guesttohypervisor

Guesttohypervisor is a term used to describe the interface and communication path by which a guest operating system running inside a virtual machine interacts with the underlying hypervisor. It is not a formal standard, but a way to refer to the set of mechanisms that enable a guest to request privileged services or perform privileged operations through the hypervisor.

The guesttohypervisor interface encompasses several mechanisms. Hypercalls are explicit calls from the guest to the hypervisor to perform privileged actions such as memory management, device access, or scheduling decisions. Paravirtualization interfaces provide the guest with optimized, cooperative ways to interact with the hypervisor, often through virtio-like devices or other paravirtual drivers. In some configurations, device I/O and other operations are trapped by the hypervisor and emulated or mediated, a process commonly described as VM exits and subsequent handling by the hypervisor.

Typical responsibilities of the hypervisor in this path include managing guest physical and host memory, handling I/O requests, scheduling CPU time, and enforcing isolation between guests. The guesttohypervisor channel is designed to minimize overhead while maintaining strong security boundaries, ensuring that a guest cannot affect the host or other guests beyond its allocated rights.

Examples of how guesttohypervisor interfaces are implemented include KVM, Xen, Hyper-V, and VMware, each with its own hypercall or driver model. KVM uses kernel-based interfaces and hypercalls exposed through the host kernel; Xen relies on a set of Xen hypercalls from DomU guests; Hyper-V and VMware provide their own mechanisms for guest-to-hypervisor communication through synthetic devices and VM-exit handling.

In summary, guesttohypervisor refers to the guest’s access path to hypervisor services, central to virtualization performance, functionality, and isolation.