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First of all, the actual MTU used by the kernel is the one set on the network adapter. The MTU for the network adapter can be set using the following command:

netsh.exe int ipv4 set subint <interface name> mtu=<mtu_value> store=persistent
# for example
netsh int ipv4 set subint "Local Area Connection" mtu=1200 store=persistent
# to show the current interface configuration
netsh.exe int ipv4 show subint

Secondly, if you have installed cloudbase-init, the MTUPlugin will query the DHCP server for the network in case and set it accordingly.

If the above does not apply to your case, you can set the device driver property Init.MTUSize using the registry keys. For example, in my Windows 2008R2 setup, using the Fedora VirtIO drivers version 0.1.102, the registry key with the MTU property value can be found at HKEYLOCALMACHINE\SYSTEM\CurrentControlSet\Control\Class{4D36E972-E325-11CE-BFC1-08002BE10318}\0007\MTU. In your scenario, the registry key can be found at another address, depending on the OS. You can search for the driver's exact registry keys using the HKEYLOCALMACHINE\SYSTEM\CurrentControlSet\Control\Class{4D36E972-E325-11CE-BFC1-08002BE10318}\0007\MatchingDeviceId value, which you can find in your netkvm.inf driver file. You can use the userdata cloudbase-init scripts to set the registry key with the wanted value.