Hypervisor console, hypervisor shell, and virtual UART

Hypervisor console

The hypervisor console is a text-based terminal accessible from UART. Figure 162 shows the workflow of the console:


Figure 162 Periodic console processing

A periodic timer is set on initialization to trigger console processing every 40ms. Processing behavior depends on whether the vUART is active:

  • If it is not active, the hypervisor shell is kicked to handle inputs from the physical UART, if there are any.
  • If the vUART is active, the bytes from the physical UART are redirected to the RX fifo of the vUART, and those in the vUART TX fifo to the physical UART.


The console is only available in the debug version of the hypervisor, configured at compile time. In the release version, the console is disabled and the physical UART is not used by the hypervisor or SOS.

Hypervisor shell

For debugging, the hypervisor shell provides commands to list some internal states and statistics of the hypervisor. It is accessible on the physical UART only when the vUART is deactivated. See ACRN Shell Commands for the list of available hypervisor shell commands.

Virtual UART

Currently UART 16550 is owned by the hypervisor itself and used for debugging purposes. Properties are configured by hypervisor command line. Hypervisor emulates a UART device with 0x3F8 address to SOS that acts as the console of SOS with these features:

  • The vUART is exposed via I/O port 0x3f8.
  • Incorporate a 256-byte RX buffer and 65536 TX buffer.
  • Full emulation of input/output bytes and related interrupts.
  • For other read-write registers the value is stored without effects and reads get the latest stored value. For read-only registers writes are ignored.
  • vUART activation via shell command and deactivate via hotkey.

The following diagram shows the activation state transition of vUART.


Figure 163 Periodic console processing


  • After initialization vUART is disabled.
  • The vUART is activated after the command “vm_console” is executed on the hypervisor shell. Inputs to the physical UART will be redirected to the vUART starting from the next timer event.
  • The vUART is deactivated after a Ctrl + Space hotkey is received from the physical UART. Inputs to the physical UART will be handled by the hypervisor shell starting from the next timer event.

The workflows are described as follows:

  • RX flow:
    • Characters are read from UART HW into a sbuf whose size is 2048 bytes, triggered by console_read
    • Characters are read from this sbuf and put to rxFIFO, triggered by vuart_console_rx_chars
    • A virtual interrupt is sent to SOS, triggered by a read from SOS. Characters in rxFIFO are sent to SOS by emulation of read of register UART16550_RBR
  • TX flow:
    • Characters are put to txFIFO by emulation of write of register UART16550_THR
    • Characters in txFIFO are read out one by one and sent to console by printf, triggered by vuart_console_tx_chars
    • Implementation of printf is based on console, which finally sends characters to UART HW by writing to register UART16550_RBR