vUART Virtualization


A vUART is a virtual 16550 UART implemented in the hypervisor. It can work as a console or a communication port. Currently, the vUART is mapped to the traditional COM port address. A UART driver in the kernel can auto detect the port base and IRQ.


Figure 206 UART virtualization architecture

Each vUART has two FIFOs: 8192 bytes Tx FIFO and 256 bytes Rx FIFO. Currently, we only provide 4 ports for use.

  • COM1 (port base: 0x3F8, irq: 4)
  • COM2 (port base: 0x2F8, irq: 3)
  • COM3 (port base: 0x3E8, irq: 6)
  • COM4 (port base: 0x2E8, irq: 7)

A VM can enable one console vUART and several communication vUARTs.

Console vUART

A vUART can be used as a console port, and it can be activated by a vm_console <vm_id> command in the hypervisor console. From Figure 207, there is only one physical UART, but four console vUARTs (green color blocks). A hypervisor console is implemented above the physical UART, and it works in polling mode. There is a timer in the hv console. The timer handler dispatches the input from physical UART to the vUART or the hypervisor shell process and gets data from vUART’s Tx FIFO and sends it to the physical UART. The data in vUART’s FIFOs will be overwritten when it is not taken out in time.


Figure 207 console vUART architecture

Communication vUART

The communication vUART is used to transfer data between two VMs in low speed. For the kernel driver, it is a general UART that can be detected and probed by 8250 serial driver. But in the hypervisor, it has a special process.

From Figure 208, the vUART in two VMs is connected according to the configuration in the hypervisor. When a user writes a byte to the communication UART in VM0:

Operations in VM0

  • VM0 UART driver puts the data to THR.
  • VM traps to hypervisor, and the vUART PIO handler is called.
  • Puts the data to its target vUART’s Rx FIFO.
  • Injects a Data Ready interrupt to VM1.
  • If the target vUART’s FIFO is not full, injects a THRE interrupt to VM0.
  • Returns.

Operations in VM1

  • Receives an interrupt, dispatches to UART driver.
  • Reads LSR register, finds a Data Ready interrupt.
  • Reads data from Rx FIFO.
  • If Rx FIFO is not full, injects THRE interrupt to VM0.

Figure 208 communication vUART architecture


  • For console vUART

    To enable the console port for a VM, change the port_base and IRQ in acrn-hypervisor/hypervisor/scenarios/<scenario name>/vm_configurations.c. If the IRQ number has been used in your system ( cat /proc/interrupt), you can choose other IRQ number. 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,

    The kernel bootargs console=ttySx should be the same with vuart[0]; otherwise, the kernel console log can not be captured by hypervisor. Then, after bringing up the system, you can switch the console to the target VM by:

    ACRN:\>vm_console 0
    ----- Entering VM 0 Shell -----
  • For communication vUART

    To enable the communication port, you should configure vuart[1] in the two VMs which want to communicate. The port_base and IRQ should not repeat with 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,


The device mode also has a virtual UART, and also uses 0x3F8 and 0x2F8 as port base. If you add -s <slot>, lpc in the launch script, the device model will create COM0 and COM1 for the post launched VM. It will also add the port info to the ACPI table. This is useful for Windows and vxworks as they probe the driver according to the ACPI table.

If the user enables both the device model UART and the hypervisor vUART at the same port address, access to the port address will be responded to by the hypervisor vUART directly, and will not pass to the device model.