Beyond 4 virtual serial ports with Linux guest

[firthmj2]

In all recent hardware versions, it is possible to add up to 32 virtual serial ports to a VM.

I can successfully add 8 serial ports to a Debian Linux 11 VM using the vCenter GUI.

However, only the first 4 serial ports are detected by the guest.

Is it possible to access the other serial ports from a Linux guest, and, if so, is there any information out there on how to do this?

I've tried quite a few Google searches, but cannot find any information on this.

[firthmj2 · Accepted solution]

To answer my own question, by default the Linux serial driver only looks for 4 ports.

It is possible to change that via a boot time parameter. For Debian 11, this was achieved by adding "8250.nr_uarts=8" to GRUB_CMDLINE_LINUX in /etc/default/grub

Now the Linux guest sees all the configured serial ports.

According to this page, for some systems you might not need the "8250." at the beginning, but I did on Debian 11.

https://tldp.org/HOWTO/Serial-HOWTO-15.html#ss15.3