To configure the port MTU while operating in SmartNIC mode, you must restrict the external host port ownership by issuing the following command on the DPU:
mlxprivhost -d /dev/mst/<pciconf0 device> r --disable_port_owner
Server cold reboot is required for this restriction to take effect.
Once the host is restricted, the port MTU is configured by changing the MTU of the uplink representor (p0 or p1).