linux-kernel - Re: [PATCH] x86: create array based interface to change page attribute

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Date:	Mon, 31 Mar 2008 09:08:02 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Thomas Hellström <thomas@...gstengraphics.com>
CC:	Andi Kleen <andi@...stfloor.org>, Dave Airlie <airlied@...hat.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH] x86: create array based interface to change page attribute

Thomas Hellström 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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