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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>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Juergen Gross <jgross@suse.com>,
        Stefan Bader <stefan.bader@canonical.com>,
        Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
        Yigal Korman <yigal@plexistor.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Date: Wed, 10 Sep 2014 13:30:28 -0600
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On Wed, 2014-09-10 at 11:26 -0700, Andy Lutomirski wrote:
> On Wed, Sep 10, 2014 at 9:51 AM, Toshi Kani <toshi.kani@hp.com> wrote:
> > This patch changes reserve_memtype() to handle the WT cache mode.
> > When PAT is not enabled, it continues to set UC- to *new_type for
> > any non-WB request.
> >
> > When a target range is RAM, reserve_ram_pages_type() fails for WT
> > for now.  This function may not reserve a RAM range for WT since
> > reserve_ram_pages_type() uses the page flags limited to three memory
> > types, WB, WC and UC.
> 
> Should it fail if WT is unavailable due to errata?  More generally,
> how are all of the do_something_wc / do_something_wt /
> do_something_nocache helpers supposed to handle unsupported types?

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.

The failure case above is a run-time error when WT is enabled and is
targeted to RAM.  In this case, reserve_memtype() fails and sets UC- to
*new_type due to the limitation in page tables.  set_memory_xzy()
interfaces do not retry with new_type, but return an error.  I think
this makes sense since the caller should receive this error as this case
is a bug in the code (while running it on an old system is not a bug).

Thanks,
-Toshi