no-copy ipc

Hi,

How does L4 ensure ipc occurs without copying?

Suppose a sender (S), receiver (R) and an other process (O) is running
on the system. I would think that during the ipc context switch from
(S) to (R), registers don't need to be pushed to memory and read back,
which stands for the "no-copy" part of ipc. Is that correct? If so,
the system must ensure no context switch to some (O) occurs during the
time which the message registers are passed from (S) to (R). Would you
agree? Or am I confusing ipc copying with context switch overhead?

Thanks,
Bahadir