(QEMU) Using a directory as rootfs

Assuming kernel has built-in (non-module) support for virtio, 9p and/or nfs, which the defconfig does.

With 9p

QEMU argument:

-virtfs local,id=root,mount_tag=/dev/root,security_model=passthrough,path=/path/to/rootfs

Append the following content to kernel command line:

rw rootfstype=9p rootflags=trans=virtio

Note

mount_tag must be /dev/root. There is no need to specify root= for kernel.

With NFS

Ensure the host is reachable through a network device in QEMU, e.g.:

-device virtio-net-device,netdev=usernet -netdev user,id=usernet

Append the following content to kernel command line:

root=/dev/nfs rw rootfstype=nfs nfsroot=<server-ip>:/path/to/nfs/export,vers=3 ip=dhcp

Note

root= must be /dev/nfs.

The vers parameter changes the NFS protocol version from 2 (the default) to higher because nfs-utils already disabled NFS v2 support in 2014. See this Gentoo forum post.