linux-kernel - Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL

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Message-ID: <mb61pplvw6grf.fsf@gmail.com>
Date: Thu, 14 Mar 2024 14:16:04 +0000
From: Puranjay Mohan <puranjay12@...il.com>
To: Björn Töpel <bjorn@...nel.org>, Mark Rutland
 <mark.rutland@....com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, Steven Rostedt
 <rostedt@...dmis.org>, "Masami
 Hiramatsu" <mhiramat@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>,
 Guo Ren <guoren@...nel.org>, Ley Foon Tan <leyfoon.tan@...rfivetech.com>,
 Deepak Gupta <debug@...osinc.com>, Sia Jee Heng
 <jeeheng.sia@...rfivetech.com>, Bjorn Topel <bjorn@...osinc.com>, "Song
 Shuai" <suagrfillet@...il.com>, Cl'ement L'eger <cleger@...osinc.com>, "Al
 Viro" <viro@...iv.linux.org.uk>, Jisheng Zhang <jszhang@...nel.org>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Andy Chiu <andy.chiu@...ive.com>
Subject: Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

Björn Töpel <bjorn@...nel.org> writes:

>
> Hmm, depending on RISC-V's CMODX path, the pro/cons CALL_OPS vs dynamic
> trampolines changes quite a bit.
>
> The more I look at the pains of patching two instruction ("split
> immediates"), the better "patch data" + one insn patching look.

I was looking at how dynamic trampolines would be implemented for RISC-V.

With CALL-OPS we need to patch the auipc+jalr at function entry only, the
ops pointer above the function can be patched atomically.

With a dynamic trampoline we need a auipc+jalr pair at function entry to jump
to the trampoline and then another auipc+jalr pair to jump from trampoline to
ops->func. When the ops->func is modified, we would need to update the
auipc+jalr at in the trampoline.

So, I am not sure how to move forward here, CALL-OPS or Dynamic trampolines?

Thanks,
Puranjay