After ptp_kvm initialized, there will be a new device node under
/dev/ptp*. A guest userspace service, like chrony, can use this
device to get host walltime, sometimes also counter cycle, which depends
on the service it calls. Then this guest userspace service can use those
data to do the time sync for guest.
here is a rough sketch to show how kvm ptp clock works.

|----------------------------|              |--------------------------|
|       guest userspace      |              |          host            |
|ioctl -> /dev/ptp%d         |              |                          |
|       ^   |                |              |                          |
|----------------------------|              |                          |
|       |   | guest kernel   |              |                          |
|       |   V      (get host walltime/counter cycle)                   |
|      ptp_kvm -> hypercall - - - - - - - - - - ->hypercall service    |
|                         <- - - - - - - - - - - -                     |
|----------------------------|              |--------------------------|