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From: John Baldwin <jhb@freebsd.org>
To: freebsd-amd64@freebsd.org
Subject: Re: PAT and identity mapped pages.
Date: Tue, 01 Dec 2015 13:43:59 -0800
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On Friday, November 20, 2015 11:14:57 AM AlexandreFressange wrote:
> Hi,
> 
> On x86_64, every memory page entry has a bit (4) to control its cache method: write-back or write-through.
> 
> But even on identity mapped pages, you still use paging. Say, I want a page to be uncached; I can use the PAT to Uncached.
> 
> In this case, what happens to the bit 4 of the accessed page? is it ignored?

Can you clarify more what you mean by an identity mapped page?  If you mean
the direct map, the OS has to ensure that all mappings of a given physical
page use the same PAT settings, so when a page is mapped with a non-default
PAT setting the page entries for the direct map have to be updated as well as
any other mappings.

-- 
John Baldwin