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

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Date:	Wed, 07 May 2008 19:28:57 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Rene Herman <rene.herman@...access.nl>,
	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

Linus Torvalds wrote:
> 
>> *Certainly* I don't want anything like this crap:
>>
>>> -    if (cpu_has_pat)
>>> +    if (cpu_has_pat && cpu_has_pat_good)
> 
> This in fact is likely the best part of it.
> 
> Because that at least guarantees that we never say we have a good PAT when 
> the hardware doesn't even report it.
> 
> As it is, we seem to just blindly override hardware. It may be correct for 
> all the models we override, but still..
> 

Yah, this is not good.  We should mask out the bit, but never, ever, set 
it if it was clear to begin with (unless we have it on really, *really*, 
good authority.)

I'm embarrassed to have let that slink by.

	-hpa
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