Queries regarding ftrace's implementation for OpenRISC-linux

[Sahil Siddiq <icegambit91@gmail.com>]

Hi,

I have been looking into implementing ftrace for OpenRISC-linux. Based on what I have
understood from the ftrace design document [1], mcount will have to be implemented in
the kernel for ftrace to work.

While this document is outdated, I did come across an article in the Linux Journal [2]
that explains the use of fentry instead of mcount in the implementation of ftrace. It
states that fentry is a powerful alternative to mcount. To make use of fentry, GCC's
-pg flag needs to be used along with the -mfentry flag. Simply calling -pg results in
mcount being used.

Should it be possible for ftrace to make use of either option (fentry and mcount) when
implementing it for OpenRISC? Or should it only use fentry (or mcount)? mcount still
seems to be used in other architectures (I have only checked MIPS, RISC-V, LoongArch
and ARM so far).

Also, or1k-linux-gcc (version 13.3.0) does not support the -mfentry flag. So, if fentry
is to be supported for OpenRISC, this will also involve changes in GCC and glibc.

Thanks,
Sahil

[1] https://www.kernel.org/doc/html/latest/trace/ftrace-design.html
[2] https://web.archive.org/web/20250215201110/https://www.linuxjournal.com/content/simplifying-function-tracing-modern-gcc