linux-kernel - Re: WT memory type on x86

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Message-ID: <518AD65E.1080509@zytor.com>
Date:	Wed, 08 May 2013 15:49:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Stefan Bader <stefan.bader@...onical.com>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: WT memory type on x86_64?

On 05/08/2013 09:37 AM, Andy Lutomirski wrote:
>>
>> Not sure whether this is the reason but UC and UC- (as well as WB) are kept like
>> the backward compatible setup. So for systems with and without PAT cache types
>> map to the same. Only WT gets replaced by WC. For that whenever PCD is set it
>> means some uncached type.
> 
> This also makes the pgprot_xyz macros faster -- they can do the same
> thing regardless of PAT settings.
> 

It also deals with errata in some very old CPUs... old enough that we
don't use PAT on them anyway AFAIK.

	-hpa


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