4. How to Debug

4.1. How to debug the security monitor (bbl) and the linux kernel?

We use QEMU+GDB to debug the security monitor or the kernel. QEMU is an effective way to debug them.

./scripts/run-qemu.sh -debug

All cores will immediately hang at the first instruction (i.e., bootrom), waiting for gdb to be attached.

Now, run a riscv gdb in another terminal. You can feed it with the bbl binary or the kernel image to add debug information. (You may want to compile them with the debugging flag -g)

For example, if you want to debug with the bbl symbols

riscv64-unknown-linux-gnu-gdb ./hifive-work/riscv-pk/bbl

If you want to debug with the kernel’s debug information

riscv64-unknown-linux-gnu-gdb ./riscv-linux/vmlinux

Then, attach to QEMU:

(gdb) target remote localhost:1234

Now, you can start debugging the SM (bbl) or the kernel. Try to set breakpoints and run.

Before setting breakpoints, you should run following command:

(gdb) set riscv use_compressed_breakpoints no

To see why we need that command, see this issue

4.2. Logging QEMU debug messages

QEMU provides a great option to collect the logs. If you add -D [filename] flag to the QEMU command, it will print out the logs into [filename].

You can also choose which kind of logs you want to print out, using -d [options] flag. For example,

./riscv-qemu/riscv64-softmmu/qemu-system-riscv64 -d in_asm -D debug.log #...etc...