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

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Message-Id: <55AFD5CB0200007800094280@prv-mh.provo.novell.com>
Date:	Wed, 22 Jul 2015 09:41:31 -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 17:23, <toshi.kani@...com> wrote:
> On Wed, 2015-07-22 at 09:17 -0600, Jan Beulich wrote:
>> > 
>> > > > 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.
> 
> Please take a look at the comments in pat_init().  WT uses slot 7 (not slot
> 1) in the regular case.

But that is an adjustment Linux makes to the default the system
comes up in. And again - in my opinion the pre-initialized table
values should reflect the mode the system comes up in (i.e.
correct prior to execution reaching pat_init()), and be updated
(which as it seems happens in all three possible cases) once the
MSR gets fiddled with.

Jan

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