Ph.D. - geht es zur Homepage der Informatik des Fachbereiches 3 ...

6.2. Hardware Virtualisation
in a virtual machine. This means, in the worst case other, components only have to
cope with the unavailability of other components in the corresponding virtual machine
or false data delivered or sent by them.
Adv.2: Each hardware device and its software interfaces are protected from direct access
from software components executed in a virtual machine. Several implementations of
hardware virtualisation hypervisors already provide for certain bus-types, like USB,
the possibility to configure the accessible devices in a certain virtual machine.
Adv.3: Virtualisation reduces dramatically the costs of partitioning analysis because software
components executed in a virtual machine can only have an impact to other components
over defined and known channels or interfaces.
Adv.4: Platform dependent source code can be executed in any (proprietary) operating system
and does not influence the choice of the host operating system.
Adv.5: Costs for hardware could be reduced because one hardware platform can host several
operating systems.
Adv.6: Virtual machines can be easily maintained or (re)installed because their hard disk
images can simply be copied.
There exist also some disadvantages and unsolved problems:
Prob.1: Hardware virtualisation itself does not implicitly provide any mechanism to protect
the bandwidth of real hardware resources, like the CPU(s) or the network interface(s)
on the host system. A hypervisor process can be assigned to a certain CPU (in
a multi-core system), which would at least protect the bandwidth of other cores.
Otherwise, if a (faulty or malicious) implementation consumes too much bandwidth,
it may influence any other executed implementation and limit its functionality.
Prob.2: Current open source hypervisors implementations are not too complex to be validated
and certified [15], but which is currently for typical multi-user operating systems
impossible because of their complexity. This means although the hypervisor can be
assumed to be trustworthy that the operating system cannot. Therefore, the operating
system itself may cause security problems.
Prob.3: Overhead for computation, main memory, and storage space is generated because
each virtual machine needs physical memory, each virtualised operating system needs
additional computation time, and each virtual operating system needs physical storage
space.
Prob.4: Access to shared memory outside a VM is not possible even if it is desired. Communication
with processes outside a VM can only be done by mechanisms also used for
communication between different computers / hardware platforms that is typically a
network or memory mapped files on a shared file system.
Prob.5: Direct access to hardware devices or their interfaces is only possible for certain
bus-types.
Possible solutions for the problems Prob.1 till Prob.5 are introduced in the following subsections.
71