>> The requirement is that the guest VM will be synced to the host clock - I'm not talking on real-time in the meaning of speed, but I need that the guest clock will measure the exact delay between packet transmission from the host to the packet received in the guest.
>This makes no sense - if you want a "reliable clock synchronized up to X microseconds", then you have to take into account any possible delay incurred by scheduling (as scheduling determines how reliable a clock read can be) into the clock reliability calculation.
I understand the scheduling latency , anyway my clock is totally not in sync. Actually - I see about 800ms delay in the guest clock relative to the host clock , also call clock_settime (any_time) in the host and did not see any impact on the guest host.
Also I've looked in the code in kvm_guest_time_update and I think that the clock is not updated because use_master_clock is false - how can I enable master_clock in KVM ?
>> My problem now is that sometimes when I receive the packet in the guest I calculate a negative delay because my clock in the guest is in delay relative to the host clock.
>Use the guest TSC and host TSC values (rdtsc in both).
>Then subtract from host TSC timestamps the tsc_offset value.
>You'll get a "perfect" synchronized RDTSC clock.
>To find out the tsc_offset value, enable kvm_write_tsc_offset tracepoint in the host (before the particular guest boots).
Do u mean that in the host/guest I will not call clock_gettime but:
In host : tx_time = tsc_to_ns * (rdtsc () - tsc_offset)
In guest : rx_time = tsc_to_ns * rdtsc()
Delay = rx_time - tx_time
How to enable kvm_write_tsc_offset ?
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