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

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Date:	Mon, 19 Oct 2009 17:07:40 +0200
From:	Thomas Schlichter <thomas.schlichter@....de>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Jan Beulich <JBeulich@...ell.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	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,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Tejun Heo <tj@...nel.org>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	x86@...nel.org, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not available

Konrad Rzeszutek Wilk wrote:
> I am not familiar with libpciaccess, but I was wondering why that library
> cannot realize it failed in its endeavour and use other means to accomplish
> its goals?

The thing is that the mmap() will succeed! Only the memory region will not be
set up as write combining when PAT is disabled. So userspace would have to
find out it PAT is enabled in kernel and accordingly set up MTRR entries. And
currently I don't know of a kernel interface that would tell userspace if PAT is
enabled.

Kind regards,
  Thomas
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