RE: Relocatable kernel for ppc44x

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2011-06-15 at 10:30 +0100, David Laight wrote:
> > The PPC440X currently uses 256M TLB entries to pin the 
> > lowmem. When we go for a relocatable kernel we have to :
> > 
> > 1) Restrict the kernel load address to be 256M aligned
> > 
> > OR
> > 
> > 2) Use 16M TLB(the next possible TLB page size supported) 
> > entries till the first
> > 256M and then use the 256M TLB entries for the rest of lowmem.
> 
> What is wrong with:
> 
> 3) Use 256M TLB entries with the lowest one including
>    addresses below the kernel base.
> 
> Clearly the kernel shouldn't be accessing the addresses
> below its base address - but that is true of a lot of
> address space mapped into the kernel.

In the case of a relocatable kernel it's perfectly kosher to access
addresses below the kernel itself... Typically this is used for kdump
where the kdump kernel excecutes in place in a reserved area but access
to the rest of memory is allowed to ... well, do the dump :-) There
could be other reasons to do that too.

Cheers,
Ben.