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Message-ID: <1410383484.28990.303.camel@misato.fc.hp.com>
Subject: Re: [PATCH v2 2/6] x86, mm, pat: Change reserve_memtype() to handle
 WT
From: Toshi Kani <toshi.kani@hp.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
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Date: Wed, 10 Sep 2014 15:11:24 -0600
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On Wed, 2014-09-10 at 14:06 -0700, Andy Lutomirski wrote:
> On Wed, Sep 10, 2014 at 1:30 PM, Toshi Kani <toshi.kani@hp.com> wrote:
> > On Wed, 2014-09-10 at 13:14 -0700, H. Peter Anvin wrote:
> >> On 09/10/2014 12:30 PM, Toshi Kani wrote:
> >> >
> >> > When WT is unavailable due to the PAT errata, it does not fail but gets
> >> > redirected to UC-.  Similarly, when PAT is disabled, WT gets redirected
> >> > to UC- as well.
> >> >
> >>
> >> But on pre-PAT hardware you can still do WT.
> >
> > Yes, if we manipulates the bits directly, but such code is no longer
> > allowed for PAT systems.  The PAT-based kernel interfaces won't work for
> > pre-PAT systems, and therefore requests are redirected to UC- on such
> > systems.
> >
> 
> Right, the PWT bit.  Forgot about that.
> 
> I wonder whether it would make sense to do some followup patches to
> replace the current support for non-PAT machines with a "PAT" and
> corresponding reverse map that exactly matches the mapping when PAT is
> disabled.  These patches are almost there.

That's possible, but the only benefit is that we can enable WT on
pre-PAT systems, which I do not think anyone cares now...  WB & UC work
on pre-PAT systems.  WC & WT need PAT.  I think this requirement is
reasonable.

Thanks,
-Toshi