Cscope in many large trees

[Neil Baylis]

What's the canonical way to use cscope under the following scenario:

I frequently check out a large source tree from some old revision of a
project. The source tree contains hundreds of directories and
thousands of source files. I'm interested in the files in maybe a few
dozen directories, scattered throughout the tree. (These are all .c
and .h files). I would have maybe 10 of these trees checked out at the
same time, and I'm switching between them frequently, to work on
different development tasks. They all have basically the same
structure, just with different versions of files in them.

How should I set up cscope to index the parts I want? Is there a way
to tell it to always index the same directories, relative to the top
of the tree?

I currently (require 'xcscope) but don't have any other cscope
settings. It seems to find things in the current drectory, but nowhere
else.