How to setup network connection for initramfs running in QEMU?

Thanks to this mail list, I have already successfully set up a simple initramfs and booted it in QEMU with busybox tools. However, there is no network connection for the initramfs, that makes me hard to exchange files between qemu and the host. Command `ip link show` tells there exists only one 'lo' interface, and I can't ping 127.0.0.1 successfully. So how to boot initramfs in qemu with full network support? Best regards.