[seL4] seL4 IPC fastpath question

Hello, I have question about seL4 IPC fastpath. I ran sel4bench on sabre lite(arm v7). 1. IPC fastpath can apply in seL4_Call and seL4_Reply_Recv, is it right? seL4_Send, seL4_Recv can't apply IPC fastpath. 2. IPC_fastpath can only send message that message's length is 0, is it right? I found these comments. /* Call slowpath, long IPC (10), same prio client to server, different address space */ /* ReplyRecv slowpath, long IPC (10), same prio server to client, on the slowpath, different address space */ 3. In seL4 IPC mechanism, IPC only use registers not memory?