Re: Uncool feature for TTM introduced by x86, pat: Use page flagsto track memtypes of RAM pages

[H. Peter Anvin]

On 02/18/2010 09:27 AM, Andi Kleen wrote:
> Jerome Glisse <glisse@xxxxxxxxxxxxxxx> writes:
>>
>> Can we modify the interface to support again changing from uc to wc
>> or wc to uc ? (i can try to do a patch for that).
> 
> At least on Intel CPUs that support self-snoop (all modern
> ones) that should really be very cheap.
> 

The UC/WC transition should be particularly trivial; I don't see any
reason it should have to go through any other procedure on *any* CPU --
selfsnoop shouldn't even figure into it, since neither UC nor WC
actually caches anything.  For the WC->UC direction, all we should need
to do is to flush the write combiners; a simple wmb() will do that.

Self-snoop is about not having to flush something that can have been
cached, as is normally needed to do WB/WT/WP -> UC/WC.

	-hpa

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