Computer system provided with hypervisor

A computer system comprising:

• a hypervisor managing at least one virtual computer;
• and
• a composite I/O (Input/Output) controller,
• wherein the composite I/O controller is provided with a plurality of I/O controllers and a plurality of I/O ports that can be shared by each I/O controller,
• at least one I/O controller of the plurality of I/O controllers is assigned to any one of virtual computers,
• any one of I/O devices and any one of I/O controllers are coupled to each I/O port, and
• the hypervisor executes the processes (a) to (c):
• (a) acquiring an I/O command that has been issued from the virtual computer;
• (b) judging whether or not a target of an I/O that complies with the I/O command is an unassigned port associated device that is a device associated with an I/O port that is coupled to an I/O controller that is not assigned to a virtual computer that is an issuing source of the I/O command;
• and
• (c) in the case in which the result of the judgment of (b) is positive, not executing an I/O to the unassigned port associated device, and returning a virtual execution result to the virtual computer that is an issuing source of the I/O command.