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

[Mark Cave-Ayland]

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

Hi Michael,

> Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk> writes:
>> On 08/02/2019 14:45, Christophe Leroy wrote:
>>
>>> Le 08/02/2019 à 15:33, Mark Cave-Ayland a écrit :
>>>> Commit 8792468da5e1 "powerpc: Add the ability to save FPU without giving it up"
>>>
>>> Expected format for the above is:
>>>
>>> Commit 123456789abc ("text")
>>
>> Hi Christophe,
>>
>> Apologies - I'm fairly new at submitting kernel patches, but I can re-send it in the
>> correct format later if required.
>>
>>>> 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.

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.

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.

>>>> Reinstate these bits to the MSR bitmask to enable MacOS guests to run under
>>>> 32-bit KVM-PR once again without issue.
>>>>
>>>> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland at ilande.co.uk>
>>>
>>> Should include a Fixes: and a Cc to stable ?
>>>
>>> Fixes: 8792468da5e1 ("powerpc: Add the ability to save FPU without giving it up")
>>> Cc: stable at vger.kernel.org
>>
>> Indeed, but there are still some questions to be asked here:
>>
>> 1) Why were these bits removed from the original bitmask in the first place without
>> it being documented in the commit message?
> 
> It was almost certainly an accident.

Heh, okay :)

>> 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.


ATB,

Mark.