Virtio Console

The Virtio Console device is essentially a virtual console device used for input and output of data, and can serve as a virtual terminal for other virtual machines. Currently, hvisor supports the VIRTIO_CONSOLE_F_SIZE and VIRTIO_F_VERSION_1 features.

Description of the Virtio Console Device

For the Virtio console device, the type field in the VirtIODevice structure is VirtioTConsole, vqs_len is 2, indicating that there are two Virtqueues, the receive virtqueue and the transmit virtqueue, used for receiving and sending data on port 0. The dev pointer points to the virtio_console_dev structure that describes specific information about the console device. In this structure, config represents the number of rows and columns of the console, master_fd is the file descriptor of the pseudo-terminal master device connected to the device, rx_ready indicates whether the receive queue is available, and event is used for the event monitor thread to monitor readable events of the pseudo-terminal master device through epoll.

typedef struct virtio_console_dev {
    ConsoleConfig config;
    int master_fd;
    int rx_ready;
    struct hvisor_event *event;
} ConsoleDev;

Pseudo Terminal

A terminal is essentially an input and output device. In the early days of computing, terminals were called teleprinters (TTY). Now, terminals have become a virtual device on computers, connected by terminal emulation programs to graphics card drivers and keyboard drivers to implement data input and output. There are two different implementations of terminal emulation programs: one is as a kernel module in Linux, exposed to user programs as /dev/tty[n]; the other is as an application running in Linux user space, known as a pseudo-terminal (PTY).

Pseudo-terminals themselves are not the focus of this article, but the two devices used by pseudo-terminals—PTY master and PTY slave—are used in this article to implement the Virtio Console device.

Applications can obtain an available PTY master by executing posix_openpt, and get the corresponding PTY slave through the ptsname function. A TTY driver connecting PTY master and PTY slave will copy data between the master and slave. Thus, when a program writes data to the master (or slave), the program can read the same data from the slave (or master).

Overall Design of Virtio Console

The Virtio Console device, as a daemon on Root Linux, opens a PTY master during device initialization and outputs the path of the corresponding PTY slave /dev/pts/x to the log file for screen session connection. Meanwhile, the event monitor thread in the Virtio daemon monitors the readable events of the PTY slave so that the PTY master can promptly receive user input data.

When a user executes screen /dev/pts/x on Root Linux, a screen session is created on the current terminal, connecting the device corresponding to the PTY slave /dev/pts/x, and taking over the input and output of the current terminal. The implementation structure diagram of the Virtio Console device is shown below.

virtio_console

Input Commands

When a user types commands on the keyboard, the input characters are passed to the Screen session through the terminal device. The Screen session writes the characters into the PTY slave. When the event monitor thread detects through epoll that the PTY slave is readable, it calls the virtio_console_event_handler function. This function reads from the PTY slave and writes the data into the Virtio Console device's Receive Virtqueue, and sends an interrupt to the corresponding virtual machine.

The corresponding virtual machine, after receiving the interrupt, passes the received character data to the Shell through the TTY subsystem for interpretation and execution.

Display Information

When a virtual machine using the Virtio Console driver wants to output information through the Virtio Console device, the Virtio Console driver writes the data to be output into the Transmit Virtqueue and writes to the QueueNotify register in the MMIO area to notify the Virtio Console device to handle the IO operation.

The Virtio Console device reads from the Transmit Virtqueue, retrieves the data to be output, and writes it into the PTY master. The Screen session then retrieves the data to be output from the PTY slave and displays the output information on the monitor through the terminal device.