use of BAT before taking over the MMU

[Segher Boessenkool]

> On the prom boot path, with the firmware supposed to
> be managing the MMU, there is a case where:
>
> 1. Linux changes some BAT registers.
> 2. Bits 0x00000070 are/become set in the MSR.
> 3. Linux takes an MMU fault.
> 4. The firmware handles it.
>
> AFAIK, you can't expect the firmware to leave the BAT alone.
> If the firmware provides mapping services by using the BAT
> as a software-filled TLB, Linux's BAT changes may be lost.
>
> You also can't expect that your BAT changes will not conflict
> with mappings that the firmware uses for itself. The firmware
> might write to your new BAT mapping, relying on those virtual
> addresses to be something else entirely.

The PowerPC OF binding requires the firmware to save and restore
the BATs on entry to / exit from the firmware.


Segher