M4: System call interface

The aim of this milestone is to design the RPC protocol for the system call interface. You should implement both the client and system side of this interface. This client side system call interface must conform to the interface provided in libs/libsos/include. You should create a sos.c that implements the sos.h interface within the library.

At this stage you will not actually be able to implement most of the system calls, however you should be able to partially implement open/close, read/write for the console device. You should also be able to implement sleep, time_stamp and process_wait, which only needs to work for the console. In other words, you need to modify your timer driver to be able to sleep and wake up sosh. This will allow you to run a simple shell on your system, which will allow you to perform interactive testing.

You should also change the implementation of sys_morecore in libs/libsos/src/sys_morecore.c to be backed by your own system call to the rootserver, in order to fetch more memory for malloc.

Other system calls should output system call not implemented.

Console device

When a program opens the file console it should access the console on the serial device. The console is a multiple writer, single reader device, i.e., more than one process can concurrently open the device for writing, but only one process can open the device for reading.

Reading the console is a blocking operation. If a process reads from the console and there is no data available it should block until data becomes available.

Be careful not to implement the console device as a 'hack'. You should think about being able to support multiple serial ports and other stream devices in your design (although not necessarily implement them). This means designing a consistent interface for interacting with all devices. You may want to read up on how Linux treats devices.

You may once again find the documentation on libserial handy.

Getting started

You will need to modify the libsos library to add implementations of the interface defined in libs/libsos/include/sos.h. You should copy the contents of tty_test/ttyout.c to libs/libsos/src/sos.c to import your sos_write implementation from M0.

You will also need to add sosh to the applications that are being built and included in the dite image. This is achieved by changing the startup application configuration parameter for SOS. Run $ make menuconfig and navigate to the SOS application configuration options. Change the application name from tty_test to sosh. You will also need to modify the top level makefile as shown below.

- sos-package: .config sos elfloader common kernel/kernel.elf kernel_elf dite tty_test
+ sos-package: .config sos elfloader common kernel/kernel.elf kernel_elf dite tty_test sosh

Design alternatives

At this stage of the project you will need to decide whether you want to have a simple single-threaded server, or to multi-thread it. A multi-threaded design could be advantageous to deal with the inherent concurrency your system will have (e.g. between paging, system calls, asynchronous I/O and clock interrupts), but it will require careful design of synchronisation in order to avoid race conditions and deadlocks. A single threaded model will require extra attention to ensure liveness.

Another design decision is how to transfer data between the kernel and user processes. Some options you have are:

• Transfer data in message registers.
• sos maps a user page in to itself.
• Set up a region of memory which is shared between the kernel and each user process and memcpy data into it when you need to transfer data.
• Use copyin-copyout method, where you lookup the page table and your system copies data in to/out of physical memory (i.e. all frames are also mapped into SOS.

Whatever you do, remember the basic engineering rule: keep it simple, stupid!(KISS).

Advice

This milestone is larger than it seems. The system call interface of an OS determines how user applications receive data they request. You will need to consider how you can move data in various quantities between your root server and clients. Scenarios to consider include:

• Getting system call arguments from client to server. These are generally a few words long.
• Getting filesystem call arguments from client to server. Mentioned specially because filenames can be long.
• Getting filesystem data from client to server (for read()) or server back to client (for write()). This can be really big. Streaming it a few words at a time over IPC will be sloooooow. Do not do it.

Think about how your OS/161 did it. Was it copy-in-copy-out? Did it reach into application address spaces based on app-supplied pointers? If so, what are the implications of this approach in the face of paging? (That is, what happens if the app's buffer has been paged out? How do you arrange to block the syscall, page the relevant data back in, and then restart the syscall?)

Assessment

For this assessment you should be able to demonstrate sosh running, and SOS outputting system call not implemented for the relevant SOS system calls.

sosh makes use of the SOS system calls in its ls and ps commands. This should be sufficient to demonstrate that your SOS system calls work. Of course, you can also write your own test code.

You should also show some sos application-level test code that uses the time_stamp and sleep system calls. You may find it useful to extend sosh to have a time and sleep command. The sleep implemention must use your clock driver.

As always, you should be able to explain both the design and your implementation.

Note: In line with the comments in the advice section, streaming large blocks of data over multiple IPCs is a sure-fire way not to get full marks!