linux-kernel - Re: [patch]pageattr: cache flush before tlb flush

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Message-ID: <48AA559A.6060903@zytor.com>
Date:	Mon, 18 Aug 2008 22:09:46 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Shaohua Li <shaohua.li@...el.com>
CC:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Subject: Re: [patch]pageattr: cache flush before tlb flush

Shaohua Li wrote:
> clflush uses a virtual address but cache line is physical indexed in
> x86. In my understanding, clflush will do some pagetable walk, so doing
> cache flush first should reduce some pagetable walk.
> 
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>

I would say NAK on this.

Doing the CLFLUSH first does cut down on page table walking, but opens a 
hole in the sequencing: first set PAT to an uncachable mode, then flush.

If an unlucky prefetch comes in during this window, then you will have a 
dirty cache again.

So no, this is not a good idea.

	-hpa
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