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

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Message-ID: <2ed8efb9-5cd4-31bf-6c7b-501b9d1925e6@ilande.co.uk>
Date:   Fri, 8 Feb 2019 14:51:43 +0000
From:   Mark Cave-Ayland <mark.cave-ayland@...nde.co.uk>
To:     Christophe Leroy <christophe.leroy@....fr>,
        benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
        npiggin@...il.com, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org
Subject: Re: [PATCH] powerpc: fix 32-bit KVM-PR lockup and panic with MacOS
 guest

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.
>>
>> 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@...nde.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@...r.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?

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.

If you think that these points don't matter, then I'm happy to resubmit the patch
as-is based upon your comments above.

ATB,

Mark.