ImageBuilder

Booting Xen from U-Boot

Booting Xen from U-Boot requires:

• loading all the required binaries, manually specifying the loading address for each of them making sure they don't overlap
  • Xen, Dom0 kernel, Dom0 ramdisk, device tree binary, any Dom0-less DomUs kernels, ramdisk and partial dtbs for passthrough

• adding relevant nodes to device tree
  • the Dom0 kernel and ramdisk loading addresses need to be specified in device tree under /chosen

See booting.txt as a reference.

ImageBuilder

The whole process can be automated with ImageBuilder:

https://gitlab.com/ViryaOS/imagebuilder

ImageBuilder can be invoked as a container for build automation, but its useful scripts can also be called manually. Specifically, script/uboot-script-gen generates a U-Boot script that loads all the necessary binaries and automatically adds the required entries to device tree at boot time. In order to use it, you need to write a config file first.

Config file

MEMORY_START="0x0"
MEMORY_END="0x80000000"

DEVICE_TREE="board.dtb"
XEN="xen-hypervisor"
DOM0_KERNEL="Image-dom0"
DOM0_RAMDISK="ramdisk-dom0.rootfs.cpio.gz"

NUM_DOMUS=2
DOMU_KERNEL[0]="dom1/Image-domU"
DOMU_RAMDISK[0]="dom1/ramdisk-domU"
DOMU_PASSTHROUGH_DTB[0]="dom1/passthrough-domU.dtb"
DOMU_KERNEL[1]="dom2/Image-domU"
DOMU_RAMDISK[1]="dom2/ramdisk-domU2"

UBOOT_SOURCE="boot.source"
UBOOT_SCRIPT="boot.scr"

The fields are self explanatory but you can find more detailed information on the readme. Make sure to use raw binaries for Xen, all the kernels and rootfs's, not U-Boot binaries (do not use the output of mkimage).

uboot-script-gen

Once you have the config file, you can call uboot-script-gen as follows:

$ bash ./scripts/uboot-script-gen -c /path/to/config -d . -t tftp

to generate a U-Boot script named boot.scr that will load all your binaries automatically using tftp. The paths specified in the config file are relative to the directory pass as -d. Now, you just need to load the generated boot.scr and source it from U-Boot:

$ tftpb 0xC00000 boot.scr; source 0xC00000

The command used to load the binaries can be customized, for instance you can have uboot-script-gen generate a U-Boot script that loads binaries from a SD card passing -t sd, which is a syntactic sugar for -t "load scsi 0:1":

$ bash ./scripts/uboot-script-gen -c /path/to/config -d . -t sd

Important Notes

Xen and Dom0 command line

ImageBuilder uses sched=null by default. If you want to change it, and also modify other Xen and/or Dom0 command line options you'll make to edit the generated U-Boot script: boot.source. Look for sched=null in the source and edit as needed. Then you need to regenerate boot.scr using mkimage and specifying the same loading address for the boot.scr as printed earlier by uboot-script-gen:

$ mkimage -A arm64 -T script -C none -a 0xC00000 -e 0xC00000 -d boot.source boot.scr

U-Boot binaries vs. raw binaries

uboot-script-gen takes only raw binaries as input. If you have a U-Boot binary in your hands and you want to convert it back to a raw binary, you can do that with the following command:

$ dd if=uboot-binary-source of=raw-binary-dest bs=64 skip=1

Xen booting as raw binary

If you use a version of Xen older than 4.12, you need to make sure to have the following commit backported in your Xen tree:

4f3d0ed5d9 xen:arm: Populate arm64 image header

It is necessary to be able to boot Xen as a raw binary from U-Boot.