Every VM must associate with a unique vTPM instance. The vTPM instance number is prepended on theserver side so that virtual machines cannot forge packet and try to get access to a vTPM instance that is not associated with them. A command’s originating virtual machine can be determined from the unique interrupt number raised by each client-side driver.Since a TPM holds unique persistent state with secret information such as keys, it is necessary that a virtual machine be associated with its virtual TPM instance throughout the lifetime of the virtual machine. To keep this association over time, we maintain a list of virtual- machine- to virtual- TPM- instance associations.