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

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Message-ID: <1437583681.3214.270.camel@hp.com>
Date:	Wed, 22 Jul 2015 10:48:01 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Jan Beulich <JBeulich@...e.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 Wed, 2015-07-22 at 10:36 -0600, Jan Beulich wrote:
> > 
> > > > On 22.07.15 at 18:06, <toshi.kani@...com> wrote:
> > On Wed, 2015-07-22 at 09:41 -0600, Jan Beulich wrote:
> > > > 
> > > > > > 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.
> > 
> > 'enum page_cache_mode' thru __cachemode2pte_tbl[] is a high-level
> > abstraction that can be used after pat_init().  For early boot-time,
> > __early_ioremap() takes pgprot_t directly.
> > 
> > Do you have a need to use __cachemode2pte_tbl[] before pat_init()?
> 
> No. I just noticed the (apparent?) inconsistency.

I will update the comment of __cachemode2pte_tbl[] to avoid such confusion.

Thanks,
-Toshi
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