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

Mark Cave-Ayland [off-list ref] writes:

On 19/02/2019 04:20, Michael Ellerman wrote:

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Mark Cave-Ayland [off-list ref] writes:

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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.

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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