linux-kernel - Re: The 3G (or nG) Kernel Memory Space Offset

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Date:	Tue, 29 Aug 2006 18:05:03 +0200
From:	Arjan van de Ven <arjan@...radead.org>
To:	Dong Feng <middle.fengdong@...il.com>
Cc:	Jan Engelhardt <jengelh@...ux01.gwdg.de>, Andi Kleen <ak@...e.de>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Paul Mackerras <paulus@...ba.org>,
	Christoph Lameter <clameter@....com>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: The 3G (or nG) Kernel Memory Space Offset

On Wed, 2006-08-30 at 00:01 +0800, Dong Feng wrote:
> 2006/8/29, Jan Engelhardt <jengelh@...ux01.gwdg.de>:
> >
> > "0-4G physical memory space" denotes RAM. Since kernelspace is resident, it
> > only seems logical to map it to 0G (that is, the start of RAM), because the
> > end of RAM can be flexible.
> >
> > IOW, you cannot map kernelspace to the physical location 0xc0000000 because
> > there might not be that much RAM.
> >
> > (Also note the PCI memory hole which is near the end of the 4G range.)
> >
> >
> > Jan Engelhardt
> > --
> >
> 
> 
> Sorry for my typo. I actually means "0-1G physical memory space." My
> question is actually why there is a 3G offset from linear kernel to
> physical kernel. Why not simply have kernel memory linear space
> located on 0-1G linear address, and therefore the physical kernel and
> linear kernel just coincide?


the price for that would be that you would have to flush all the tlb's
on each syscall. That's seen as a quite hefty price by many kernel
developers.


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