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Re: [RFC PATCH 0/4] ppc: nested TCG migration (KVM-on-TCG)

From: Fabiano Rosas
Subject: Re: [RFC PATCH 0/4] ppc: nested TCG migration (KVM-on-TCG)
Date: Fri, 25 Feb 2022 13:11:02 -0300

Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> writes:

> On 24/02/2022 18:58, Fabiano Rosas wrote:
>> This series implements the migration for a TCG pseries guest running a
>> nested KVM guest. This is just like migrating a pseries TCG guest, but
>> with some extra state to allow a nested guest to continue to run on
>> the destination.
>> Unfortunately the regular TCG migration scenario (not nested) is not
>> fully working so I cannot be entirely sure the nested migration is
>> correct. I have included a couple of patches for the general migration
>> case that (I think?) improve the situation a bit, but I'm still seeing
>> hard lockups and other issues with more than 1 vcpu.
>> This is more of an early RFC to see if anyone spots something right
>> away. I haven't made much progress in debugging the general TCG
>> migration case so if anyone has any input there as well I'd appreciate
>> it.
>> Thanks
>> Fabiano Rosas (4):
>>    target/ppc: TCG: Migrate tb_offset and decr
>>    spapr: TCG: Migrate spapr_cpu->prod
>>    hw/ppc: Take nested guest into account when saving timebase
>>    spapr: Add KVM-on-TCG migration support
>>   hw/ppc/ppc.c                    | 17 +++++++-
>>   hw/ppc/spapr.c                  | 19 ++++++++
>>   hw/ppc/spapr_cpu_core.c         | 77 +++++++++++++++++++++++++++++++++
>>   include/hw/ppc/spapr_cpu_core.h |  2 +-
>>   target/ppc/machine.c            | 61 ++++++++++++++++++++++++++
>>   5 files changed, 174 insertions(+), 2 deletions(-)
> FWIW I noticed there were some issues with migrating the decrementer on Mac 
> machines 
> a while ago which causes a hang on the destination with TCG (for MacOS on a 
> x86 host 
> in my case). Have a look at the following threads for reference:
> https://lists.gnu.org/archive/html/qemu-devel/2016-01/msg00546.html
> https://lists.gnu.org/archive/html/qemu-devel/2016-01/msg04622.html

Thanks, Mark! There's a lot of helpful information in these threads.

> IIRC there is code that assumes any migration in PPC is being done
>live, and so adjusts the timebase on the destination to reflect wall
>clock time by recalculating tb_offset. I haven't looked at the code for
>a while but I think the outcome was that there needs to be 2 phases in
>migration: the first is to migrate the timebase as-is for guests that
>are paused during migration, whilst the second is to notify
>hypervisor-aware guest OSs such as Linux to make the timebase
>adjustment if required if the guest is running.

> ATB,
> Mark.

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