Multi gem5 is an extension to gem5 to enable parallel simulation of a

distributed system (e.g. simulation of a pool of machines

connected by Ethernet links). A multi gem5 run consists of seperate gem5

processes running in parallel (potentially on different hosts/slots on

a cluster). Each gem5 process executes the simulation of a component of the

simulated distributed system (e.g. a multi-core board with an Ethernet NIC).
The patch implements the "distributed" Ethernet link device

(dev/src/multi_etherlink.[hh.cc]). This device will send/receive

(simulated) Ethernet packets to/from peer gem5 processes. The interface

to talk to the peer gem5 processes is defined in dev/src/multi_iface.hh and

in tcp_iface.hh.
There is also a central message server process (util/multi/tcp_server.[hh,cc])

which acts like an Ethernet switch and transfers messages among the gem5 peers.
A multi gem5 simulations can be kicked off by the util/multi/gem5-multi.sh

wrapper script.
Checkpoint support will follow in a subsequent patch.