Enable vUART Configurations¶

Introduction¶

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	Service VM (vuart enable)	Post-launched	Post-launched
Hybrid	Pre-launched (Zephyr) (vuart enable)	Service VM (vuart enable)	Post-launched
Industry	Service VM (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)

Example:

.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])

Example:

/* 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	Service VM	Post-launched	Post-launched
Hybrid	Pre-launched (Zephyr) (vuart enable COM2)	Service VM (vuart enable COM2)	Post-launched
Industry	Service VM (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:

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.
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.
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.

Note

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.