Virtio-console¶

The Virtio-console is a simple device for data input and output. The console’s virtio device ID is 3 and can have from 1 to 16 ports. Each port has a pair of input and output virtqueues used to communicate information between the Front End (FE) and Back end (BE) drivers. Currently the size of each virtqueue is 64 (configurable in the source code). The FE driver will place empty buffers for incoming data onto the receiving virtqueue, and enqueue outgoing characters onto the transmitting virtqueue.

A Virtio-console device has a pair of control IO virtqueues as well. The control virtqueues are used to communicate information between the device and the driver, including: ports being opened and closed on either side of the connection, indication from the host about whether a particular port is a console port, adding new ports, port hot-plug/unplug, indication from the guest about whether a port or a device was successfully added, or a port opened or closed.

The virtio-console architecture diagram in ACRN is shown below.

Figure 226 Virtio-console architecture diagram

Virtio-console is implemented as a virtio legacy device in the ACRN device model (DM), and is registered as a PCI virtio device to the guest OS. No changes are required in the frontend Linux virtio-console except that the guest (UOS) kernel should be built with CONFIG_VIRTIO_CONSOLE=y.

The virtio console FE driver registers a HVC console to the kernel if the port is configured as console. Otherwise it registers a char device named /dev/vportXpY to the kernel, and can be read and written from the user space. There are two virtqueues for a port, one is for transmitting and the other is for receiving. The FE driver places empty buffers onto the receiving virtqueue for incoming data, and enqueues outgoing characters onto the transmitting virtqueue.

The virtio console BE driver copies data from the FE’s transmitting virtqueue when it receives a kick on the virtqueue (implemented as a vmexit). The BE driver then writes the data to the backend, and can be implemented as PTY, TTY, STDIO, and a regular file. BE driver uses mevent to poll the available data from the backend file descriptor. When new data is available, the BE driver reads it to the receiving virtqueue of the FE, followed by an interrupt injection.

The feature bits currently supported by the BE device are:

Table 10 Feature bits supported by BE drivers¶
VTCON_F_SIZE(bit 0)	configuration columns and rows are valid.
VTCON_F_MULTIPORT(bit 1)	device supports multiple ports, and control virtqueues will be used.
VTCON_F_EMERG_WRITE(bit 2)	device supports emergency write.

Virtio-console supports redirecting guest output to various backend devices. Currently the following backend devices are supported in ACRN device model: STDIO, TTY, PTY and regular file.

The device model configuration command syntax for virtio-console is:

virtio-console,[@]stdio|tty|pty|file:portname[=portpath]\
   [,[@]stdio|tty|pty|file:portname[=portpath]]

Preceding with @ marks the port as a console port, otherwise it is a normal virtio serial port
The portpath can be omitted when backend is stdio or pty
The stdio/tty/pty is tty capable, which means TAB and BACKSPACE are supported, as on a regular terminal
When tty is used, please make sure the redirected tty is sleeping, (e.g., by sleep 2d command), and will not read input from stdin before it is used by virtio-console to redirect guest output.
Claiming multiple virtio serial ports as consoles is supported, however the guest Linux OS will only use one of them, through the console=hvcN kernel parameter. For example, the following command defines two backend ports, which are both console ports, but the frontend driver will only use the second port named pty_port as its hvc console (specified by console=hvc1 in the kernel command line):
```
-s n,virtio-console,@tty:tty_port=/dev/pts/0,@pty:pty_port \
-B "root=/dev/vda2 rw rootwait maxcpus=$2 nohpet console=hvc1 console=ttyS0 ..."
```

Console Backend Use Cases¶

The following sections elaborate on each backend.

STDIO¶

Add a pci slot to the device model (acrn-dm) command line:
```
-s n,virtio-console,@stdio:stdio_port
```
Add the console parameter to the guest OS kernel command line:
```
console=hvc0
```

PTY¶

Add a pci slot to the device model (acrn-dm) command line:
```
-s n,virtio-console,@pty:pty_port
```
Add the console parameter to the guest os kernel command line:
```
console=hvc0
```
One line of information, such as shown below, will be printed in the terminal after acrn-dm is launched (/dev/pts/0 may be different, depending on your use case):
Use a terminal emulator, such as minicom or screen, to connect to the tty node:
```
minicom -D /dev/pts/0
```
or
```
screen /dev/pts/0
```

TTY¶

Identify your tty that will be used as the UOS console:
- If you’re connected to your device over the network via ssh, use the linux tty command, and it will report the node (may be different in your use case):
```
/dev/pts/0
sleep 2d
```
- If you do not have network access to your device, use screen to create a new tty:
```
screen
tty
```
  you will see (depending on your use case):
```
/dev/pts/0
```
  Prevent the tty from responding by sleeping:
```
sleep 2d
```
  and detach the tty by pressing CTRL-A d.
Add a pci slot to the device model (acrn-dm) command line (changing the dev/pts/X to match your use case):
```
-s n,virtio-console,@tty:tty_port=/dev/pts/X
```
Add the console parameter to the guest OS kernel command line:
```
console=hvc0
```
Go back to the previous tty. For example, if you’re using screen, use:
```
screen -ls
screen -r <pid_of_your_tty>
```

FILE¶

The File backend only supports console output to a file (no input).

Add a pci slot to the device model (acrn-dm) command line, adjusting the </path/to/file> to your use case:
```
-s n,virtio-console,@file:file_port=</path/to/file>
```
Add the console parameter to the guest OS kernel command line:
```
console=hvc0
```