Unwind custom RSP in x64 MASM

I pass a custom stack to an asm function.

In the asm function, it sets the custom stack to be its RSP and call external function.

If an exception occurs in the external function, I can't find a way to unwind RSP from any of the operations listed in

http://msdn.microsoft.com/en-us/library/ms235231%28v=vs.100%29.aspx

Please help me to find out how to unwind a custom stack.