host-timesync-daemon

host-timesync-daemon

Some hypervisors (e.g. hyperkit / xhyve) don't provide a good way to keep the VM's clock in sync with the Host's clock. NTP will usually keep the clocks together, but after the host or VM is suspended and resumed the clocks can be suddenly too far apart for NTP to work properly.

This simple daemon listens on an AF_VSOCK port. When a connection is received, the daemon

• reads the (hypervisor's virtual) hardware clock via RTC_RD_TIME on /dev/rtc0
• calls settimeofday to set the time in the VM
• closes the connection

Note the hardware clock has only second granularity so you will still need NTP (or some other process) to keep the clocks closely synchronised.

To use this, simply connect to the AF_VSOCK port when you want the clock to be resynchronised. If you want to wait for completion then read from the socket -- EOF means the resychronisation is complete.