QEMU is incredibly helpful when debugging issues very early in OS startup (like before console prints work). This sort of debugging was invaluable when working

QEMU is incredibly helpful when debugging issues very early in OS startup (like before console prints work). This sort of debugging was invaluable when working on FreeBSD NUMA stuff and doing things like backing the vm page array with node-local physical memory. My colleague, who is working on kboot for FreeBSD (to run the FreeBSD bootloader as a linux binary that kexec's into FreeBSD from a linuxboot environment) also found QEMU an invaluable tool. In both cases, I think QEMU probably cut development time by 50% or more.