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Re: [qemu-s390x] [PATCH v1] s390x/tod: properly stop the KVM TOD while t

From: Christian Borntraeger
Subject: Re: [qemu-s390x] [PATCH v1] s390x/tod: properly stop the KVM TOD while the guest is not running
Date: Tue, 27 Nov 2018 14:11:23 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 27.11.2018 14:06, Thomas Huth wrote:
> On 2018-11-27 13:43, Christian Borntraeger wrote:
>> On 27.11.2018 12:41, David Hildenbrand wrote:
>>> Just like on other architectures, we should stop the clock while the guest
>>> is not running. This is already properly done for TCG. Right now, doing an
>>> offline migration (stop, migrate, cont) can easily trigger stalls in the
>>> guest.
>>> Even doing a
>>>     (hmp) stop
>>>     ... wait 2 minutes ...
>>>     (hmp) cont
>>> will already trigger stalls.
>>> So whenever the guest stops, backup the KVM TOD. When continuning to run
>>> the guest, restore the KVM TOD.
>> We do a similar thing for managedsave so it probably makes sense to solve
>> the stall warnings. Now: At the same time, we actually want to have the guest
>> see the real time and maybe even share the TOD clock with the host in some
>> way, while at the same time avoid the stall warnings.
> Hmm, by the way, do we also have to consider the "-rtc clock=host|rt|vm"
> option from the commandl line, or is this an orthogonal concept?

I think this option does not work at all in how time works on s390.

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