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

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Message-ID: <479A5689.80505@zytor.com>
Date:	Fri, 25 Jan 2008 13:37:13 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	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()

Jeremy Fitzhardinge wrote:
> PAE mode requires that we reload cr3 in order to guarantee that
> changes to the pgd will be noticed by the processor.  This means that
> in principle pud_clear needs to reload cr3 every time.  However,
> because reloading cr3 implies a tlb flush, we want to avoid it where
> possible.

It only matters (for a processor which supports PGE) if we actually use 
the namespace in between.

> In huge_pmd_unshare, it is followed by flush_tlb_range, which always
> results in a full cr3-reload tlb flush.

This one makes me nervous, as it feels like a side effect of 
implementation, and not a guarantee by design.

	-hpa
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