Is Zircon Boot Image (ZBI) essential for bootin the Zircon kernel?

Question

The kernel entrypoint for the Zircon kernel calls platform_early_init() where it gets the zbi_paddr from the .S file and then passes to pdev_init

void* zbi_vaddr = paddr_to_physmap(zbi_paddr);
zbi_header_t* header = (zbi_header_t*)zbi_vaddr;
ramdisk_base = header;
zbi_root = reinterpret_cast<zbi_header_t*>(ramdisk_base);
pdev_init(zbi_root);

Where pdev_init is responsible for starting drivers like uart.

It looks like the drivers through pdev_run_hooks (on the same file), where it walks through the zbi file and finds each driver information.

So it looks like ZBI is essential for booting?

I ask because I compiled the zircon kernel and got

lz@vm:~/fuchsia/out/default/kernel_arm64$ ls gen
image.build-id.stamp kernel.zbi toolchain.ninja zircon.elf.build-id.stamp zircon.elf.map image image.map
obj zircon.elf zircon.elf-gdb.py

as you see it produces a zircon.elf, which theoretically could be ran, but I think ZBI information is essential for booting otherwise it would panic. Am I right?

Answer 1

Zircon expects the bootloader to pass up a Zircon Boot Image (ZBI) object that it can parse.

This image contains a lot of information that is useful for system startup, for example, it contains physical memory ranges, the kernel cmdline, cpu topology information, peripherals information, and even a read-only filesystem used for bootstrapping userspace.

You can read more in the "Zircon Kernel to userspace bootstrapping" documentation. The ZBI Format is described in zircon/system/public/zircon/boot/image.h.

In the vim2 branch for the u-boot repo, you can see how the bootloader assembles and appends the zbi items before booting into zircon.