kvm masterclock

kvm masterclock

[Mika Penttilä]

While using the master clock among guests, when is it updated? Thought it had to be in 

pvclock_gtod_notify() but it only triggers update of :

  master_kernel_ns, master_cycle_now

if clocksource changes away from TSC :

      if (gtod->clock.vclock_mode != VCLOCK_TSC &&
            atomic_read(&kvm_guest_has_master_clock) != 0)
                queue_work(system_long_wq, &pvclock_gtod_work);

And gtod work would set KVM_REQ_MASTERCLOCK_UPDATE if queued, which is not the case if always TSC.

So kvm_guest_time_update() always uses same values is use_master_clock ?

Maybe I missed some piece?

--Mika

Re: kvm masterclock

[Paolo Bonzini]

On 05/09/2016 17:50, Mika Penttilä wrote:
> While using the master clock among guests, when is it updated? Thought it had to be in 
> 
> pvclock_gtod_notify() but it only triggers update of :
> 
>   master_kernel_ns, master_cycle_now
> 
> if clocksource changes away from TSC :
> 
>       if (gtod->clock.vclock_mode != VCLOCK_TSC &&
>             atomic_read(&kvm_guest_has_master_clock) != 0)
>                 queue_work(system_long_wq, &pvclock_gtod_work);
> 
> And gtod work would set KVM_REQ_MASTERCLOCK_UPDATE if queued, which is not the case if always TSC.
> 
> So kvm_guest_time_update() always uses same values is use_master_clock ?

Yes, if the master clock is in use the kvmclock does _not_ adjust itself
and, as a consequence, doesn't follow host NTP adjustments.

Paolo