1.29. VirtIO Support

This document describes the information about barebox support for VirtIO_ devices, including supported boards, build instructions, driver details etc.

1.29.1. What’s VirtIO?

VirtIO is a virtualization standard for network and disk device drivers where just the guest’s device driver “knows” it is running in a virtual environment, and cooperates with the hypervisor. This enables guests to get high performance network and disk operations, and gives most of the performance benefits of paravirtualization. In the barebox case, the guest is barebox itself, while the virtual environment will normally be QEMU targets like ARM, MIPS, RISC-V or x86.

1.29.2. Status

VirtIO can use various different buses, aka transports as described in the spec. While VirtIO devices are commonly implemented as PCI devices on x86, embedded devices models like ARM/RISC-V, which does not normally come with PCI support might use simple memory mapped device (MMIO) instead of the PCI device. The memory mapped virtio device behaviour is based on the PCI device specification. Therefore most operations including device initialization, queues configuration and buffer transfers are nearly identical. Both MMIO and non-legacy PCI are supported in barebox.

The VirtIO spec defines a lots of VirtIO device types, however at present only block, console and RNG devices are supported.

1.29.3. Build Instructions

Building barebox for QEMU targets is no different from others. For example, we can do the following with the CROSS_COMPILE environment variable being properly set to a working toolchain for ARM:

$ make vexpress_defconfig
$ make

1.29.4. Testing

The following QEMU command line is used to get barebox up and running with a VirtIO console on ARM:

$ qemu-system-arm -m 256M -M virt -nographic             \
      -kernel ./images/barebox-dt-2nd.img                \
      -device virtio-serial-device                       \
      -chardev socket,path=/tmp/foo,server,nowait,id=foo \
      -device virtconsole,chardev=foo,name=console.foo

To access the console socket, you can use socat /tmp/foo -.

Note the use of -kernel ./images/barebox-dt-2nd.img instead of -bios ./images/barebox-$BOARD.img. -kernel will cause QEMU to pass barebox a fixed-up device tree describing the virtio-mmio rings.

Except for the console, multiple instances of a VirtIO device can be created by appending more ‘-device’ parameters. For example to extend a MIPS malta VM with one HWRNG and 2 block VirtIO PCI devices:

$ qemu-system-mips -m 256M -M malta -serial stdio         \
      -bios ./images/barebox-qemu-malta.img -monitor null \
      -device virtio-rng-pci,disable-legacy=on            \
      -drive if=none,file=image1.hdimg,format=raw,id=hd0  \
      -device virtio-blk-pci,drive=hd0,disable-legacy=on  \
      -drive if=none,file=image2.hdimg,format=raw,id=hd1  \
      -device virtio-blk-pci,drive=hd1,disable-legacy=on

Note the use of disable-legacy=on. barebox doesn’t support legacy or transitional VirtIO devices. Some versions of QEMU may need to have ,disable-modern=off specfied as well.