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

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

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.