Linux Kernel Module Cheat

All of 9P and NFS (and sshfs) allow sharing directories between guest and host.

Advantages of 9P

requires sudo on the host to mount
we could share a guest directory to the host, but this would require running a server on the guest, which adds simulation overhead

Furthermore, this would be inconvenient, since what we usually want to do is to share host cross built files with the guest, and to do that we would have to copy the files over after the guest starts the server.
QEMU implements 9P natively, which makes it very stable and convenient, and must mean it is a simpler protocol than NFS as one would expect.

This is not the case for gem5 7bfb7f3a43f382eb49853f47b140bfd6caad0fb8 unfortunately, which relies on the diod host daemon, although it is not unfeasible that future versions could implement it natively as well.

Advantages of NFS:

way more widely used and therefore stable and available, not to mention that it also works on real hardware.
the name does not start with a digit, which is an invalid identifier in all programming languages known to man. Who in their right mind would call a software project as such? It does not even match the natural order of Plan 9; Plan then 9: P9!