low-level development tooling
#Qemu can load the kernel (ELF) and RAM-disk (CPIO) for you, without any firmware or bootloader. You only have to provide it with a root partition image, where the first process' binary resides.
Then, you can make it drive some of the hosts' devices (not a trivial configuration, but still).
And also, you can use it as emulating debugger (server to which you connect remotely with #GDB). No special kernel modules are required.
So it's a pretty comprehensive development tool, and not just something where you run Windows when you absolutely have to…
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