linux-kernel - Re: [PATCH] x86: introduce a new Linux defined feature flag for PAT support

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Date:	Thu, 08 May 2008 15:39:47 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Rene Herman <rene.herman@...access.nl>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Adrian Bunk <bunk@...nel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, Pavel Machek <pavel@...e.cz>
Subject: Re: [PATCH] x86: introduce a new Linux defined feature flag for PAT
 support

Rene Herman wrote:
> On 08-05-08 12:19, Andi Kleen wrote:
> 
>>> Use a new Linux defined X86_FEATURE_PAT_GOOD feature flag to
>>
>> Better would be PAT_TESTED or PAT_RANDOMLY_APPROVED. Most of these
>> CPUs without PAT_GOOD have actually perfectly good PAT, as Windows
>> proves every day.
>>
>> The main flaw in all of this of course is that there is no procedure
>> to test CPUs which do not have the flag set yet.
> 
> Quite. And hiding the fact that the CPU _should_ have perfectly good
> PAT doesn't help any at all. The discussion turned into a mini-flame
> war enough that now noone would even consider backing down, but this
> current PAT setup just sucks plain and simple.

For old CPUs it is actually ok (after all they worked for years without
PAT), I just don't like it for new CPUs. It's a bad idea there and
in the x86 world it is a reasonable expectation that CPU features
generally work.

That said I am not aware of that many PAT erratas even on old CPUs.
There are a few, but they are known and well understood and could
well be black listed.

-Andi

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