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To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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References: <20190208143319.11980-1-mark.cave-ayland@ilande.co.uk>
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From:   Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
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Message-ID: <46205b6a-7671-5d90-9507-b5b20045b99d@ilande.co.uk>
Date:   Mon, 11 Feb 2019 21:39:26 +0000
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Subject: Re: [PATCH] powerpc: fix 32-bit KVM-PR lockup and panic with MacOS
 guest
Sender: linux-kernel-owner@vger.kernel.org
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On 11/02/2019 00:30, Benjamin Herrenschmidt wrote:

> On Fri, 2019-02-08 at 14:51 +0000, Mark Cave-Ayland wrote:
>>
>> 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.
> 
> We should write a test case to verify that FE0/FE1 are properly
> preserved/context-switched etc... I bet if we accidentally wiped them,
> we wouldn't notice 99.9% of the time.

Right I guess it's more likely to cause in issue in the KVM PR case because the guest
can alter the flags in a way that doesn't go through the normal process switch mechanism.

The original patchset at
https://www.mail-archive.com/linuxppc-dev@lists.ozlabs.org/msg98326.html does include
some tests in the first few patches, but AFAICT they are concerned with the contents
of the FP registers rather than the related MSRs.

Who is the right person to ask about fixing issues related to context switching with
KVM PR? I did add the original author's email address to my first few emails but have
had no response back :/


ATB,

Mark.