Search notes:

arch/x86/include/asm/syscall.h

Access to user system call parameters and results (See asm-generic/syscall.h for description of what we must do here).

sys_call_ptr_t

sys_call_ptr_t is a syscall function pointer: 
typedef long (*sys_call_ptr_t)(const struct pt_regs *);
See also the sys_call_table array

sys_call_table

sys_call_table is an array of sys_call_ptr_t. In syscall.h, it is defined as 
extern const sys_call_ptr_t sys_call_table[];
sys_call_table is populated in arch/x86/um/sys_call_table_64.c and/or arch/x86/entry/syscall_64.c.
The function corresponding to a sys call number is looked up in the array and called in do_syscall_x64().
A note in the source file says that sys_call_table[] is no long used for system calls but that kernel/trace/trace_syscalls.c still wants to know the system call address.

See also

include/linux/syscalls.h

Index