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Date:	Fri, 18 Jan 2008 22:28:49 -0500
From:	Dave Jones <>
To:	Ingo Molnar <>
Cc:	Yinghai Lu <Yinghai.Lu@....COM>,,
	LKML <>
Subject: Re: [PATCH] X86: fix typo PAT to X86_PAT

On Fri, Jan 18, 2008 at 10:02:10PM +0100, Ingo Molnar wrote:
 > * Dave Jones <> wrote:
 > >  > you mean modifies MTRRs? Which code is that? (besides the 
 > >  > /proc/mtrr userspace API)
 > > 
 > > This exclusion is going to be a real pain in the ass for distro 
 > > kernels. It's impossible for example to build a kernel that will now 
 > > support the MTRR-alike registers on the AMD K6/early Cyrix etc and 
 > > also support PAT.
 > > 
 > > Additionally, given people tend to update their kernels a lot more 
 > > often than they update to a whole new version of X, it means until 
 > > userspace has caught up, we can't ship a kernel with PAT supported, or 
 > > else X gets a lot slower due to the missing mtrr support.
 > there's no exclusion enforced right now, and if a CPU is PAT-incapable 
 > (or if the kernel is booted nopat) then the MTRR bits should be usable. 
 > But if we boot with PAT enabled, and Xorg gets /proc/mtrr wrong, we'll 
 > see nasty crashes. If it gets them right, it should all still work just 
 > fine. Is this ok? Then, in a year or two, distros can disable write 
 > support to /proc/mtrr. Hm?

A crazy idea just occured to me..  We could make /proc/mtrr an interface
to set PAT on a range of memory.  This would make it transparently work
without any changes in X or anything else that sets them in userspace.


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