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Re: [Qemu-ppc] [RFC PATCH] spapr: support time base offset migration

From: Alexander Graf
Subject: Re: [Qemu-ppc] [RFC PATCH] spapr: support time base offset migration
Date: Thu, 5 Sep 2013 15:39:55 +0200

On 05.09.2013, at 15:36, Benjamin Herrenschmidt wrote:

> On Thu, 2013-09-05 at 14:37 +0200, Alexander Graf wrote:
>> Hrm, I think I'm starting to understand what this is about. So what we want 
>> is
>>  - timebase in guest
>>  - timebase frequency in guest
>>  - wall clock time in host
>> That way the receiving end can then take the timebase and add (new_timebase 
>> - old_timebase) * tb_freq to the guest's time base.
>> Which gets me to the next question. Can we modify the tb frequency in guests?
> No. It's architected at 512Mhz however since P7 I think. Not sure how we
> did before, it's possible that P6 was the same (at least it's sourced
> from more/less the same chip TOD facility).

I think we should transmit the tb frequency as well to at least allow us to 
adjust if a later chip derives here.

But yes, without frequency adjustment I see where you're coming from. We still 
only need the timebase the guest sees, not the offset. But we also need the 
host wall clock to allow for adjustments :).


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