How can I build a loopback filesystem?

["Ian S. Nelson" <ian.nelson@xxxxxxxxxxxx>]

Okay, I'm stupid.  I've been trying to build an XFS loopback filesystem
to do some playing with, any hints?

I did it like I do it for all the others, I dd a blank file of about
100M then I try to build a filesystem in it.  the xfs mkfs utility
doesn't want to detect the size and I haven't been able to get the size
parameters to work.

Thanks,
Ian Nelson