  This paper studies the problem of reasoning about flow security properties in
virtualised computing networks with mobility from perspective of formal
language. We propose a distributed process algebra CSP_{4v} with security
labelled processes for the purpose of formal modelling of virtualised computing
systems. Specifically, information leakage can come from observations on
process executions, communications and from cache side channels in the
virtualised environment. We describe a cache flow policy to identify such
flows. A type system of the language is presented to enforce the flow policy
and control the leakage introduced by observing behaviours of communicating
processes and behaviours of virtual machine (VM) instances during accessing
shared memory cache.
