linux-kernel - Re: [PATCH 1/4] x86_64: Switch to SPARSE

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Message-ID: <Pine.LNX.4.64.0704021032150.31053@schroedinger.engr.sgi.com>
Date:	Mon, 2 Apr 2007 10:34:38 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andi Kleen <ak@...e.de>
cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Martin Bligh <mbligh@...gle.com>, linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 1/4] x86_64: Switch to SPARSE_VIRTUAL

On Mon, 2 Apr 2007, Andi Kleen wrote:

> > No no no. For the gazillions time: All of 1-1 mapped kernel memory on 
> > x86_64 needs a 2 MB page table entry. The virtual memmap uses the same. 
> > There are *no* additional TLBs used.
> 
> But why do you reserve an own virtual area then if you claim to not use any
> additional mappings? 

The 1-1 area using mappings for 2MB pages right? So it uses a virtual 1-1 
area. It already has a virtual mapping.

What we do for virtual memmap here is also use 2MB pages but order the 
pages a bit different so that they provide a linear memory map.

So the number of TLBs in use stays the same. There are a few additional 
higher level page table pages that are needed to provide the alternate 
view that generates the linear mapping but that is just a couple of pages.

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