linux-kernel - Re: [RFC PATCH v3 02/20] x86: Set the write-protect cache mode for full PAT support

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Date:   Fri, 11 Nov 2016 01:26:48 +0000
From:   "Kani, Toshimitsu" <toshi.kani@....com>
To:     "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "bp@...en8.de" <bp@...en8.de>
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Subject: Re: [RFC PATCH v3 02/20] x86: Set the write-protect cache mode for
 full PAT support

On Thu, 2016-11-10 at 14:14 +0100, Borislav Petkov wrote:
> + Toshi.
> 
> On Wed, Nov 09, 2016 at 06:34:48PM -0600, Tom Lendacky wrote:
> > 
> > For processors that support PAT, set the write-protect cache mode
> > (_PAGE_CACHE_MODE_WP) entry to the actual write-protect value
> > (x05).

Using slot 6 may be more cautious (for the same reason slot 7 was used
for WT), but I do not have a strong opinion for it.

set_page_memtype() cannot track the use of WP type since there is no
extra-bit available for WP, but WP is only supported by
early_memremap_xx() interfaces in this series.  So, I think we should
just document that WP is only intended for temporary mappings at boot-
time until this issue is resolved.  Also, we need to make sure that
this early_memremap for WP is only called after pat_init() is done.

A nit - please add WP to the function header comment below.
"This function initializes PAT MSR and PAT table with an OS-defined
value to enable additional cache attributes, WC and WT."

Thanks,
-Toshi