linux-kernel - Re: [PATCH] x86: adjust default caching mode translation tables

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Date:	Wed, 22 Jul 2015 09:17:27 -0600
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Toshi Kani" <toshi.kani@...com>
Cc:	<bp@...en.de>, <mingo@...e.hu>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86: adjust default caching mode translation
 tables

>>> On 22.07.15 at 00:29, <toshi.kani@...com> wrote:
> On Mon, 2015-07-20 at 08:46 +0100, Jan Beulich wrote:
>> Make WT really mean WT (rather than UC).
>> 
>> I can't see why commit 9cd25aac1f ("x86/mm/pat: Emulate PAT when it is
>> disabled") didn't make this match its changes to pat_init().
> 
> No, the default values need to be set to the fallback types, i.e. minimal
> supported mode.  For WC and WT, UC is the fallback type.

But why would that be?

> When PAT is disabled, pat_init() does update the tables below to enable WT 
> per the default BIOS setup.  However, when PAT is enabled, but CPU has PAT
> -errata, WT falls back to UC per the default values. 

PAT related errata I'm aware of are related to either page size or
the number of bits used to index into the PAT MSR, but never to
a particular memory type. Are you saying there are errata which
make use of WT or WC impossible altogether? Otherwise I would
have thought (even more so in the absence of any comment
saying otherwise - "minimal supported modes" doesn't really say
on what basis the set is the minimal one) that the mode systems
come up in (compatible with pre-PAT) ought to be what the tables
express.

Jan

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