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

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Date:	Thu, 22 Oct 2009 16:41:19 +0200
From:	Thomas Schlichter <thomas.schlichter@....de>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Jan Beulich <JBeulich@...ell.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"dri-devel@...ts.sourceforge.net" <dri-devel@...ts.sourceforge.net>,
	Robert Hancock <hancockrwd@...il.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"H. Peter Anvin" <hpa@...or.com>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Tejun Heo <tj@...nel.org>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	"x86@...nel.org" <x86@...nel.org>, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not available

Suresh Siddha wrote:
> > > There is no need to go through num_var_ranges etc.
> > 
> > Well I have to remember wich file added which MTRR entries. Because I have
> > to remove them if the file is being closed. Therefore I need an array of size
> > "num_var_ranges" (or MTRR_MAX_VAR_RANGES which is the uper bound).
> 
> No. the private data  for example can keep track of a struct containing
> mtrr number and ref count etc. Exporting var_ranges and going through
> var ranges elements in an array is not clean, especially when you are
> populating only one element.

OK, I should have written that num_var_ranges is neccessary if I do copy a
algorithm for exactly the same purpose from an other place. And I don't see
anything better in having a dynamically growing list that makes the operation
of incrementing MTRR entries an O(n) operation where it now is a O(1) operation.

Additionally, the worst case memory requirement would be
2*sizeof(int)*num_var_ranges, where it is now 1*sizeof(int)*num_var_ranges.

So for me, this would be a step back, but if you want this, you should additionally
change arch/x86/kernel/cpu/mtrr/if.c from where I reused the algorithm.

Regards,
  Thomas
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