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

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Message-ID: <4CA23C6F.1040807@zytor.com>
Date:	Tue, 28 Sep 2010 12:05:19 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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 11:58 AM, Jeremy Fitzhardinge wrote:
>  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.
> 

Yeah, that should all be fixed now.

	-hpa
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