• How to measure cache usage

[1] Cache load measurements create very effective covert channels between cooperating processes running in different VMs. 

In practise, this is not a major threat for current deployments since in most cases the cooperating processes can simply talk to each other over a network. 

However, covert channels become significant when communication is forbidden by information flow control (IFC) mechanisms such as sandboxing and IFC kernels. The latter is a promissing emerging approach to improving security (e.g., web-server functionality). 

[1] explains more on the covert-channel in Section 8.1.