Lab: System calls

This lab shows us how to create a system call and use definitions in kernel code.

How to activate a system call in xv6?

1. ecall <num> - There is a function declaration in user/user.h, a system call number in kernel/syscall.h and a stub to user/usys.pl (which generates user-level syscall instructions).
2. jump to syscall() - ecall will jump to the syscall() function in kernel/syscall.c with the calling number saved in a7 register.
3. syscall() calls corresponding kernel code - syscall() use the number as an index of a function pointer array to get the address of its kernel code and call it.
4. execute kernel code - Execute the actual kernel system call, ex. sys_fork(), in kernel/sysproc.c and other files.
5. resume user code

System Call Tracing (moderate)

Add an mask integer in process structure. When the actual system call returns to syscall(), test the process mask with the system call number and print a line.

How to pass the mask to child processes?

• Initial every process with an empty mask while allocating.
• The sys_trace() writes the mask integer of process from its argument. If the user doesn't call trace(), all processes will have empty masks and hence avoid printing lines.
• fork() will copy mask number to the child process.
• code

Sysinfo (moderate)

• freemem - Traverse the free page list and get the result by multiplying PGSIZE.
• nproc - Traverse the process array and count the number of used ones.
• sys_sysinfo() copy the result numbers to the given address by calling copyout().
• code