Abstract: In order to solve the semantic gap problem between virtual machine monitor (VMM) and guest virtual machine (VM), an up-call mechanism is proposed, with which the service request is launched by VMM, and the guest-VM is responsible to provide response to the request from VMM. This makes it possible for VMM to synchronously call guest-VM services to get guest semantic information, bringing convenience for monitor agents constructed out of guest-VM address space to get guest information exactly. The up-call mechanism implemented as a synchronous communication channel is able to make up the problem of duplicate definitions and implementations inside VMM brought by semantic reconstruction. A prototype system on the para-virtualization platform of Xen is implemented. The result shows that the method proposed in this paper is able to help VMM to call guest functions to get guest services and semantic information instantly.