1. A virtual machine system, comprising: a memory; and a processor configured to execute a host VMM (Virtual Machine Monitor), first-stage VMs (Virtual Machine) on the host VMM, a guest VMM of the first-stage VMs, and a second-stage VM generated on the guest VMM, the processor being coupled to the memory, wherein the processor is configured to: perform a first determination as to whether to execute a physical VMENTRY caused by a virtual VMENTRY of the second-stage VM and a second determination as to whether a physical VMEXIT is detected as a result of a virtual VMEXIT of the second-stage VM, and calculate an execution time of the second-stage VM based on results of the first determination and the second determination, wherein the physical VMENTRY is a VMENTRY executed by the host VMM, wherein the virtual VMENTRY is a VMENTRY executed by the guest VMM, wherein the physical VMEXIT is a trigger of starting the host VMM, and wherein the virtual VMEXIT is an end of execution of the second-stage VM.