vUART Configuration


The virtual universal asynchronous receiver-transmitter (vUART) supports two functions: one is the console, the other is communication. vUART only works on a single function.

Currently, only two vUART configurations are added to the hypervisor/scenarios/<xxx>/vm_configuration.c file, but you can change the value in it.

.vuart[0] = {
        .type = VUART_LEGACY_PIO,
        .addr.port_base = INVALID_COM_BASE,
.vuart[1] = {
        .type = VUART_LEGACY_PIO,
        .addr.port_base = INVALID_COM_BASE,

vuart[0] is initiated as the console port.

vuart[1] is initiated as a communication port.

Console enable list

Scenarios vm0 vm1 vm2 vm3
SDC SOS (vuart enable) Post-launched Post-launched  
SDC2 SOS (vuart enable) Post-launched   Post-launched
Hybrid Pre-launched (Zephyr) (vuart enable) SOS (vuart enable) Post-launched  
Industry SOS (vuart enable) Post-launched Post-launched (vuart enable) Post-launched
Logic_partition Pre-launched (vuart enable) Pre-launched RTVM (vuart enable) Post-launched RTVM  

How to configure a console port

To enable the console port for a VM, change only the port_base and irq. If the irq number is already in use in your system (cat /proc/interrupt), choose another irq number. If you set the .irq =0, the vuart will work in polling mode.

  • COM1_BASE (0x3F8) + COM1_IRQ(4)
  • COM2_BASE (0x2F8) + COM2_IRQ(3)
  • COM3_BASE (0x3E8) + COM3_IRQ(6)
  • COM4_BASE (0x2E8) + COM4_IRQ(7)


.vuart[0] = {
                     .type = VUART_LEGACY_PIO,
                     .addr.port_base = COM1_BASE,
                     .irq = COM1_IRQ,

How to configure a communication port

To enable the communication port, configure vuart[1] in the two VMs that want to communicate.

The port_base and irq should differ from the vuart[0] in the same VM.

t_vuart.vm_id is the target VM’s vm_id, start from 0. (0 means VM0)

t_vuart.vuart_id is the target vuart index in the target VM. start from 1. (1 means vuart[1])


/* VM0 */
/* VM1 */
.vuart[1] = {
                     .type = VUART_LEGACY_PIO,
                     .addr.port_base = COM2_BASE,
                     .irq = COM2_IRQ,
                     .t_vuart.vm_id = 2U,
                     .t_vuart.vuart_id = 1U,
/* VM2 */
.vuart[1] = {
                     .type = VUART_LEGACY_PIO,
                     .addr.port_base = COM2_BASE,
                     .irq = COM2_IRQ,
                     .t_vuart.vm_id = 1U,
                     .t_vuart.vuart_id = 1U,

Communication vUART enable list

Scenarios vm0 vm1 vm2 vm3
SDC SOS Post-launched Post-launched  
SDC2 SOS Post-launched Post-launched Post-launched
Hybrid Pre-launched (Zephyr) (vuart enable COM2) SOS (vuart enable COM2) Post-launched  
Industry SOS (vuart enable COM2) Post-launched Post-launched RTVM (vuart enable COM2) Post-launched
Logic_partition Pre-launched Pre-launched RTVM    

Launch script

  • -s 1:0,lpc -l com1,stdio This option is only needed for WaaG and VxWorks (and also when using OVMF). They depend on the ACPI table, and only acrn-dm can provide the ACPI table for UART.
  • -B ” ….,console=ttyS0, …” Add this to the kernel-based system.

Test the communication port

After you have configured the communication port in hypervisor, you can access the corresponding port. For example, in Clear Linux:

  1. With echo and cat

    On VM1: # cat /dev/ttyS1

    On VM2: # echo "test test" > /dev/ttyS1

    you can find the message from VM1 /dev/ttyS1.

    If you are not sure which port is the communication port, you can run dmesg | grep ttyS under the Linux shell to check the base address. If it matches what you have set in the vm_configuration.c file, it is the correct port.

  2. With minicom

    Run minicom -D /dev/ttyS1 on both VM1 and VM2 and enter test in VM1’s minicom. The message should appear in VM2’s minicom. Disable flow control in minicom.

  3. Limitations

    • The msg cannot be longer than 256 bytes.
    • This cannot be used to transfer files because flow control is not supported so data may be lost.

vUART design

Console vUART


Communication vUART (between VM0 and VM1)


COM port configurations for Post-Launched VMs

For a post-launched VM, the acrn-dm cmdline also provides a COM port configuration:

-s 1:0,lpc -l com1,stdio

This adds com1 (0x3f8) and com2 (0x2f8) modules in the Guest VM, including the ACPI info for these two ports.

Data Flows

Three different data flows exist based on how the post-launched VM is started, as shown in the diagram below.

Figure 1 data flow: The post-launched VM is started with the vUART enabled in the hypervisor configuration file only.

Figure 2 data flow: The post-launched VM is started with the acrn-dm cmdline of -s 1:0,lpc -l com1,stdio only.

Figure 3 data flow: The post-launched VM is started with both vUART enabled and the acrn-dm cmdline of -s 1:0,lpc -l com1,stdio.



For operating systems such as VxWorks and Windows that depend on the ACPI table to probe the uart driver, adding the vuart configuration in the hypervisor is not sufficient. Currently, we recommend that you use the configuration in the figure 3 data flow. This may be refined in the future.