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K-Boot

K-Boot is a bootloader based on Linux kexec.

There are several benefits with this kind of bootloader solution:

  • no need to duplicate/integrate drivers from the Kernel
  • no need to maintain another bootloader project
  • same kernel source code used between Linux and K-Boot
  • better boot/flashing performance
  • easier to extend or debug (many tools available)
  • use/contribute existing reliable projects

For now K-Boot only targets arm64 hardware.

Setup environment

Fetch K-Boot project:

$ git clone --recurse-submodules [email protected]:baylibre/k-boot.git

Download toolchains and add to $PATH:

# aarch64-linux-musl toolchain for userspace binaries
$ wget https://musl.cc/aarch64-linux-musl-cross.tgz
$ tar zxvf aarch64-linux-musl-cross.tgz

# aarch64-none-linux toolchain for linux
$ wget https://developer.arm.com/-/media/Files/downloads/gnu-a/10.3-2021.07/binrel/gcc-arm-10.3-2021.07-x86_64-aarch64-none-linux-gnu.tar.xz
$ tar -xvf gcc-arm-10.3-2021.07-x86_64-aarch64-none-linux-gnu.tar.xz

Linux requirements

K-Boot doesn't manage the linux project nor provide the defconfig.

However the user must set in the defconfig:

CONFIG_INITRAMFS_SOURCE="usr/initramfs/setup"
CONFIG_KEXEC=y

Build K-Boot

$ ./build/build_all.sh --linux=~/src/linux --defconfig=kboot_defconfig

All other build scripts can be run independently (--help for more infos).

Intregration with Arm Trusted Firmware (ATF)

In the ARM boot flow, K-Boot will be used as BL33.

According to Booting AArch64 Linux:

Primary CPU general-purpose register settings:

  • x0 = physical address of device tree blob (dtb) in system RAM.

Thus the device tree must be loaded before booting to K-Boot and x0 must be set to the physical address where the device tree has been loaded.

This task can be fulfilled by the BL2.

ATF Boot Flow example with K-Boot:

  1. MMC BOOT0 contains BL2 + Device Tree for K-Boot
  2. BL1 (ROM code) copy MMC BOOT0 to SRAM
  3. BL1 jump to SRAM and execute BL2
  4. BL2 initialize DDR, extract from fip image: BL31 (pm runtime services), BL32 (OP-TEE) and BL33 (K-Boot)
  5. BL2 copy the Device Tree for K-Boot found in SRAM to DDR and set x0 register
  6. BL2 jump to BL31
  7. BL31 boot and jump to OP-TEE
  8. OP-TEE boot and jump to K-Boot
  9. K-Boot kernel start, find init process in initramfs and execute K-Boot daemon (kbootd)

Tips

Send kbootd over serial:

# On host
$ sx kbootd < /dev/ttyUSB0 > /dev/ttyUSB0

# On device, make sure kbootd is not running
# rx /bin/kbootd

Contributions

kbootd coding style:

$ clang-format -i kbootd/src/*