Re: [B2-devel] DUDA: HWMMU PARA SHEEPSHAVER?

[Gwenole B. <gb....@fr...>]

Le jeudi, 7 avr 2005, =E0 21:39 Europe/Paris, Steven Knoxville a =E9crit =
:

> Okay, pero entiendo que para el software PearPC existe un Parche de=20
> nombre HWMMU que se encarga de utilizar la MMU i386 para emular una=20
> MMU PowerPC, mi pregunta es =BFEs Posible modificar este parche para =
que=20
> trabaje con SheepShaver y asi poder emular MacOS 9.1 y X que requeren=20=

> de una mmu para trabajar?

QEMU already implemented an HWMMU implementation, nothing new in=20
PearPC. Basically, you use mmap() to map shared pages at specified=20
locations (expected by MacOS). The problem is that on Linux/x86=20
architectures with a default 3G/1.5G split, you will be limited by up=20
to 3GB address space in user mode (the case here). Full HWMMU support=20
on 32-bit systems requires a patch to host kernels and people may not=20
be willing to do that. This limitation can be workarounded in the=20
emulator but you still get suboptimal performance.

All in all, you don't want HWMMU. However, you may want MMU emulation=20
to help understand how MacOS VM works and provide better native=20
replacements. See, SheepShaver is a higher level emulator that=20
virtually can replace any system function with native implementations=20
for improved performance. *All* PowerPC MacOS version require an MMU,=20
there are just ways to do without it.

I don't think MMU will ever be implemented into SheepShaver. However, I=20=

already experimented MOL with a CPU emulator in the past. My MMU=20
emulation was inacurate so I had started to use QEMU implementation but=20=

time terribly dropped and when time came back I moved on to SheepShaver=20=

again with some bug fixes and other features.

BTW, adding MMU emulation to SheepShaver wouldn't have made it run=20
MacOS X either. It's a totally different architecture. Emulating the=20
MacOS X System is not interesting but enabling MacOS X Applications to=20=

run on another system is. That's the key difference in speed and=20
integration.

If you look carefully, it's easier to emulate MacOS X than MacOS 9,=20
simply because you have the MacOS X kernel sources: xnu. And, if you=20
look carefully at xnu, you can find ways to virtualize the Mach VM=20
layer, but that's not trivially simple but not that difficult either.=20
At first, you will have the same limitations than HWMMU. But at least,=20=

this time, you can also restrict a little the user address space so=20
that another trick can be used so that you don't get performance loss.=