Another issue I found that might help others: I had compilation errors trying to do some integer multiplication on my firmware test:

undefined reference to '__mulsi3'

It seems the example firmware Makefile doesn't include libgcc, where soft implementations of multiplication, mod, float, etc are located. Adding

-lgcc

to the end of the compilation command solved the issue:

$(TOOLCHAIN_PATH)$(TOOLCHAIN_PREFIX)-gcc -I../ -I../generated/ -O0 -mabi=ilp32 -march=$(ARCH) -D__vexriscv__ -Wl,-Bstatic,-T,../sections.lds,--strip-debug -ffreestanding -nostdlib -o $@ ../crt0_vex.S ../isr.c ../stub.c $< -lgcc

I created an issue on the caravel_board repo so they can check if it's a useful change: https://github.com/efabless/caravel_board/issues/101