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Virtual machine Time Synchronization

Virtual machine Time Synchronization

IEC 61850 Requirement

The IEC 61850 standard specifies that each IED (Intelligent Electronic Device) must be synchronized using PTP. In a physical deployment, this is straightforward: every device connects to the PTP network and aligns its hardware clock with the Grand Master.

Challenges in Virtual Environments

When IEDs are virtualized, enforcing IEC 61850 compliance becomes complicated:

  • The PTP network must be propagated into the VM, requiring careful configuration of virtual NICs.

  • The VM must receive hardware timestamps to achieve precise synchronization. This usually requires PCI passthrough of the network interface.

  • SR-IOV cannot be used, because virtual functions do not support hardware timestamping, which is critical for PTP accuracy.

  • Each VM would need its own PTP setup (ptp4l, phc2sys), VLANs, and careful network isolation, making the configuration complex and fragile.

It is a totally viable solution to let the VM receive PTP frames on a pci_passthrough interface and synchronize itself directly on this source.
However, this solution is usually less precise and less practical than the host synchronization described below

PTP synchronisation through the host (ptp_kvm)

SEAPATH takes a simpler and more reliable approach:

  • Only the physical host is synchronized to the PTP Grand Master.

  • The host’s PTP clock is exposed to the VM using the ptp_kvm kernel module.

  • This allows the VM to inherit the host’s synchronized time without requiring direct access to the PTP network.

Steps to use ptp_kvm in the VM:

  1. Load the ptp_kvm kernel module (at boot or manually).

  2. Identify the created PTP device under /dev/ptp* by checking /sys/class/ptp/ptp*/clock_name, which should show "KVM virtual PTP". Example: /dev/ptp0.

  3. Synchronize the VM system clock to the PTP clock using chrony (for example by creating the /etc/chrony/conf.d/ptp.conf file):

    refclock PHC /dev/ptp0 poll 2

PTP Status Propagation to Virtual IEDs

IEC 61850 Requirement

IEC 61850 specifies that each Sampled Value (SV) message should include a SmpSynch field, indicating the PTP synchronization status of the source device. For a physical IED, this is straightforward: the device knows whether it is synchronized.

In a virtual environment, the VM itself does not have a direct PTP connection, so it cannot know its own PTP sync status.

SEAPATH Solution: ptp_status_vsock Role

SEAPATH provides the ptp_status_vsock Ansible role to bridge this gap.

  • The role runs on the host and retrieves the host’s PTP synchronization status every second.

  • It exposes this status to the guest VM via a vsock interface.

More information on this role on Ansible Galaxy.

What is a vsock?

A vsock (virtual socket) is a communication channel between the hypervisor and its guests. It allows data to flow between host and VM without going through a network stack, making it fast and reliable.

  • In libvirt, you define a vsock device in the VM XML, for example:

    <devices> <vsock model='virtio' cid='3'/> </devices>

How to use it in the guest

  • The VM can open the vsock device using standard libraries (e.g., libvirt or python-vsock) to read the host’s PTP status.

Example python code:

#!/usr/bin/env python3 import socket,sys PORT = int(sys.argv[1]) message = sys.argv[2] CID = socket.VMADDR_CID_HOST s = socket.socket(socket.AF_VSOCK, socket.SOCK_STREAM) s.connect((CID, PORT)) s.send(str.encode(message)) buf = s.recv(10000) if buf: print(buf.decode(),end='')

And call the script this way : python3 test.py 9999 STATUS and python3 test.py 9999 DETAILS

  • This value can then be propagated into the SmpSynch field of the Virtual IED’s SV messages.

With this mechanism, virtual IEDs effectively inherit the PTP status of the host, maintaining IEC 61850 compliance without needing the guest to manage its own PTP stack.