Re: [OpenBIOS] Re: [RFC PATCH 0/2] QEMU/openbios: PPC Software TLB support in the G4 family

[Fabiano Rosas <farosas@linux.ibm.com>]

Cédric Le Goater <clg@kaod.org> writes:

> On 11/26/21 13:13, Fabiano Rosas wrote:
>> Segher Boessenkool <segher@kernel.crashing.org> writes:
>> 
>>> Hi!
>>>
>>> On Fri, Nov 26, 2021 at 09:34:44AM +0100, Cédric Le Goater wrote:
>>>> On 11/25/21 10:38, Segher Boessenkool wrote:
>>>>> On Thu, Nov 25, 2021 at 01:45:00AM +0100, BALATON Zoltan wrote:
>>>>>> As for guests, those running on the said PowerMac G4 should have support
>>>>>> for these CPUs so maybe you can try some Mac OS X versions (or maybe
>>>>>
>>>>> OSX uses hardware pagetables.
>>>>>
>>>>>> MorphOS but that is not the best for debugging as there's no source
>>>>>> available nor any help from its owners but just to see if it boots it may
>>>>>> be sufficient, it should work on real PowerMac G4).
>>>>>
>>>>> I have no idea what MorphOS uses, but I bet HPT as well.  That is
>>>>> because HPT is fastest in general.  Software TLB reloads are good in
>>>>> special cases only; the most common is real-time OSes, which can use its
>>>>> lower guaranteed latency for some special address spaces (and can have a
>>>>> simpler address map in general).
>>>>
>>>> The support was added to QEMU knowing that Linux didn't handle soft TLBs.
>>>> And the commit says that it was kept disabled initially. I guess that was
>>>> broken these last years.
>>>
>>> Ah :-)  So when was it enabled, do you know?
>> 
>> Hm.. That commit message does not match the code. They simply added the
>> software TLB implementation to an already existing SOFT_74xx MMU
>> model. I don't see anything that would keep it disabled at that time.
>> 
>
> because most of the cpu definitions in ppc_defs[] are protected by a :
>
> #if defined (TODO)
>
> See below. commit 8ca3f6c3824c ("Allow selection of all defined PowerPC
> 74xx (aka G4) CPUs.") removed the TODO without a reason :/ This is old,
> when SVN was in used.

Ah nice catch!