Re: [QEMU PATCH v2] kvmclock: advance clock by time window between vm_stop and pre_save

Paolo Bonzini
2016-11-09T16:23:55Z

On 08/11/2016 11:22, Dr. David Alan Gilbert wrote:
> * Marcelo Tosatti (mtosatti@redhat.com) wrote:
>> On Mon, Nov 07, 2016 at 08:03:50PM +0000, Dr. David Alan Gilbert wrote:
>>> * Marcelo Tosatti (mtosatti@redhat.com) wrote:
>>>> On Mon, Nov 07, 2016 at 03:46:11PM +0000, Dr. David Alan Gilbert wrote:
>>>>> * Marcelo Tosatti (mtosatti@redhat.com) wrote:
>>>>>> This patch, relative to pre-copy migration codepath,
>>>>>> measures the time between vm_stop() and pre_save(),
>>>>>> which includes copying the remaining RAM to destination,
>>>>>> and advances the clock by that amount.
>>>>>>
>>>>>> In a VM with 5 seconds downtime, this reduces the guest
>>>>>> clock difference on destination from 5s to 0.2s.
>>>>>>
>>>>>> Tested with Linux and Windows 2012 R2 guests with -cpu XXX,+hv-time.
>>>>>
>>>>> One thing that bothers me is that it's only this clock that's
>>>>> getting corrected; doesn't it cause things to get upset when
>>>>> one clock moves and the others dont?
>>>>
>>>> If you are correlating the clocks, then yes.
>>>>
>>>> Older Linux guests get upset (marking the TSC clocksource unstable
>>>> because the watchdog checks TSC vs kvmclock), but there is a workaround for it
>>>> in newer guests
>>>> (kvmclock interface to notify watchdog to not complain).
>>>>
>>>> Note marking TSC clocksource unstable on older guests is harmless
>>>> because kvmclock is the standard clocksource.
>>>>
>>>> For Windows guests, i don't know that Windows correlates between different
>>>> clocks.
>>>>
>>>> That is, there is relative control as to which software reads kvmclock
>>>> or Windows TIMER MSR, so i don't see the need to advance every clock
>>>> exposed.
>>>>
>>>>> Shouldn't the pause delay be recorded somewhere architecturally
>>>>> independent and then be a thing that kvm-clock happens to use and
>>>>> other clocks might as well?
>>>>
>>>> In theory, yes. In practice, i don't see the need for this...
>>>
>>> It seems unlikely to me that x86 is the only one that will want
>>> to do something similar.
>>
>> Can't they copy what kvmclock is doing today?
>
> We shouldn't have copies of code all over should we?

Let's cross the bridge when we get there.

For now I'm more interested in having a version of the patch that is
clean and doesn't need advance_clock.

Paolo