Re: [qemu-s390x] [PATCH v1] s390x/tod: properly stop the KVM TOD while the guest is not running

[David Hildenbrand]

On 27.11.18 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?
> 
>  Thomas
> 

-rtc [base=utc|localtime|date][,clock=host|vm][,driftfix=none|slew]
       Specify base as "utc" or "localtime" to let the RTC start at the current 
UTC or local time, respectively. "localtime" is required for
       correct date in MS-DOS or Windows. To start at a specific point in time, 
provide date in the format "2006-06-17T16:01:21" or
       "2006-06-17". The default base is UTC.

       By default the RTC is driven by the host system time. This allows using 
of the RTC as accurate reference clock inside the guest,
       specifically if the host time is smoothly following an accurate external 
reference clock, e.g. via NTP.  If you want to isolate the
       guest time from the host, you can set clock to "rt" instead.  To even 
prevent it from progressing during suspension, you can set it to
       "vm".

Most of the stuff here is not supported (should we error out if specified?)
and I think this is only considered for the RTC clock on x86, not "ordinary" 
clocks
(e.g. kvmclock)

-- 

Thanks,

David / dhildenb