QEMU has a lot of interfaces (like command line options or HMP commands) and old features (like certain devices) which are considered as deprecated since other more generic or better interfaces/features have been established instead. While the QEMU developers are generally trying to keep each QEMU release compatible with the previous ones, the old legacy sometimes gets into the way when developing new code and/or causes quite some burden of maintaining it.

If you used QEMU in the past, you are probably familiar with the -net command line option, which can be used to configure a network connection for the guest, or with with the -netdev option, which configures a network back-end. Yet, QEMU v2.12 introduces a third way to configure NICs, the -nic option.

Sometimes, there is a VM disk image whose contents you want to manipulate without booting the VM. One way of doing this is to use libguestfs, which can boot a minimal Linux VM to provide the host with secure access to the disk’s contents. For example, guestmount allows you to mount a guest filesystem on the host, without requiring root rights.