microvm

The x86_64 microvm implementation targets QEMU's microvm machine type. See: https://www.qemu.org/docs/master/system/i386/microvm.html

There is no bootloader: QEMU boots the kernel directly (-kernel) using the PVH ELF note (see asm/notes.asm), and hands over boot information (command line, initrd, memory map) via a hvm_start_info structure (see core/pvh.c).

Compared to pc, this board is headless (serial console only, no VGA/ framebuffer, no keyboard) and has no PCI bus (no networking yet).

Config file

ARCH  = x86_64
BOARD = microvm
 
# This is only useful when QEMU is used.
CONFIG_SEMIHOSTING = 1

Boot options

It is possible to change the command line by passing the CMDLINE variable to make (or add it to your config file):

$ make clean run-debug CMDLINE="/bin/init foo bar"

Early boot sequence

QEMU loads the kernel ELF directly and finds the PVH entry point
QEMU builds the hvm_start_info structure (command line, initrd, memory map) and jumps to the entry point (start), passing its address in ebx
Various checks are performed to make sure we can continue the boot process
The page tables and paging are configured and enabled
A 64-bit Global Descriptor Table is loaded
A Task State Segment is loaded
All the data segment registers are reloaded
The C kmain() function is called, which validates the hvm_start_info structure and parses the boot information
Eventually, the kernel switches to user mode and calls /bin/init by default

See the x86_64 architecture page for memory management/layout details shared with other boards.