[OpenBIOS] Running client with MMU off

[Mark Cave-Ayland]

On 08/06/14 13:15, BALATON Zoltan wrote:

> On Sat, 7 Jun 2014, BALATON Zoltan wrote:
>> I'm not sure how it should work but on real hardware the mapped area
>> seems to be 0x0000-0x3000 while with OpenBIOS it's only 0x1000-0x3000.
>> Could we just modify it to be 0x2000-0x3000 to avoid the exception? Or
>> can we match what's on real hardware?
>
> Alternatively maybe the translation could be locked in using dbat
> registers as maybe the DSI happens because the entry is not in the TLB
> any more by the time it is accessed. Please decide on one solution and
> let me know which one you prefer.

Actually just thinking about this, I've just remembered that in OpenBIOS 
we use a round-robin hash table slot eviction scheme that forces a tlbie 
(i.e. invalidate the TLB entry) at the end so perhaps we are 
invalidating the entry early? See arch/ppc/qemu/ofmem.c in hash_page32().

Quick and dirty hack alert: you may be able to tweak the code so that 
any hash entry with a virtual address which references physical page 0 
is never chosen for eviction (and hence never forcibly invalidated with 
tlbie) and see if that helps at all? Have a play and see what happens.

> I'd really like to get this in before the freeze because it is probably
> the biggest obstacle now preventing more people to try running MorphOS
> on QEMU. (The other one is in QEMU but that's less prohibiting because
> unlike building OpenBIOS that does not need a cross-compiler environment.)

Understood. I appreciate the time you have spent working on this, we 
just need to make sure that we don't just scatter MorphOSisms around the 
code in a way that makes maintenance harder for those of us who won't 
run it everyday, and isn't detrimental to all the other supported 
architectures.


ATB,

Mark.