Re: [PATCH 11 of 11] x86: defer cr3 reload when doing pud_clear()

[Keir Fraser]

On 25/1/08 22:54, "Jeremy Fitzhardinge" <jeremy@xxxxxxxx> wrote:

> The only possibly relevant comment I can find in vol3a is:
> 
>     Older IA-32 processors that implement the PAE mechanism use uncached
>     accesses when loading page-directory-pointer table entries. This
>     behavior is
>     model specific and not architectural. More recent IA-32 processors may
>     cache page-directory-pointer table entries.

Go read the Intel application note "TLBs, Paging-Structure Caches, and Their
Invalidation" at http://www.intel.com/design/processor/applnots/317080.pdf

Section 8.1 explains about the PDPTR cache in 32-bit PAE mode, which can
only be refreshed by appropriate tickling of CR0, CR3 or CR4.

It is also important to note that *any* valid page directory entry at *any*
level in the page-table hierarchy can become cached at *any* time. Basically
TLB lookup is performed as a longest-prefix match on the linear address to
skip as many levels in a page-table walk as possible (where a walk is
needed, because there is no full-length match on the linear address). So, if
you modify a directory entry from present to not-present, or change the page
directory that a valid pde points to, you probably need to flush the pde
caching structure. One piece of good news is that all pde caches are flushed
by any arbitrary INVLPG.

 -- Keir


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