This is something I wanted to implement a long time ago, but held off until there was a good reason to do so. Now it appears that having access to the arguments of the function by default is very useful. As a bonus, because arguments must be saved regardless before calling a callback, because they need to be restored before returning back to the start of the traced function, there's not much work to do to have them always be there for normal function callbacks.
The basic idea is that if CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS is set, then all callbacks registered to ftrace can use the regs parameter for the stack and arguments (kernel_stack_pointer(regs), regs_get_kernel_argument(regs, n)), without the need to set REGS that causes overhead by saving all registers as REGS simulates a breakpoint. Only the first patch is to be applied. The second patch is just to show how this could work. Steven Rostedt (VMware) (2): ftrace/x86: Allow for arguments to be passed in to REGS by default ftrace: Test arguments by adding trace_printk in function tracer ---- arch/x86/Kconfig | 1 + arch/x86/kernel/ftrace_64.S | 12 +++++++----- kernel/trace/Kconfig | 9 +++++++++ kernel/trace/trace_functions.c | 13 ++++++++++++- 4 files changed, 29 insertions(+), 6 deletions(-)