linux-kernel - Re: [PATCH 11 of 11] x86: defer cr3 reload when doing pud

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Message-ID: <479A7463.7070801@goop.org>
Date:	Fri, 25 Jan 2008 15:44:35 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Keir Fraser <Keir.Fraser@...cam.ac.uk>
CC:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>, Andi Kleen <ak@...e.de>,
	Jan Beulich <jbeulich@...ell.com>,
	Eduardo Pereira Habkost <ehabkost@...hat.com>,
	Ian Campbell <ijc@...lion.org.uk>,
	William Irwin <wli@...omorphy.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 11 of 11] x86: defer cr3 reload when doing pud_clear()

Keir Fraser wrote:
> 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.
>   

Yeah, I found that document, and mentioned it a little lower in the mail ;)

> 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.
>   

Hm, but then chapter 10 goes and makes things confusing with 
"Alternative INVLPG Behavior"; but I guess if software needs to 
explicitly enable this behaviour in a yet-to-be-determined way, its OK...

Is there any guide about the tradeoff of when to use invlpg vs flushing 
the whole tlb?  1 page?  10?  90% of the tlb?

    J
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