[PATCH 0/2] riscv/ftrace: add WITH_DIRECT_CALLS support

[Song Shuai <suagrfillet@gmail.com>]

This series adds DYNAMIC_FTRACE_WITH_DIRECT_CALLS support for RISC-V.
SAMPLE_FTRACE_DIRECT and SAMPLE_FTRACE_DIRECT_MULTI are also included
here as the samples for testing DIRECT_CALLS related interface.

First, select the DYNAMIC_FTRACE_WITH_DIRECT_CALLS to provide 
register_ftrace_direct[_multi] interfaces allowing user to register 
the customed trampoline (direct_caller) as the mcount for one or 
more target functions. And modify_ftrace_direct[_multi] are also 
provided for modify direct_caller.

At the same time, the samples in ./samples/ftrace/ can be built
as kerenl module for testing these interfaces with SAMPLE_FTRACE_DIRECT
and SAMPLE_FTRACE_DIRECT_MULTI selected.

Second, to make the direct_caller and the other ftrace hooks
(eg. function/fgraph tracer, k[ret]probes) co-exist, a temporary register
are nominated to store the address of direct_caller in ftrace_regs_caller.
After the setting of the address direct_caller by direct_ops->func and
the RESTORE_REGS in ftrace_regs_caller, direct_caller will be jumped to
by the `jr` inst.

The following tests have been passed in my local qemu-riscv64 virt machine. 

1. tests with CONFIG_FTRACE_STARTUP_TEST
2. tests of samples/ftrace/ftrace*.ko
3. manual tests with any combination of the following hooks
  - function/function_graph tracer 
  - ftrace*.ko
  - kprobe/kretprobe

For your reference, here is the log when function tracer, kretprobe and 
ftrace-direct-too.ko co-hooks the handle_mm_fault function.

```
[root@stage4 tracing]# echo handle_mm_fault > set_ftrace_filter
[root@stage4 tracing]# echo 'r:myr handle_mm_fault' > kprobe_events
[root@stage4 tracing]# echo function > current_tracer 
[root@stage4 tracing]# echo 1 > events/kprobes/myr/enable 
[root@stage4 tracing]# insmod /root/ftrace-direct-too.ko 
[root@stage4 tracing]# 
[root@stage4 tracing]# cat trace | tail
             cat-388     [000] ...1.   583.051438: myr: (do_page_fault+0x16c/0x5f2 <- handle_mm_fault)
             cat-388     [000] ...2.   583.057930: handle_mm_fault <-do_page_fault
             cat-388     [000] .....   583.057990: my_direct_func: handle mm fault vma=000000002d9fe19c address=ffffffae9b7000 flags=215
             cat-388     [000] ...1.   583.058284: myr: (do_page_fault+0x16c/0x5f2 <- handle_mm_fault)
            tail-389     [001] ...2.   583.059062: handle_mm_fault <-do_page_fault
            tail-389     [001] .....   583.059104: my_direct_func: handle mm fault vma=0000000017f3c48e address=aaaaaabebf3000 flags=215
            tail-389     [001] ...1.   583.059325: myr: (do_page_fault+0x16c/0x5f2 <- handle_mm_fault)
            tail-389     [001] ...2.   583.060371: handle_mm_fault <-do_page_fault
            tail-389     [001] .....   583.060410: my_direct_func: handle mm fault vma=0000000017f3c48e address=aaaaaabebf1000 flags=255
            tail-389     [001] ...1.   583.060996: myr: (do_page_fault+0x16c/0x5f2 <- handle_mm_fault)
```

Note1: 

This series is based on (riscv/ftrace: make ftrace_caller call ftrace_graph_func)
in this repo -> https://github.com/guoren83/linux/commits/ftrace_fixup_v3 .

Note2:

The checkpatch.pl will output some warnings on this series, like this

```
WARNING: Prefer using '"%s...", __func__' to using 'my_direct_func2', this function's name, in a string
111: FILE: samples/ftrace/ftrace-direct-multi-modify.c:48:
+"       call my_direct_func2\n"
```

The reason is that checkpatch depends on patch context providing the
function name. In the above warning, my_direct_func2 has some codeline
distance with the changed trunk, so its declaration doesn't come into
the patch, and then the warning jumps out.

You may notice the location of `my_ip` variable changes in the 2nd patch.
I did that for reducing the warnings to some extent. But killing all the 
warnings will makes the patch less readable, so I stopped here.

--
Song

Song Shuai (2):
  riscv/ftrace: add DYNAMIC_FTRACE_WITH_DIRECT_CALLS support
  samples/ftrace: add riscv support for SAMPLE_FTRACE_DIRECT[_MULTI]

 arch/riscv/Kconfig                          |  3 ++
 arch/riscv/include/asm/ftrace.h             |  6 ++++
 arch/riscv/kernel/mcount-dyn.S              |  4 +++
 samples/ftrace/ftrace-direct-modify.c       | 35 ++++++++++++++++++-
 samples/ftrace/ftrace-direct-multi-modify.c | 37 +++++++++++++++++++++
 samples/ftrace/ftrace-direct-multi.c        | 22 ++++++++++++
 samples/ftrace/ftrace-direct-too.c          | 26 +++++++++++++++
 samples/ftrace/ftrace-direct.c              | 22 ++++++++++++
 8 files changed, 154 insertions(+), 1 deletion(-)

-- 
2.20.1