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From: Laurent Vivier <lvivier@redhat.com>
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Date: Fri, 27 Jan 2017 10:23:42 +0100
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Subject: Re: [Qemu-devel] [PATCH] spapr: clock should count only if vm is
 running
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, David Gibson <david@gibson.dropbear.id.au>, Alexander Graf <agraf@suse.de>, Thomas Huth <thuth@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-ppc@nongnu.org, Alexey Kardashevskiy <aik@ozlabs.ru>

On 27/01/2017 09:52, Paolo Bonzini wrote:
> 
>> This is a port to ppc of the i386 commit:
>>     00f4d64 kvmclock: clock should count only if vm is running
>>
>> We remove timebase_/pre_save/post_load/ functions,
>> and use the VM state change handler to save and restore
>> the guest_timebase (on stop and continue).
>>
>> Time base offset has originally been introduced by commit
>>     98a8b52 spapr: Add support for time base offset migration
>>
>> So while VM is paused, the time is stopped. This allows to have
>> the same result with date (based on Time Base Register) and
>> hwclock (based on "get-time-of-day" RTAS call).
>>
>> Moreover in TCG mode, the Time Base is always paused, so this
>> patch also adjust the behavior between TCG and KVM.
>>
>> VM state field "time_of_the_day_ns" is now useless but we keep
>> it to be able to migrate to older version of the machine.
>>
>> As vmstate_ppc_timebase structure (with timebase_pre_save() and
>> timebase_post_load() functions) was only used by vmstate_spapr,
>> we register the VM state change handler only in ppc_spapr_init().
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> 
> I think you should keep the pre_save handler, otherwise after
> migration the timebase register will be off by as long as the
> time needed to do the final RAM transfer.  See commit 6053a86
> ("kvmclock: reduce kvmclock difference on migration", 2016-12-22).

I will.

Thank you,
Laurent