Implementing new system call

1. Define interface of system call

For example, assume we are implementing sched_setaffinity(2). You must find the definition of sched_setaffinity in shim_syscalls.c, which will be the following code:

SHIM_SYSCALL_RETURN_ENOSYS(sched_setaffinity, 3, int, pid_t, pid, size_t,
                           len, __kernel_cpu_set_t*, user_mask_ptr)

Change this line to DEFINE_SHIM_SYSCALL(...) to name the function that implements this system call: shim_do_sched_setaffinity (this is the naming convention, please follow it):

DEFINE_SHIM_SYSCALL(sched_setaffinity, 3, shim_do_sched_setaffinity, int, pid_t, pid, size_t, len,
                    __kernel_cpu_set_t*, user_mask_ptr)

2. Add definitions to system call table

To implement system call sched_setaffinity, two functions need to be defined in shim_table.h: __shim_sched_setaffinity and shim_do_sched_setaffinity. The first one should already be defined. Add the second in respect to the system call you are implementing, with the same prototype as defined in shim_syscalls.c:

int shim_do_sched_setaffinity(pid_t pid, size_t len, __kernel_cpu_set_t* user_mask_ptr);

3. Implement system call

You can add the function body of shim_do_sysinfo (or the function name defined earlier) in a new source file or any existing source file in LibOS/shim/src/sys.

For example, in LibOS/shim/src/sys/shim_sched.c:

int shim_do_sched_setaffinity(pid_t pid, size_t len, __kernel_cpu_set_t* user_mask_ptr) {
   /* code for implementing the semantics of sched_setaffinity */
}

4. Add new PAL Calls (optional)

The concept of Graphene library OS is to keep the PAL interface as simple as possible. So, you should not add new PAL calls if the features can be fully implemented inside the library OS using the existing PAL calls. However, sometimes the OS features needed involve low-level operations inside the host OS and cannot be emulated inside the library OS. Therefore, you may have to add a few new PAL calls to the existing interface.

To add a new PAL call, first modify Pal/include/pal/pal.h. Define the PAL call in a platform-independent way:

PAL_BOL DkThreadSetCPUAffinity(PAL_NUM cpu_num, PAL_IDX* cpu_indexes);

Make sure you use the PAL-specific data types, including PAL_BOL, PAL_NUM, PAL_PTR, PAL_FLG, PAL_IDX, and PAL_STR. The naming convention of a PAL call is to start functions with the Dk prefix, followed by a comprehensive name describing the purpose of the PAL call.

5. Export new PAL calls from PAL binaries (optional)

For each directory in PAL/host/, there is a pal.map file. This file lists all the symbols accessible to the library OS. The new PAL call needs to be listed here in order to be used by your system call implementation.

6. Implement new PAL calls (optional)

Todo

(Not finished…)