Hypercallsit

Hypercalls, sometimes referred to as "hypercallsit", are a mechanism by which a virtual machine (VM) can communicate directly with the hypervisor. Unlike traditional system calls which a user-space program makes to the operating system kernel, hypercalls are a way for guest operating system code, or even applications running within a VM, to request services or information from the hypervisor layer. This layer is responsible for managing and allocating hardware resources to multiple VMs.

The primary purpose of hypercalls is to provide a more efficient and secure way for VMs to interact with the underlying hardware and hypervisor than relying solely on emulated hardware devices or complex software translation layers. When a VM needs to perform an operation that requires privileged access or direct interaction with hardware managed by the hypervisor, it can issue a hypercall. The hypervisor then intercepts this call, performs the requested operation on behalf of the VM, and returns the result.

Common examples of hypercall functionality include requesting memory allocation, scheduling CPU time, accessing network interfaces, or managing storage. By using hypercalls, hypervisors can offer a more streamlined and performant environment for virtualized systems. The specific set of available hypercalls varies between different hypervisor implementations, such as Xen, KVM, or VMware ESXi, each having its own defined interface for guest interaction. This direct communication channel enhances the overall efficiency of virtualization.