On Tue, 2014-07-15 at 20:40 -0400, Konrad Rzeszutek Wilk wrote:

On July 15, 2014 5:23:24 PM EDT, Toshi Kani [off-list ref] wrote:

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On Tue, 2014-07-15 at 13:09 -0700, H. Peter Anvin wrote:

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On 07/15/2014 12:34 PM, Toshi Kani wrote:

 :

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I have given this piece of feedback at least three times now,

possibly

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to different people, and I'm getting a bit grumpy about it:

We already have an issue with Xen, because Xen assigned mappings
differently and it is incompatible with the use of PAT in Linux.  As

a

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result we get requests for hacks to work around this, which is

something

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I really don't want to see.  I would like to see a design involving a
"reverse PAT" table where the kernel can hold the mapping between

memory

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types and page table encodings (including the two different ones for
small and large pages.)

Thanks for pointing this out! (And sorry for making you repeat it three
time...)  I was not aware of the issue with Xen.  I will look into the
email archive to see what the Xen issue is, and how it can be
addressed.

https://lkml.org/lkml/2011/11/8/406

Thanks Konrad for the pointer!

Since [__]change_page_attr_set_clr() and __change_page_attr() have no
knowledge about PAT and simply work with specified PTE flags, they do
not seem to fit well with additional PAT abstraction table...

I think the root of this issue is that the kernel ignores the PAT bit.
Since __change_page_attr() only supports 4K pages, set_memory_<type>()
can set the PAT bit into the clear mask.

Attached is a patch with this approach (apply on top of this series -
not tested).  The kernel still does not support the PAT bit, but it
behaves slightly better.

Thanks,
-Toshi