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

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Date:	Wed, 10 Sep 2014 12:14:30 -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 11:26 AM, Andy Lutomirski wrote:
> On Wed, Sep 10, 2014 at 9:51 AM, Toshi Kani <toshi.kani@...com> wrote:
>> This patch changes reserve_memtype() to handle the WT cache mode.
>> When PAT is not enabled, it continues to set UC- to *new_type for
>> any non-WB request.
>>
>> When a target range is RAM, reserve_ram_pages_type() fails for WT
>> for now.  This function may not reserve a RAM range for WT since
>> reserve_ram_pages_type() uses the page flags limited to three memory
>> types, WB, WC and UC.
> 
> Should it fail if WT is unavailable due to errata?  More generally,
> how are all of the do_something_wc / do_something_wt /
> do_something_nocache helpers supposed to handle unsupported types?
> 

Errata, or because it is pre-PAT hardware.  Keep in mind that even
pre-PAT hardware supports using page tables for cache types, it is only
that the only types supposed are WB, WT, UC.

	-hpa


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