Boot Parameter

iso-scan/filename=

example

To boot a livecd iso with grub2 for example if you placed the ubuntu-xxx.iso under the usb drive /boot/ubuntu-xxx.iso

menuentry LiveISO {
loopback iso (hd0,1)/boot/ubuntu-xxx.iso
linux (iso)/casper/vmlinuz boot=casper iso-scan/filename=/boot/ubuntu-xxx.iso --
initrd (iso)/casper/initrd.gz
}