====
Changes from V1:
* fixed missing change to error log message pointed out by abologna
* added a validation check to assure that shared memory is enabled
if there is a type='vhostuser' interface in the domain definition
* included a patch documenting differences between type='user' SLIRP
and passt behaviors (because I had to do it anyway, and the
reorganization made documenting type='vhostuser' passt slightly
easier.
* added documentation for type='vhostuser' backend type='passt'
=====
passt (https://passt.top) provides a method of connecting QEMU virtual
machines to the external network without requiring special privileges
or capabilities of any participating processes - even libvirt itself
can run unprivileged and create an instance of passt (which *always*
runs unprivileged) that is then connected to the qemu process (and
thus the virtual machine) with a unix socket.
Originally passt used its own protocol for this socket, sending both
control messages and data packets over the socket. This works, and is
already much more efficient than the previously
only-unprivileged-networking-solution slirp.
But recently passt added support for using the vhost-user protocol for
communication between the passt process (which is connected to the
external network) and the QEMU process (and thus the VM). vhost-user
also uses a unix socket, but only for control plane messages - all
data packets are "sent" between the VM and passt process via a shared
memory region. This is unsurprisingly much more efficient.
