Message-ID: <47F10C62.7040500@linux.intel.com>
Date: Mon, 31 Mar 2008 09:08:02 -0700
From: Arjan van de Ven <arjan@linux.intel.com>
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To: =?ISO-8859-1?Q?Thomas_Hellstr=F6m?= <thomas@tungstengraphics.com>
CC: Andi Kleen <andi@firstfloor.org>, Dave Airlie <airlied@redhat.com>,
       linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com
Subject: Re: [PATCH] x86: create array based interface to change page attribute
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Thomas Hellstr?m wrote:

> Let me rehprase. Not really time-critical but it is of some importance 
> that CPA is done quickly.
> We're dealing with the tradeoff of reading from uncached device memory 

uncached or write combining ?

> vs taking the pages out of
> AGP, setting up a cache-coherent mapping, read and then change back. 
> What we'd really would like to set up is a pool of completely unmapped 
> (like highmem) pages. Then we could, to a large extent, avoid the CPA 
> calls.

changing attributes by nature means a tlb flush and a bunch of expensive cache work.
That's never going to be cheap, I guess it all depends on how much work you do
on the memory for it to pay off or not...
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