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Re: [OpenBIOS] Re: [RFC PATCH 0/2] QEMU/openbios: PPC Software TLB suppo

From: Cédric Le Goater
Subject: Re: [OpenBIOS] Re: [RFC PATCH 0/2] QEMU/openbios: PPC Software TLB support in the G4 family
Date: Fri, 26 Nov 2021 09:34:44 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0


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.


$ git show 7dbe11acd807
commit 7dbe11acd807
Author: Jocelyn Mayer <l_indien@magic.fr>
Date:   Mon Oct 1 05:16:57 2007 +0000

    Handle all MMU models in switches, even if it's just to abort because of 
      of supporting code.
    Implement 74xx software TLB model.
    Keep 74xx with software TLB disabled, as Linux is not able to handle TLB 
      on those processors.


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