[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 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?

>>> 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.

> 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.

cheers