Running U-Boot & Linux Kernel in QEMU


A disk image is ready with Linux kernel. We need a way to make QEMU aware of the presence of this disk. If QEMU can correctly identify the disk, it will pass the information to U-Boot and U-Boot will be able to find the partitions in it.

How? When QEMU is compiled, it creates a device tree binary (dtb) and passes it to U-Boot. A device tree is a description of all hardware of the host. In memory the device tree is organized as flattened device tree (FDT).

QEMU -- passes dtb --> OpenSBI -- passes dtb -->
  U-Boot -- parses dtb > finds disk > partitions > filesystem > kernel file

Run the following commands from the root working directory to add additional parameters to QEMU so that QEMU recognizes the disk image:

cd qemu/
gedit ./ &

Append the following lines to the end of the file:

-blockdev driver=file,filename=../disk.img,node-name=disk \
-device virtio-blk-device,drive=disk

Save and close the file. Run the script:


Observe that U-Boot has detected the disk as seen in something like the following console output:

Hit any key to stop autoboot:  0 

Device 0: QEMU VirtIO Block Device
            Type: Hard Disk
            Capacity: 128.0 MB = 0.1 GB (262144 x 512)
... is now current device
Scanning virtio 0:1...
** File not found ubootefi.var **
Failed to load EFI variables
BootOrder not defined
EFI boot manager: Cannot load any image
scanning bus for devices...

What's Happening?

disk.img is a block device. In QEMU a block device should be represneted by two things: a device back end and a device front end. The blockdev parameter specifies a device back end and the device parameter specifies a device front end that the guest sees. When the guest operates on the front end, e.g., reading a file, it actually reads some bytes from the back end.


Device Back End

-blockdev driver=file,filename=../disk.img,node-name=disk

  • driver=file = Indicates that the back end is a file
  • filename = Path to the disk image that will act as the back end
  • node-name = An identifier so that this back end driver can be referred in other nodes

Device Front End

-device virtio-blk-device,drive=disk

  • virtio-blk-device = This is a block device for virt machine

To get a list of devices QEMU supports, run ./build/qemu-system-riscv64 -device help

To get further help on virtio-blk-device, run ./build/qemu-system-riscv64 -device virtio-blk-device,help

  • drive = Name of the back end that will be manipulated by the front end
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