Where is entry of hypercalls in KVM

Introduction

Kernel-based Virtual Machine (KVM) is a virtualization infrastructure for Linux that allows multiple operating systems to run simultaneously on a single host machine. It is a popular open-source virtualization technology that leverages hardware virtualization support, such as Intel VT-x and AMD-V, to run multiple virtual machines on same physical hardware. KVM has a modular architecture, with different components responsible for different aspects of virtualization. In this article, we will explore entry of hypercalls in KVM.

Hypercalls in KVM

A hypercall is a privileged instruction that allows a virtual machine to interact with hypervisor, which is software layer that controls access to physical hardware. Hypercalls are similar to system calls in a non-virtualized environment, in that they allow a virtual machine to request services from hypervisor, such as creating or destroying virtual devices, allocating memory, and configuring virtual network interfaces. Hypercalls are an essential component of KVM virtualization infrastructure, as they allow virtual machines to access physical hardware in a secure and controlled manner.

In KVM, hypercalls are implemented using a mechanism called KVM API. KVM API is a set of system calls and data structures that allow a user-space process to interact with KVM kernel module. KVM kernel module is responsible for managing virtual machines and providing virtualized hardware interfaces to guest operating systems. KVM API allows a user-space process to create and configure virtual machines, attach virtual devices, and send hypercalls to hypervisor.

The entry of Hypercalls in KVM

The entry of hypercalls in KVM is a critical component of virtualization infrastructure, as it determines how a virtual machine communicates with hypervisor. In KVM, entry of hypercalls is implemented using a mechanism called vCPU run loop. vCPU run loop is a piece of code that runs in context of a virtual CPU and is responsible for executing instructions on behalf of guest operating system.

When a virtual machine sends a hypercall to hypervisor, vCPU run loop is interrupted, and control is transferred to hypervisor. hypervisor then processes hypercall and performs requested action, such as creating a virtual device or allocating memory. Once hypercall has been processed, control is returned to vCPU run loop, and guest operating system continues executing as if nothing happened.

The vCPU run loop is implemented in KVM kernel module and is responsible for managing execution of guest code in a virtualized environment. vCPU run loop is a loop that executes guest instructions until an interrupt or exception occurs. When an interrupt or exception occurs, control is transferred to hypervisor, which decides how to handle interrupt or exception. If interrupt or exception is a hypercall, hypervisor processes hypercall and returns control to vCPU run loop.

Examples of Hypercalls in KVM

Hypercalls in KVM are implemented using a set of hypercall numbers that are defined in KVM API. These hypercall numbers are used by guest operating system to specify type of hypercall it is making. Here are some examples of hypercalls in KVM −

• KVM_CREATE_VM − This hypercall is used to create a new virtual machine. It takes no arguments and returns a file descriptor that can be used to interact with newly created virtual machine.

• KVM_CREATE_VCPU − This hypercall is used to create a new virtual CPU for a virtual machine. It takes an argument that specifies ID of virtual CPU and returns a file descriptor that can be used to interact with virtual CPU.

• KVM_SET_USER_MEMORY_REGION − This hypercall is used to set up a memory region in a virtual machine. It takes arguments that specify guest physical address and size of memory region, as well as a file descriptor that represents memory area in host operating system.

• KVM_IRQFD − This hypercall is used to connect a virtual interrupt to a file descriptor in host operating system. It takes arguments that specify interrupt number, file descriptor, and type of interrupt.

• KVM_CREATE_IRQCHIP − This hypercall is used to create a virtual interrupt controller for a virtual machine. It takes no arguments and returns a file descriptor that can be used to interact with interrupt controller.

Advanced Topics

In addition to basics of hypercalls in KVM, there are also several advanced topics related to entry of hypercalls that are worth exploring. Here are a few examples −

Handling Hypercalls in Hypervisor

When a hypercall is sent from a virtual machine to hypervisor, hypervisor needs to handle request and perform requested action. This can involve a significant amount of processing, especially for complex hypercalls such as creating a new virtual machine. Hypervisors such as KVM are designed to handle hypercalls efficiently, but there are still potential bottlenecks and performance considerations to keep in mind.

One way to optimize hypercall handling in hypervisor is to use event-driven processing, where hypervisor waits for hypercalls to arrive and then processes them as quickly as possible. This can be more efficient than continuously polling for hypercalls, as it reduces amount of CPU time wasted on idle polling.

Another technique for optimizing hypercall handling is to use a dedicated thread pool for handling hypercalls. This allows hypervisor to prioritize hypercall handling over other tasks and ensures that hypercalls are processed as quickly as possible.

Hypercall Interception

Another advanced topic related to entry of hypercalls in KVM is hypercall interception. Hypercall interception is process of intercepting hypercalls and modifying them before they are processed by hypervisor. This can be useful for a variety of purposes, such as adding new virtual device drivers, implementing custom security policies, or modifying behavior of virtual machines.

Hypercall interception can be implemented using several techniques, including modifying guest operating system to redirect hypercalls to a custom handler, using a kernel module to intercept hypercalls at system call level, or using a hypervisor-level technique such as KVM's kernel-based Virtual Machine Introspection (kVMI) interface.

Hypercall Emulation

Hypercall emulation is another advanced topic related to entry of hypercalls in KVM. Hypercall emulation is process of emulating hypercalls that are not natively supported by hypervisor. This can be necessary if a virtual machine is running an older operating system that does not support newer hypercalls, or if a virtual machine is running an operating system that is not fully compatible with hypervisor.

Hypercall emulation can be implemented using a variety of techniques, such as modifying guest operating system to use a different set of hypercalls, using a translation layer to map unsupported hypercalls to supported hypercalls, or using a dynamic binary translator to emulate hypercalls at runtime.

Conclusion

In conclusion, entry of hypercalls in KVM is a critical component of virtualization infrastructure that allows virtual machines to interact with hypervisor in a secure and controlled manner. Hypercalls are implemented using a mechanism called vCPU run loop, which is responsible for executing guest instructions and handling interrupts and exceptions. KVM API defines a set of hypercall numbers that are used by guest operating system to specify type of hypercall it is making. Examples of hypercalls in KVM include creating a new virtual machine, creating a virtual CPU, setting up a memory region, and connecting a virtual interrupt to a file descriptor. By understanding entry of hypercalls in KVM, developers and administrators can better understand how virtual machines interact with hypervisor and how to configure and manage virtualized environments.