linux-kernel - Re: [PATCh] x86: overmapped fix when 4K pages on tail

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Date:	Mon, 14 Jul 2008 08:39:55 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Ingo Molnar <mingo@...e.hu>, Yinghai Lu <yhlu.kernel@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCh] x86: overmapped fix when 4K pages on tail - 64bit

H. Peter Anvin wrote:
> Andi Kleen wrote:
>>
>> First I was only commenting on one specific patch, nothing more.
>>
>> My point is full rounding to 4K on all corners is wasteful because the
>> CPUs have to handle that case anyways and every split costs precious
>> TLB entries in direct mapping accesses.
>>
> 
> Well, the CPU *does* handle them... by splitting the larger pages into
> smaller pages.  They still end up in the small-page TLB, so there is no
> real difference if done in the CPU or in software.

There's actually a difference in some cases, but that's a different
issue.

Only when the hole is a real hole. But when it's just some firmware
area or similar that's not needed.

> Memory state transitions are (fortunately) relatively rare and
> long-lived,

That's not true today with several 3d driver setups. Also in general
I would expect more PAT use in the future and that is usually a split.

-Andi
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