linux-kernel - Re: Uncool feature for TTM introduced by x86, pat: Use page flags to track memtypes of RAM pages

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Message-ID: <4B7D7AF9.9060600@zytor.com>
Date:	Thu, 18 Feb 2010 09:38:01 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Jerome Glisse <glisse@...edesktop.org>, suresh.b.siddha@...el.com,
	venkatesh.pallipadi@...el.com, thellstrom@...are.com,
	airlied@...ux.ie, currojerez@...eup.net,
	linux-kernel@...r.kernel.org
Subject: Re: Uncool feature for TTM introduced by x86, pat: Use page flags
 to track memtypes of RAM pages

On 02/18/2010 09:27 AM, Andi Kleen wrote:
> Jerome Glisse <glisse@...edesktop.org> 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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