The guest loader is similar to the generic-loader although it is aimed at a particular use case of loading hypervisor guests. This is useful for debugging hypervisors without having to jump through the hoops of firmware and boot-loaders.
The guest loader does two things:
- load blobs (kernels and initial ram disks) into memory
- sets platform FDT data so hypervisors can find and boot them
This is what is typically done by a boot-loader like grub using it’s multi-boot capability. A typical example would look like:
qemu-system-x86_64 -kernel ~/xen.git/xen/xen -append "dom0_mem=1G,max:1G loglvl=all guest_loglvl=all" -device guest-loader,addr=0x42000000,kernel=Image,bootargs="root=/dev/sda2 ro console=hvc0 earlyprintk=xen" -device guest-loader,addr=0x47000000,initrd=rootfs.cpio
In the above example the Xen hypervisor is loaded by the -kernel parameter and passed it’s boot arguments via -append. The Dom0 guest is loaded into the areas of memory. Each blob will get /chosen/module@<addr> entry in the FDT to indicate it’s location and size. Additional information can be passed with by using additional arguments.
Currently the only supported machines which use FDT data to boot are the ARM and RiscV virt machines.
The full syntax of the guest-loader is:
- This is mandatory and indicates the start address of the blob.
- Indicates the filename of the kernel or initrd blob. Both blobs will have the “multiboot,module” compatibility string as well as “multiboot,kernel” or “multiboot,ramdisk” as appropriate.
- This is an optional field for kernel blobs which will pass command like via the /chosen/module@<addr>/bootargs node.