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Chapter 6. Security in Open Source Software
guest mode is used for VMs. In the guest mode, the virtualised operating system still sees the
four privilege levels of x86 CPUs in protected mode. Additionally, in guest mode, a virtualised
operating system can be executed in the highest privilege level. This does not mean a security
risk because in guest mode all privileged executions are trapped and the control returned to
the hypervisor, which handles then the execution in the host mode. The advantage of the two
modes is that the hypervisor does not need to provide a software implemented virtual CPU.
This can drastically reduce the computational overhead [1].
The usage of host and guest mode still leaves an overhead problem with the memory
management inside a VM: The memory management unit (MMU) of a CPU, which is used for
paging and segmentation [77, pp. 353-453], has to be provided by the hypervisor as software
implementation. For this problem also exists a hardware assisted solution because a virtual
MMU can also be provided by x86 CPUs by a feature known as Rapid Virtualisation Indexing
(RVI) or Extended Page Table (EPT) [1]. The combination of both CPU features in a hypervisor
may provide the best performance or rather overhead reduction that is currently available.
6.2.4. Process Communication
The limitation of the communication to network mechanisms (Prob.4) could be directly
integrated in the software design to reduce the drawbacks. For example, CORBA [41] can
be used for user or supplier implementations. Simply and generally expressed, the typical
CORBA development case is that an interface [41] is defined, which has to be implemented in
an object-oriented programming language, e.g. C++ or Java. The implementation is called
Servant [41] and is a software class. Additionally, CORBA provides a so-called Proxy [41],
which is generated from the interface and can be used to access the Servant. Figure 6.4 shows
a simplified example of a CORBA development case.
Interface
void Method()
implements
Servant
void Method
generates
calls
Proxy
void Method()
Figure 6.4.: Simple CORBA usage example
This Servant-Proxy mechanism could be directly included in the software model by, for
example, defining each part of the open model (from Figure 6.3) that could or should be
(re)implemented as supplier parts as CORBA interface. Consequently, each supplier implementation
would be done by implementing a CORBA Servant for a certain interface. Besides
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