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

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Date:	Tue, 28 Sep 2010 18:19:37 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"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 Tue, 28 Sep 2010, Jeremy Fitzhardinge wrote:
>  On 09/28/2010 07:00 AM, Stefano Stabellini wrote:
> > On Tue, 28 Sep 2010, Ingo Molnar wrote:
> >> * stefano.stabellini@...citrix.com <stefano.stabellini@...citrix.com> wrote:
> >>
> >>> From: Stephen Tweedie <sct@...hat.com>
> >>>
> >>> Add a Xen mtrr type, and reorganise mtrr initialisation slightly to
> >>> allow the mtrr driver to set up num_var_ranges (Xen needs to do this by
> >>> querying the hypervisor itself.)
> >>>
> >>> [ Impact: add basic MTRR support ]
> >>>
> >>> Signed-off-by: Stephen Tweedie <sct@...hat.com>
> >>> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> >>> Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> >>> ---
> >>>  arch/x86/kernel/cpu/mtrr/Makefile |    2 +-
> >>>  arch/x86/kernel/cpu/mtrr/main.c   |    3 +
> >>>  arch/x86/kernel/cpu/mtrr/mtrr.h   |    7 ++
> >>>  arch/x86/kernel/cpu/mtrr/xen.c    |  110 +++++++++++++++++++++++++++++++++++++
> >>>  4 files changed, 121 insertions(+), 1 deletions(-)
> >>>  create mode 100644 arch/x86/kernel/cpu/mtrr/xen.c
> >> Still NAK, for the very same reasons as we NAK-ed it the previous time: 
> >> /proc/mtrr is a problematic and complicated legacy interface that should 
> >> die. Any modern X server will do the right thing via PAT.
> >>
> > Sorry I should have read the original thread more carefully: I didn't
> > realize this patch had been NAK-ed.
> >
> > However it is not a problem because we can easily disable MTRRs from Xen
> > and with no cpu_has_mtrr the kernel would still boot fine on Xen.
> > Also I think we do have PAT support nowadays but I'll let Jeremy comment
> > on that.
> 
> 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.
 
Yeah, I have already tested a prototype patch to xen to mask the MTRR
cpu features and everything seems to run fine on my testbox.

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