NetBSD|EdgeBSD/lm32

What it does so far:

• Boots the kernel all the way up to mounting rootfs from ramdisk
• Loads /sbin/init from the ramdisk and starts executing it in user space
• /sbin/init executes entirely, it contains 3 open() and 1 write() syscalls
• init opens /dev/console 3 times (stdin, stdout, stderr) and writes "hello, world!" to stdout which shows up in the uart console
• Then init "returns to 0x0" and crashes the machine

How to build EdgeBSD/lm32:

• Fetch the source: git clone http://git.edgebsd.org/EdgeBSD/edgebsd-src.git
• Go to my branch: cd edgebsd-src && git checkout fallen-port-milkymist
• Fetch the submodule (init_src): git submodule update --init
• Build the lm32 cross toolchain: ./build.sh -m milkymist -U tools
• Build EdgeBSD kernel: ./build.sh -m milkymist -U kernel=GENERIC

Run EdgeBSD/lm32 in qemu:

• ./lm32-softmmu/qemu-system-lm32 -M milkymist -cpu lm32-full-mmu -nographic -kernel /path/to/edgebsd-src/sys/arch/milkymist/compile/obj/GENERIC/netbsd

Build Qemu for Milkymist SoC with support for lm32-mmu:

• git clone https://github.com/fallen/qemu.git && cd qemu && ./configure --target-list=lm32-softmmu && make

Debug with Qemu and GDB:

• Build EdgeBSD kernel with debug information: DEBUG=-g ./build.sh -m milkymist -U kernel=GENERIC
• ./lm32-softmmu/qemu-system-lm32 -M milkymist -cpu lm32-full-mmu -nographic -s -S -kernel /path/to/edgebsd-src/sys/arch/milkymist/compile/obj/GENERIC/netbsd.gdb
• gdb -x /path/to/edgebsd-src/sys/arch/milkymist/compile/obj/GENERIC/.gdbinit /path/to/edgebsd-src/sys/arch/milkymist/compile/obj/GENERIC/netbsd.gdb
• (inside gdb) target remote :1234

Embed a ramdisk (MFS : memory file system) inside the kernel with a simple static init binary:

• Build EdgeBSD kernel with MFS support: DEBUG=-g ./build.sh -m milkymist -U kernel=GENERIC_MFS
• Build init binary: make -C init_src
  • Beware, if you are not under Mac OS X you need to tell the makefile where your lm32 crosstoolchain is by prefixing the previous command with: TOOLCHAIN_DIR=$PWD/obj/tooldir.NetBSD-7.0-i386/
• Install init binary inside the kernel image: debug=1 make -C init_src install
  • you still need the TOOLCHAIN_DIR variable if not under OS X
  • you will need to enter your SUDO password
• Run the kernel with Qemu and wait for GDB to attach: ./lm32-softmmu/qemu-system-lm32 -M milkymist -cpu lm32-full-mmu -nographic -kernel /path/to/edgebsd-src/sys/arch/milkymist/compile/obj/GENERIC_MFS/netbsd.gdb -s -S
• Attach GDB: gdb -x /path/to/edgebsd-src/sys/arch/milkymist/compile/obj/GENERIC/.gdbinit /path/to/edgebsd-src/sys/arch/milkymist/compile/obj/GENERIC_MFS/netbsd.gdb
• Inside GDB:
  • (gdb) target remote :1234
  • 0x10000000 is the virtual address where init binary gets loaded (you can inspect the binary with readelf)
    (gdb) break *0x10000000
  • (gdb) continue
  • 0x47eb4000 is the physical address where init binary gets loaded in RAM
    (gdb) x/10i 0x47eb4000
  • Then you can single step using "si" and see the registers changing values according to the instructions you printed at the previous step
  • (gdb) break syscall
    (gdb) continue
  • So far the init program will do 3 open("/dev/console") and one write(0, "hello, world\n", 12), you can break on sys_open and sys_write to catch them or just single step from the break to syscall