Unified hypercall interface across processors in virtualized computing systems

An example method of interfacing with a hypervisor in a computing system is described, which includes a processor having at least three hierarchical privilege levels including a third privilege level more privileged than a second privilege level, the second privilege level more privileged than a first privilege level. The method includes: identifying an input/output (I/O) space instruction, not supported by the processor, to be performed for backdoor communication between the hypervisor and guest software executing in a virtual machine (VM) managed by the hypervisor, the hypervisor executing at the third privilege level; writing one or more parameters to one or more registers of the processor that are mapped to one or more unsupported registers used by the I/O space instruction; writing a value indicative of the I/O space instruction to a designated register of the processor; executing an instruction, by the guest software executing at the first or second privilege level, which is trapped to the third privilege level.