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

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Date:	Thu, 18 Feb 2010 21:27:54 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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 Thu, Feb 18, 2010 at 09:38:01AM -0800, H. Peter Anvin wrote:
> 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.

I'm not sure that is sanctioned by the SDM rules; AFAIK they don't
make any special exception for this case.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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