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From: Puranjay Mohan <puranjay12@gmail.com>
To: =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>, Mark Rutland
 <mark.rutland@arm.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt
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 linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
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Subject: Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS
In-Reply-To: <8734suqsth.fsf@all.your.base.are.belong.to.us>
References: <20240306165904.108141-1-puranjay12@gmail.com>
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 <8734suqsth.fsf@all.your.base.are.belong.to.us>
Date: Thu, 14 Mar 2024 14:16:04 +0000
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Bj=C3=B6rn T=C3=B6pel <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 ju=
mp
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