was just
concerned about possible repercussions.  Certainly when I first did this
work, I was using Fedora 8 whose X server did depend on /proc/mtrr for
good performance.

    J
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