Virtual resources

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1 Virtual resources The main contribution of a VMM is handling virtual resources: it presents to the guest OS the interface if a real resource, understands the OS requests and emulates them. Virtualisation maps virtual resources to a set of managed ones, which are often physical and in rare cases are virtual, it can provide 1. The same interface (virtualisation): advantage is the quality of performances, but it is not always possible 2. A different interface (emulation): compatibility, interoperability and flexibility but also lower performances Physical resources can be mapped into virtual ones in three ways - One to one - One to many: sharing, the main benefits are isolation, flexibility and resource management, it can be implemented using time or space multiplexing - Many to one: aggregation, the main benefits are scalability, reliability and simplification The CPU CPU: the host must be able to execute the guest instruction, via emulation if there is a different ISA which prevents direct native execution Usually, a core is shared among any virtual machines by partitioning it; it should be avoided to assign each machine a number of virtual cores that is greater than the total number of cores of the physical device, as the only way in which it can be implemented time sharing a code and it leads to bad performances. However, when more than one virtual machine is running, the total amount of their core can be greater than the physical one, as usually the machines are in a waiting state and therefore the cores are sufficient to execute something on one machine at a time. It is also possible to set an execution cap, that is the maximum amount of time a core can be used by a machine, it allows to select less important machines and ensure they don’t interfere with the…