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

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Message-Id: <1256225689.32075.6.camel@gaiman.anholt.net>
Date:	Thu, 22 Oct 2009 08:34:49 -0700
From:	Eric Anholt <eric@...olt.net>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Thomas Schlichter <thomas.schlichter@....de>,
	Ingo Molnar <mingo@...e.hu>, Jan Beulich <JBeulich@...ell.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Robert Hancock <hancockrwd@...il.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Tejun Heo <tj@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"dri-devel@...ts.sourceforge.net" <dri-devel@...ts.sourceforge.net>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	Thomas Hellstrom <thellstrom@...are.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not
 available

On Thu, 2009-10-22 at 02:53 -0700, Suresh Siddha wrote:
> On Wed, 2009-10-21 at 13:01 -0700, Thomas Schlichter wrote:
> > OK, so I think the attached patches should do it. Hopefully I have addressed 
> > all your comments.
> 
> Thomas,
> 
> I have couple of issues with this patchset still. pci_mmap_page_range()
> doesn't get called for each fork(). So, we won't be ref counting the
> mtrr usage properly.
> 
> I need to think a bit more carefully on this. Can I get back to you
> early next week on this, as I am traveling and need to think through
> this?
> 
> We already keep track of some of the PAT ref counting using
> track_pfn_vma_copy(). And we need to extend/use something similar here.
> 
> Even if we need to extend sysfs or pci vma ops, we need to increment and
> decrement the ref count of the mtrr register that gets used. There is no
> need to go through num_var_ranges etc.

Can we just not create the _wc sysfs entry if we don't have PAT?  I
don't think there's userland relying on its presence as opposed to the
non-_wc entry.

-- 
Eric Anholt
eric@...olt.net                         eric.anholt@...el.com



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