virtio

Virtio is a virtualization standard for network and disk I/O devices that enables efficient, paravirtualized communication between a guest operating system and the hypervisor or host back end. It was designed to provide a simple, high-performance interface that avoids the overhead of fully emulated hardware, while remaining portable across different virtualization platforms.

In a typical virtio setup, a guest driver (the front end) exposes a set of virtio devices and negotiates a feature set with the host back end. Communication occurs through virtqueues, which are ring buffers of descriptors describing I/O requests. The host back end processes those requests and places results back in the rings. A lightweight transport, commonly implemented as a PCI-based device in the guest, carries the descriptor rings and notifications between guest and host.

Virtio includes support for optimizations such as the vhost and vhost-user mechanisms. Vhost moves the back end into the host kernel to reduce context switches and improve throughput, while vhost-user enables back ends to run in userspace and communicate with the guest via sockets. These approaches help achieve higher performance and scalability for virtual I/O workloads.

Common virtio devices include virtio-net for networking, virtio-blk for block storage, virtio-scsi for SCSI devices, virtio-balloon for memory ballooning, virtio-rng for random number generation, virtio-serial for communication channels, and virtio-fs for shared filesystems. The virtio standard is maintained as an open standard and has seen widespread adoption in Linux-based virtualization environments and hypervisors such as QEMU/KVM, Xen, and others.

The standardification and broad support have made virtio a de facto cornerstone of modern virtualized I/O, balancing compatibility with performance and efficiency.