[OpenBIOS] PPC: QEMU breaks static variables in OpenBIOS

[Mark Cave-Ayland]

Hi all,

Futher to John's report of next_grab_slot not changing value within the 
PPC MMU hash_page_32() function, I've done some more experimentation and 
determined that there is indeed a problem with all OpenBIOS static 
variables on PPC.

The issue is that OpenBIOS uses static variables in several places to 
keep track of various bits of information, and at compile-time these 
variables are given addresses within the OpenBIOS image itself, e.g.

(gdb) p &next_grab_slot
$1 = (int *) 0xfffdd000

However as John points out, the MemoryRegion associated with OpenBIOS is 
set to read-only. Single-stepping using gdb shows that while the store 
to update the value is executed, it is discarded by QEMU and so 
therefore static variables never change from their initial value.

Rather amusingly, it seems that setting the variable value via gdb 
bypasses the MemoryRegion read-only check which made this more 
interesting to debug.

I've done a quick hack on QEMU which is to comment out the following 
line from hw/ppc_oldworld.c in order to make the region read/write again:

memory_region_set_readonly(bios, true);

I've confirmed in gdb that this fixes the issue with static variables 
being able to update their value, and now with the latest round of OS X 
patches applied, the Darwin kernel boots although panics fairly early on.

At this point, it seems that we have a decision to make. Is the bug here 
that the OpenBIOS ROM is marked as read-only for PPC, or OpenBIOS's use 
of static variables? If it's the latter, I think there is going to have 
to be a fairly major patch to remove static variables completely from 
the target image codebase.

Thoughts?

Mark.