Re: [PATCH 1/2] powerpc: enable the relocatable support for the fsl booke 32bit kernel

On 06/27/2013 08:36:37 PM, Kevin Hao wrote:

On Thu, Jun 27, 2013 at 02:58:34PM -0500, Scott Wood wrote:

quoted

On 06/26/2013 09:00:33 PM, Kevin Hao wrote:

quoted

This is based on the codes in the head_44x.S. Since we always =20

align to

quoted

quoted

256M before mapping the PAGE_OFFSET for a relocatable kernel, we =20

also

quoted

quoted

change the init tlb map to 256M size.

Why 256M?

=20
For two reasons:
  1. This is the size which both e500v1 and e500v2 support.
  2. Since we always use the PAGE_OFFSET as 0xc0000000, the 256M is
     max alignment value we can use for this virtual address.

Is there any reason why 64M won't continue to work here?

quoted

This tightens the alignment requirement for dynamic memstart.

=20
Yes. But since RELOCATABLE is a superset of DYNAMIC_MEMSTART, we can =20
always
use RELOCATABLE instead of DYNAMIC_MEMSTART for fsl booke board in =20
any cases.

The extra flexibility of RELOCATABLE may help some use cases, but you'd =20
still require the entire 256M naturally aligned region containing the =20
kernel to be present and owned by this instance of Linux.

So DYNAMIC_MEMSTART will seem not so useful after we enable this =20
feature.

Then why doesn't this patch remove it?

quoted

 And
what about boards with less than 256 MiB of RAM?

=20
It should be fine. We just create the map in the tlb. The MM still use
the real size of memory.

No, you must not map anything that is not present with a mapping that =20
is executable and/or not guarded, or you could get speculative accesses =20
to who-knows-what.  Even if RAM is present there but owned by some =20
other entity, you could be creating illegal aliases if that other =20
entity mapped it cache-inhibited or similar.

-Scott=