linux-kernel - Re: [RFC Patch] use MTRR for write combining if PAT is not available

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Message-Id: <1255988967.2576.36.camel@sbs-t61>
Date:	Mon, 19 Oct 2009 14:49:27 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Schlichter <thomas.schlichter@....de>,
	Jan Beulich <JBeulich@...ell.com>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"dri-devel@...ts.sourceforge.net" <dri-devel@...ts.sourceforge.net>,
	"hancockrwd@...il.com" <hancockrwd@...il.com>,
	"hmh@....eng.br" <hmh@....eng.br>, "hpa@...or.com" <hpa@...or.com>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	"jeremy.fitzhardinge@...rix.com" <jeremy.fitzhardinge@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"thellstrom@...are.com" <thellstrom@...are.com>,
	"tj@...nel.org" <tj@...nel.org>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"yinghai@...nel.org" <yinghai@...nel.org>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not
	available

On Mon, 2009-10-19 at 08:31 -0700, Ingo Molnar wrote:
> * Thomas Schlichter <thomas.schlichter@....de> wrote:
> 
> > I don't think this is a good idea, Robert Hancock wrote there may be 
> > millions of such Laptops (Core Solo/Duo erratum AE7, Pentium M erratum 
> > Y31) :
> >
> >   http://marc.info/?l=linux-kernel&m=125537136105246&w=2
> > 
> > > or Perhaps just try to add mtrr only for the pci mmap case like the 
> > > 4th patch in this series..
> > 
> > I'd prefer this! ;-)
> 
> Hm, we could perhaps do that - but i think we should only do that on 
> systems that have PAT disabled.
> 
> Which brings up the question of how to properly QA such a narrow segment 
> of the market. Maybe disabling CONFIG_X86_PAT should enable that logic 
> too.

Also, I think we can simplify and avoid mtrr_add_unaligned() and worry
only about the pci mmap aligned cases etc. Those are the only
interesting ones.

thanks,
suresh

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