How KVM_RUN Powers Virtual Machines: The VM Exit Loop Explained

The ioctl(KVM_RUN) system call is the core mechanism that drives virtual machine execution in the Linux KVM hypervisor stack, with tools like QEMU and Libvirt essentially acting as scaffolding around this single looped call. To understand it, one must first grasp file descriptors — non-negative integers the kernel assigns to every open resource a process holds, whether a file, socket, or virtual CPU. Three descriptors (stdin, stdout, stderr) are pre-allocated by default, and all subsequent resources receive the next lowest available integer. Standard read() and write() calls handle raw data transfers but cannot perform device control operations, such as ejecting a CD drive or changing a serial port's baud rate. Unix addresses this gap through ioctl, a generic system call that lets processes send arbitrary commands to any device or resource identified by a file descriptor.