linux-kernel - Re: [PATCH] X86: fix typo PAT to X86

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Date:	Fri, 18 Jan 2008 22:02:10 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Dave Jones <davej@...hat.com>, Yinghai Lu <Yinghai.Lu@....COM>,
	venkatesh.pallipadi@...el.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] X86: fix typo PAT to X86_PAT


* Dave Jones <davej@...hat.com> 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?

	Ingo
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