What is CONFIG_RTAS ? Which CPUs are concerned

Hello All,

I'm wondering what CONFIG_RTAS is. It makes use of one of the SPRN_SPRG, ie SPRN_SPRG2.

What are the CPUs concerned by RTAS ? Is there any of the old CPUs which have only 4 SPRGs (eg the 601), or could we use one in SPRG4-7 for it and reuse SPRG2 for something else ?

The idea behind this question is to store physical address of PGDIR in SPRG2 and then put virtual address of thread_struct instead of its physical address in SPRG3, especially for when CONFIG_VMAP_STACK is set.

Thanks
Christophe

On Mon, Jan 28, 2019 at 07:20:43PM +0100, Christophe Leroy wrote:

I'm wondering what CONFIG_RTAS is. It makes use of one of the SPRN_SPRG, 
ie SPRN_SPRG2.

What are the CPUs concerned by RTAS ? Is there any of the old CPUs which 
have only 4 SPRGs (eg the 601), or could we use one in SPRG4-7 for it 
and reuse SPRG2 for something else ?

RTAS (run-time abstraction services) is as old as PowerPC itself.  Yes there
is RTAS on various 6xx, and those do not have any SPRGs not defined in the
architecture.

RTAS is a feature of the firmware, or of the platform you could say.  Not a
feature of CPUs.


Segher