VNC Troubleshoot Cannot access vdagent virtio channel /dev/virtio-ports/

On Centos 7, the vncserver started for each service is able to launch and users can login remotely. Within 1 minute, this service crashes and the usual command (see below) to start the vncserver fails.

$ sudo systemctl start vncserver@:1

The reason for this failure is capture in the command;

$ sudo systemctl status vncserver@:1

There were 2 items to take note,

1. Users were launching the vncviewer almost immediately upon the server boot up.
2. The following error appears "Cannot access vdagent virtio channel /dev/virtio-ports/" upon checking the status.

Whats on the Linux?

Centos Linux version 7.4.1708 (Core)
Tigervnc version 1.8.0

Spice [reference Fedora]

Spice aims to provide a complete open source solution for interaction with virtualized desktop.  The Simple Protocol for Independent Computing Environments (SPICE) is used for client-server communication. Spice adds a QXL display device to QEMU and provides drivers for this device for both X and Windows. It also provides a Spice server and VDAgent to handle connections and transmit the information via the SPICE protocol to clients. The Spice project provides a Linux, Windows, and HTML5 client. There are experimental Mac OS X and Android clients as well.

This /dev/virtio-ports is created in this feature that modifies the current single-port virtio-console device to guests running on top of qemu and kvm. It exposes multiple ports to the guest in the form of simple char devices for simple IO between the guest and host userspaces. It also allows for multiple such devices to be exposed, lifting the current single device restriction.

Somehow, these were installed on the server.

$ rpm -qa \*spice\*
spice-vdagent-0.14.0-14.el7.x86_64
spice-server-0.12.8-2.el7.1.x86_64
spice-gtk3-0.33-6.el7_4.1.x86_64
spice-glib-0.33-6.el7_4.1.x86_64

The RPM documentation on spice-vdagent states
"* Client mouse mode (no need to grab mouse by client, no mouse lag)
this is handled by the daemon by feeding mouse events into the kernel via uinput. This will only work if the active X-session is running a spice-vdagent process so that its resolution can be determined.
* Automatic adjustment of the X-session resolution to the client resolution
* Support of copy and paste (text and images) between the active X-session and the client"

I make an assumption that if no virtualisations are needed on the server, its a matter of deleting these packages. However, I have an Android Emulator that have dependency on the virtio-ports. Its still a question mark if VNC depends on spice-vdagent.

Solution

Step 1. Start SPICE agent.

At the command line on the host server, start the spice agent.
$ sudo systemctl start spice-vdagentd.service

Check and ensure that this service runs at boot up.
$ sudo systemctl show spice-vdagentd |grep enable
UnitFileState=enabled
UnitFilePreset=enabled

Step 2. Give host server time

Allow the Linux server to fully boot up all processed before attempting to start a VNC client session.


Done