What is CONFIG_RTAS ? Which CPUs are concerned

[Christophe Leroy]

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