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

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Date:   Tue, 19 Feb 2019 08:15:36 +0000
From:   Mark Cave-Ayland <mark.cave-ayland@...nde.co.uk>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>, paulus@...ba.org,
        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 19/02/2019 04:55, Michael Ellerman wrote:

> Mark Cave-Ayland <mark.cave-ayland@...nde.co.uk> writes:
>> 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.
> 
> fpu_preempt.c should be able to be adapted to also check the MSR bits.
> 
>> Who is the right person to ask about fixing issues related to context switching with
>> KVM PR?
> 
> KVM PR doesn't really have a maintainer TBH. Feel like volunteering? :)

Well I only have a 32-bit Mac Mini here which I'm using to help flush out bugs in
QEMU's emulation, so I can keep an occasional eye on the 32-bit side of things but as
it's a hobby project time is quite limited.

As/when time allows I'd be interested to figure out what MacOS 9 does that causes KVM
PR to bail, and if it's possible to run KVM PR on an SMP kernel but certainly I'd
need some help from the very knowledgable people on these lists.

>> I did add the original author's email address to my first few emails but have
>> had no response back :/
> 
> Cyril who wrote the original FPU patch has moved on to other things.

Ah okay then.


ATB,

Mark.