Practical Exercise Inter-Process Communication

Transcription

1 Faculty of Computer Science Institute for System Architecture, Operating Systems Group Practical Exercise Inter-Process Communication Björn Döbel Dresden,

2 Exercise Outline IPC Recap & Implementation Details ADD A Simple Short-IPC-based server PRINT Long IPC, Opcodes, Marshalling...

3 IPC - recap Basic communication mechanism Copying data between threads Mapping resources Synchronization Sleeping Handling exceptional conditions Timeouts L4_IPC_NEVER L4_IPC_{SEND RECV BOTH}_TIMEOUT_0 l4util_micros2l4to(unsigned int us);

4 server L4 IPC Flavors Basics Special cases for client/server IPC S R send S? R R receive from (closed wait) receive any (open wait) client? call := send + recv from reply and wait := send + recv any

5 L4 IPC Message Descriptor Send/recv operations are described by a message descriptor Flags: message state (e.g., fpage included?) Dwords: directly copied data String descr.: pointers to indirect buffers Flags GP Registers No. of DWords DWords No. of Strings String descriptors Short IPC Direct String IPC Indirect String IPC

6 L4 IPC Types Sender AS Receiver AS indirect copy String descr. DWords Send descr. direct copy Registers String descr. DWords Rcv descr. Short IPC: no copying, just switch to receiver and leave register contents as is (In)direct String IPC: copy data according to msg descriptor

7 L4 Kernel ABI Linux: INT 0x80 + System Call # in EAX L4: INT 0x30 ipc() INT 0x31 nchief() / myself() INT 0x32 fpage_unmap() INT 0x33 thread_switch() INT 0x34 thread_schedule() INT 0x35 thread_ex_regs() INT 0x36 task_new() Parameters in registers -> see Lava Nucleus Reference Manual:

8 L4: System call Bindings Programming against an ABI is tedious Solution: C-API that wraps the kernel ABI L4v2 system call bindings: Example: IPC call: l4_ipc_call(l4_threadid_t dest, const void *msg, l4_umword_t dw0, l4_umword_t dw1, void *recv_msg, l4_umword_t *recv_dw0, l4_umword_t *recv_dw1, l4_timeout_t to, l4_msgdope_t *result);

9 ADD Using simple short IPC Make sure you have the Fiasco + L4Env build setup from last exercise available. Write a new program just as you did in the last exercise. Start a 2 nd thread (server) using l4util_create_thread(). In the main thread (client): write a loop sending 2 numbers to the server and print the result sent by the server. Use l4_ipc_call() and short IPC only. In the server thread: wait for a short IPC message, calculate the sum of the 2 numbers and return the result to the client.

10 Implementation Issues How does the client find the server's thread ID? Normally, we'd have a name service running. In our case, server is the 2 nd thread, so just take the client thread ID (l4_myself()) and increment the thread number. Functions to look at: l4_ipc_call() l4_ipc_wait() l4_ipc_reply_and_wait()

11 So this was easy... Real servers implement multiple functions. Need to determine which function to invoke Usually assign opcode to each function and pass it as additional parameter through IPC. May have different number and size of parameters depending on the opcode. Data structures having multiple layouts e.g., unions in C/C++ Marshalling and unmarshalling

12 Client-Server RPC Broken down Client Server Marshall data Assign Opcode IPC call Unmarshall exception or reply IPC wait Unmarshall Opcode Unmarshall Data Execute function Marshall return value or error IPC reply Goto begin

13 PRINT A Console Server Our next server is going to provide 2 functions: print(char *message); reverse_print(char *message); You may reuse the client/server skeleton from the first exercise. Use (direct or indirect) long IPC to send message from client to server.

14 Implementation Issues Message layout: LN Reference: Messages 2.3 ipc system call L4v2 Syscall bindings Documentation on IPC Documentation on IPC-related data types Hint: Indirect Long IPC might be the more natural choice for sending a string.

15 Conclusion Writing IPC code manually can be a pain in the ass. Automation and frameworks help a lot. (See IPC lecture and next practical exercise.)