Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

From: Puranjay Mohan <puranjay12@gmail.com>
To: "Björn Töpel" <bjorn@kernel.org>, "Mark Rutland" <mark.rutland@arm.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Steven Rostedt <rostedt@goodmis.org>,
	"Masami Hiramatsu" <mhiramat@kernel.org>,
	Sami Tolvanen <samitolvanen@google.com>,
	Guo Ren <guoren@kernel.org>,
	Ley Foon Tan <leyfoon.tan@starfivetech.com>,
	Deepak Gupta <debug@rivosinc.com>,
	Sia Jee Heng <jeeheng.sia@starfivetech.com>,
	Bjorn Topel <bjorn@rivosinc.com>,
	"Song Shuai" <suagrfillet@gmail.com>,
	Cl'ement L'eger <cleger@rivosinc.com>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	Jisheng Zhang <jszhang@kernel.org>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-trace-kernel@vger.kernel.org,
	Andy Chiu <andy.chiu@sifive.com>
Subject: Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS
Date: Thu, 14 Mar 2024 14:16:04 +0000	[thread overview]
Message-ID: <mb61pplvw6grf.fsf@gmail.com> (raw)
In-Reply-To: <8734suqsth.fsf@all.your.base.are.belong.to.us>

Björn Töpel <bjorn@kernel.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