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

[Arjan van de Ven <arjan@linux.intel.com>]

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...