Re: Qemu / KVM On Powermac G5 MP970

[BALATON Zoltan]

On Sun, 15 Jan 2023, Daniel Henrique Barboza wrote:
> Ccing Mark
>
> On 1/12/23 11:18, Dan Whitehouse wrote:
> > Hi all,
> > I have a Powermac G5 running Debian (from Ports) with kernel version:
> >
> > Linux powermac-g5 6.0.0-6-powerpc64 #1 SMP Debian 6.0.12-1 (2022-12-09) 
> > ppc64 GNU/Linux
> >
> > My aim is to get Qemu working with KVM acceleration, although I am unsure 
> > if that is even possible.
> > Any attempt to do so seems to result in the behaviour described here:
> >
> > Re: [PATCH 2/9] target/ppc: add errp to kvmppc_read_int_cpu_dt() 
> > <https://www.mail-archive.com/qemu-devel@nongnu.org/msg898334.html>
> > mail-archive.com 
> > <https://www.mail-archive.com/qemu-devel@nongnu.org/msg898334.html>
> > 	apple-touch-icon-114x114.png 
> > <https://www.mail-archive.com/qemu-devel@nongnu.org/msg898334.html>
> >
> > <https://www.mail-archive.com/qemu-devel@nongnu.org/msg898334.html>
> >
> > I have managed (after a fair bit of struggle to get Debian working under 
> > qemu (without kvm) and I also got MacOS 9 working as well (albeit with 
> > horribly garbled colours).
> >
> > Ive seen some posts on the KVM mailing list that may refer to whatever is 
> > going wrong here:
> >
> > 'Re: [PATCH] kvm-pr: manage illegal instructions' 
> > <https://marc.info/?l=kvm-ppc&m=146364192030854&w=2>
> > marc.info <https://marc.info/?l=kvm-ppc&m=146364192030854&w=2>
> >         <https://marc.info/?l=kvm-ppc&m=146364192030854&w=2>
> >
> > <https://marc.info/?l=kvm-ppc&m=146364192030854&w=2>
> >
> >
> > And it may well be the case that this is a KVM issue more than a Qemu wrong 
> > and I’m therefore posting in the wrong list (in which case apologies). As 
> > you can see though there has been no activity since 2016.
>
> I took a look at the thread and I'm not entirely convinced that QEMU is 100%
> in the clear. The root issue seems to be kvm-pr kernel code though.
>
> Mark managed to run kvm-pr in a G5 in the past so I believe he can help you
> with any pointers. What I can say that you'll have a bumpy road ahead -

I think he did that on G4 but maybe also on G5, I don't know. I only know 
about this blog which shows it running on newer CPU (in that case POWER9):

https://www.talospace.com/search/label/KVM

which may have some useful info also for G5 but some of that like KMV-HV 
and newer MMU modes are not applicable to G5.

> > I’d be grateful if either someone could confirm that this is a non starter 
> > or give me any pointers.
>
> It's not a non-starter - it's feasible and it was done before. But I'd 
> say you're in for some heavy kernel (and probably QEMU) hacking to make 
> it work properly.

You'll probably have to dig deeper into KVM to understand what's causing 
the problem. I don't know much about how to debug that but here are some 
links that may help:

https://arkivm.github.io/kernel%20development/2020/11/01/qemu-kvm-1gb-pages/

https://www.linux-kvm.org/page/Perf_events

I don't know how relevant or up to date these are, I've only tried to 
debug this once on G5 emulated by QEMU on X86_64 as I don't have PPC 
hardware but hit a QEMU problem in emiulated KVM that I could not 
overcome:

https://lists.nongnu.org/archive/html/qemu-ppc/2021-06/msg00146.html

Not sure if that helps but that's all I have. If you find out something it 
might be helpful for others to post it here.

Regards,
BALATON Zoltan