Virtual I/O (VIRTIO)
Overview
Virtual I/O (VIRTIO) is a protocol used for communication with various devices, typically used in virtualized environments. Its main goal is to provide an efficient and standardized mechanism for interfacing with virtual devices from within a virtual machine. The communication relies on virtqueues and standard transfer methods like PCI or MMIO.
Concepts
Virtio defines various components used during communication and initialization. It specifies both the host (named “device” in the specification) and guest (named “driver” in the specification) sides. Currently Zephyr can only work as a guest. On top of the facilities exposed by the Virtio driver, a driver for a specific device (e.g. network card) can be implemented.
A high-level overview of a system with a Virtio device is shown below.
Virtual I/O overview
Configuration space
Each device provides configuration space, used for initialization and configuration. It allows selection of device and driver features, enabling specific virtqueues and setting their addresses. Once the device is configured, most of its configuration cannot be changed without resetting the device. The exact layout of the configuration space depends on the transfer method.
Driver and device features
The configuration space provides a way to negotiate feature bits, determining some non-mandatory capabilities of the devices. The exact available feature bits depend on the device and platform.
Device-specific configuration
Some of the devices offer device-specific configuration space, providing additional configuration options.
Virtqueues
The main mechanism used for transferring data between host and guest is a virtqueue. Specific devices have different numbers of virtqueues, for example devices supporting bidirectional transfer usually have one or more tx/rx virtqueue pairs. Virtio specifies two types of virtqueues: split virtqueues and packed virtqueues. Zephyr currently supports only split virtqueues.
Split virtqueues
A split virtqueue consists of three parts: descriptor table, available ring and used ring.
The descriptor table holds descriptors of buffers, that is their physical addresses, lengths and flags. Each descriptor is either device writeable or driver writeable. The descriptors can be chained, creating descriptor chains. Typically a chain begins with descriptors containing the data for the device to read and ends with the device writeable part, where the device places its response.
The main part of the available ring is a circular buffer of references (in the form of indexes) to the descriptors in the descriptor table. Once the guest decides to send the data to the host, it adds the index of the head of the descriptor chain to the top of the available ring.
The used ring is similar to the available ring, but it’s used by the host to return descriptors to the guest. In addition to storing descriptor indexes, it also provides information about the amount of data written to them.
Common Virtio libraries
Zephyr provides an API for interfacing with Virtio devices and virtqueues, which allows performing necessary operations over the lifetime of the Virtio device.
Device initialization
Once the Virtio driver finishes performing low-level initialization common to the all devices using a given transfer method, like finding device on the bus and mapping Virtio structures, the device specific driver steps in and performs the next stages of initialization with the help of the Virtio API.
The first thing the device-specific driver does is feature bits negotiation. It uses virtio_read_device_feature_bit()
to determine which features the device offers, and then selects the ones it needs using virtio_write_driver_feature_bit().
After all required features have been selected, the device-specific driver calls virtio_commit_feature_bits(). Then, virtqueues
are initialized with virtio_init_virtqueues(). This function enumerates the virtqueues, invoking the provided callback
virtio_enumerate_queues to determine the required size of each virtqueue. Initialization process is finalized by calling
virtio_finalize_init(). From this point, if none of the functions returned errors, the virtqueues are operational. If the
specific device provides one, the device-specific config can be obtained by calling virtio_get_device_specific_config().
Virtqueue operation
Once the virtqueues are operational, they can be used to send and receive data. To do so, the pointer to the nth
virtqueue has to be acquired using virtio_get_virtqueue(). To send data consisting of a descriptor chain,
virtq_add_buffer_chain() has to be used. Along the descriptor chain, it takes pointer to the callback that
will be invoked once the device returns the given descriptor chain. After that, the virtqueue has to be notified using
virtio_notify_virtqueue() from the Virtio API.