TASK_SIZE default 0x80000000 ?

[Kumar Gala]

On Oct 6, 2007, at 11:06 PM, Paul Mackerras wrote:

> 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.

Can you explain (a) further -- I'm assuming the BAT mapping is 1:1  
for that region?

For (b) it looks like:
* 40x, 44x, fsl-booke compare against TASK_SIZE in their software  
handlers.
* 8xx still tests 0x80000000
* 6xx (603) compares against KERNELBASE

It would seem like we should set the default on 8xx & PReP to  
0x80000000 and not allow it to be modified since that will break the  
world and for everything else move it to match KERNELBASE.  Any  
issues with that?

- k