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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 11:58:05 -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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References: <alpine.DEB.2.00.1009281217540.2864@kaball-desktop> <1285676218-26218-12-git-send-email-stefano.stabellini@xxxxxxxxxxxxx> <20100928123925.GA18208@xxxxxxx> <alpine.DEB.2.00.1009281455300.2864@kaball-desktop> <4CA2223B.9040400@xxxxxxxx> <4CA22C3C.3020209@xxxxxxxxx> <4CA2302C.3000201@xxxxxxxx> <4CA231A8.5000100@xxxxxxxxx> <4CA232EF.3080906@xxxxxxxx> <4CA23800.4020409@xxxxxxxxx>
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User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100907 Fedora/3.1.3-1.fc13 Lightning/1.0b3pre Thunderbird/3.1.3
 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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