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

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Date:	Fri, 18 Jan 2008 23:58:19 -0800
From:	Yinghai Lu <Yinghai.Lu@....COM>
To:	Dave Jones <davej@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, venkatesh.pallipadi@...el.com,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] X86: fix typo PAT to X86_PAT

On Friday 18 January 2008 07:28:49 pm Dave Jones wrote:
> On Fri, Jan 18, 2008 at 10:02:10PM +0100, Ingo Molnar wrote:
>  > 
>  > * 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?
> 
> 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.

goog idea...

we need to make X86_PAT depend on MTRR in arch/x86/Kconfig

YH
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