linux-kernel - Re: [PATCH v2 2/6] x86, mm, pat: Change reserve

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Message-ID: <5410C4F7.8050403@zytor.com>
Date:	Wed, 10 Sep 2014 14:39:03 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Toshi Kani <toshi.kani@...com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Juergen Gross <jgross@...e.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Yigal Korman <yigal@...xistor.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v2 2/6] x86, mm, pat: Change reserve_memtype() to handle
 WT

On 09/10/2014 02:27 PM, Andy Lutomirski wrote:
>>
>> That's possible, but the only benefit is that we can enable WT on
>> pre-PAT systems, which I do not think anyone cares now...  WB & UC work
>> on pre-PAT systems.  WC & WT need PAT.  I think this requirement is
>> reasonable.
> 
> It might end up being a cleanup, though.  A whole bunch of
> rarely-exercised if (!pat_enabled) things would go away.
> 

Yes.  Don't think of it as PAT vs non-PAT.  Think of it as a specific
set of cache types available on different processors.  The fact that you
may have to frob an MSR to initialize it is almost trivial in comparison.

	-hpa


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