You might have to use xm mem-max to set the maximum memory for the guest prior to using xm mem-set.  Also, if you look at the man page for xm, you will see the following for mem-set:

"Because this operation requires cooperation from the domain operating system, there is no guarantee that it will succeed.  This command will definitely not work unless the domain has the required paravirt driver."

So, there is no guarantee that you can increase/decrease the ram of a guest without restarting it.

Matt

--
Mathew S. McCarrell
Clarkson University '10

mccarrms@gmail.com
mccarrms@clarkson.edu
1-518-314-9214