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Date: Sun, 7 Oct 2007 14:06:43 +1000
From: Paul Mackerras <paulus@samba.org>
To: Kumar Gala <kumar.gala@freescale.com>
Subject: Re: TASK_SIZE default 0x80000000 ?
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Cc: PowerPC dev list <linuxppc-dev@ozlabs.org>,
	Benjamin Herrenschmidt <benh@au1.ibm.com>
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Kumar Gala writes:

> In a discussion with Hollis over beer he raised the question why  
> TASK_SIZE is 0x80000000 on ppc32.
> 
> I was wondering if anyone know why this was still the case?  Seems  
> like we have a 1Gb whole between TASK_SIZE & KERNELBASE.

Two reasons: (a) PReP used to map PCI stuff with BATs in the
0x80000000 - 0xbfffffff region, and (b) some embedded ports with
software-loaded TLBs test just the high bit of the address to
determine whether to use the kernel or user page tables.

Paul.