[PATCH] powerpc: fix 32-bit KVM-PR lockup and panic with MacOS guest

[Michael Ellerman]

Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk> writes:
> On 19/02/2019 04:20, Michael Ellerman wrote:
>> Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk> writes:
>>>>> unexpectedly removed the MSR_FE0 and MSR_FE1 bits from the bitmask used to
>>>>> update the MSR of the previous thread in __giveup_fpu() causing a KVM-PR MacOS
>>>>> guest to lockup and panic the kernel.
>> 
>> Which kernel is panicking? The guest or the host?
>
> It's the host kernel. As long as you occasionally tap a few keys to keep the screen
> blanking disabled then you can see the panic on the physical console.

Ah crap I assumed you meant the guest kernel.

> I've uploaded a photo I took during the bisection containing the panic when booting
> MacOS X 10.2 under qemu-system-ppc to
> https://www.ilande.co.uk/tmp/qemu/macmini-kvm.jpg in case you find it useful.

OK. That's a host crash, but only because init died (systemd). Though
the reason it died is because we didn't clear FE0/1 properly, so still a
kernel bug.

> Given that it's really easy to recreate, let me know if you want me to do a git
> pull/rebuild and/or if you need any debugging information as it's easy for me to
> reproduce.

I think that's OK. It's reasonably clear what's going on.


>>> 2) Is this the right fix? I'm told that MacOS guests already run without this patch
>>> on a G5 under 64-bit KVM-PR which may suggest that this is a workaround for another
>>> bug elsewhere in the 32-bit powerpc code.
>> 
>> That's slightly worrying. It's hard to say without more detail on why
>> the guest is crashing.
>> 
>> I think your patch looks OK based just on the fact that it restores the
>> previous behaviour, so I'll pick it up and pass it through my usual
>> testing. If nothing breaks I'll merge it.
>
> That would be great! Does it need a CC to stable too? It would be great if this would
> get picked up in the next set of Debian ports kernels, for example.

I'll add Cc stable.

cheers