Running U-Boot & Linux Kernel in QEMU


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U-Boot needs to be configured so that it can load the Linux kernel from the disk.img. The kernel is the file named Image in the first partition of the disk.

Run the following from the root directory to start U-Boot configuration utility:

cd u-boot
make menuconfig

U-Boot configuration options will be loaded to the terminal. Navigate to Boot options -> bootcmd value and write the following as the bootcmd value:

ext4load virtio 0:1 84000000 Image; booti 0x84000000 - ${fdtcontroladdr}

Save the configuration and exit the configuration utility. Now build U-Boot, build OpenSBI embedding U-Boot binary and run U-Boot in QEMU:

make -j$(nproc)
cd ../opensbi/
make PLATFORM=generic FW_PAYLOAD_PATH=../u-boot/u-boot.bin -j$(nproc)
cd ..

The autoboot feature of U-Boot will proceed and start the kernel! Well, the kernel will crash and not usable, but at least U-Boot is able to load and run it.

What's Happening?

As autoboot is set, it executed the commands set in the environment variable bootcmd. Commands can be separated by semicolon. We've set 2 commands: ext4load and booti.

ext4load virtio 0:1 84000000 Image

This command loads data from ext4 filesystem.

  • virtio = Name of the interface to read from
  • 0:1 = Read from 1st available device's 1st partition (the device number starts from 0, the partition number starts from 1)
  • 84000000 = A hexadecimal number, this is the address in RAM where the file will be loaded
  • Image = Name of the file to be loaded into RAM
booti 0x84000000 - ${fdtcontroladdr}

This command boots Linux Image format from memory.

  • 0x84000000 = Location where Linux Image is stored
  • - = Unused parameter, used to be the address of initrd (initial RAM disk)
  • ${fdtcontroladdr} = A variable holding the address of the flattened device tree in memory
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