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

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

Hi Michael,

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

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On 08/02/2019 14:45, Christophe Leroy wrote:

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Le 08/02/2019 à 15:33, Mark Cave-Ayland a écrit :

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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.

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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.

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.

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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 <redacted>

Should include a Fixes: and a Cc to stable ?

Fixes: 8792468da5e1 ("powerpc: Add the ability to save FPU without giving it up")
Cc: stable@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 :)

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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.


ATB,

Mark.