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Date: Wed, 10 Sep 2014 14:39:03 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andy Lutomirski <luto@amacapital.net>, Toshi Kani <toshi.kani@hp.com>
CC: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Juergen Gross <jgross@suse.com>,
        Stefan Bader <stefan.bader@canonical.com>,
        Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
        Yigal Korman <yigal@plexistor.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: [PATCH v2 2/6] x86, mm, pat: Change reserve_memtype() to handle
 WT
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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