linux-kernel - Re: [PATCH 1/5] x86, mm, pat: Set WT to PA4 slot of PAT MSR

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Date:	Thu, 04 Sep 2014 17:27:35 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	hpa@...or.com, tglx@...utronix.de, mingo@...hat.com,
	akpm@...uxfoundation.org, arnd@...db.de, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, jgross@...e.com,
	stefan.bader@...onical.com, luto@...capital.net,
	konrad.wilk@...cle.com
Subject: Re: [PATCH 1/5] x86, mm, pat: Set WT to PA4 slot of PAT MSR

On Thu, 2014-09-04 at 14:31 -0600, Toshi Kani wrote:
> On Thu, 2014-09-04 at 17:11 -0300, Henrique de Moraes Holschuh wrote:
> > On Thu, 04 Sep 2014, Toshi Kani wrote:
> > > This patch sets WT to the PA4 slot in the PAT MSR when the processor
> > > is not affected by the PAT errata.  The upper 4 slots of the PAT MSR
> > > are continued to be unused on the following Intel processors.
> > > 
> > >   errata           cpuid
> > >   --------------------------------------
> > >   Pentium 2, A52   family 0x6, model 0x5
> > >   Pentium 3, E27   family 0x6, model 0x7
> > >   Pentium M, Y26   family 0x6, model 0x9
> > >   Pentium 4, N46   family 0xf, model 0x0
> > > 
> > > For these affected processors, _PAGE_CACHE_MODE_WT is redirected to UC-
> > > per the default setup in __cachemode2pte_tbl[].
> > 
> > There are at least two PAT errata.  The blacklist is in
> > arch/x86/kernel/cpu/intel.c:
> > 
> >         if (c->x86 == 6 && c->x86_model < 15)
> >                 clear_cpu_cap(c, X86_FEATURE_PAT);
> > 
> > It covers model 13, which is not in your blacklist.
> > 
> > It *is* possible that PAT would work on model 13, as I don't think it has
> > any PAT errata listed and it was blacklisted "just in case" (from memory. I
> > did not re-check), but this is untested, and unwise to enable on an aging
> > platform.
> > 
> > I am worried of uncharted territory, here.  I'd actually advocate for not
> > enabling the upper four PAT entries on IA-32 at all, unless Windows 9X / XP
> > is using them as well.  Is this a real concern, or am I being overly
> > cautious?
> 
> The blacklist you pointed out covers a different PAT errata, and is
> still effective after this change.  pat_init() will call pat_disable()
> and the PAT will continue to be disabled on these processors.  There is
> no change for them.
> 
> My blacklist covers the PAT errata that makes the upper four bit
> unusable when the PAT is enabled.

I checked more carefully, and it turns out that the processors that have
the WC bug with PAT/MTRR also have the upper four bit bug in PAT as
well.  The updated blacklist is:

   errata               cpuid
   --------------------------------------
   Pentium 2, A52       family 0x6, model 0x5
   Pentium 3, E27       family 0x6, model 0x7, 0x8
   Pentium 3 Xeon, G26  family 0x6, model 0x7, 0x8, 0xa
   Pentium M, Y26       family 0x6, model 0x9
   Pentium M 90nm, X9   family 0x6, model 0xd
   Pentium 4, N46       family 0xf, model 0x0
                
So, the check can be the same as cpu/intel.c, except that early Pentium
4 steppings also have the upper four bit bug.  I will update the check.
In any case, this check is only meaningful for P4 since the PAT is
disabled for P2/3/M. 

Thanks Henrique for pointing this out!
-Toshi

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