linux-kernel - Re: [PATCH v3 3/7] riscv: ftrace: prepare ftrace for atomic code patching

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Message-ID: <Z1OKPxHBJ0kV5TbA@archie.me>
Date: Sat, 7 Dec 2024 06:35:27 +0700
From: Bagas Sanjaya <bagasdotme@...il.com>
To: Björn Töpel <bjorn@...nel.org>,
	Andy Chiu <andybnac@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Robbin Ehn <rehn@...osinc.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org, bjorn@...osinc.com,
	puranjay12@...il.com, alexghiti@...osinc.com,
	yongxuan.wang@...ive.com, greentime.hu@...ive.com,
	nick.hu@...ive.com, nylon.chen@...ive.com, tommy.wu@...ive.com,
	eric.lin@...ive.com, viccent.chen@...ive.com, zong.li@...ive.com,
	samuel.holland@...ive.com
Subject: Re: [PATCH v3 3/7] riscv: ftrace: prepare ftrace for atomic code
 patching

On Fri, Dec 06, 2024 at 11:02:29AM +0100, Björn Töpel wrote:
> Adding Robbin for input, who's doing much more crazy text patching in
> JVM, than what we do in the kernel. ;-)
> 
> Let's say we're tracing "f". Previously w/ stop_machine() it was
> something like:
> 
> f:
> 1: nop
>    nop
>    ...
>    ...
> 
> ftrace_caller:
>    ...
>    auipc	a2, function_trace_op
>    ld		a2, function_trace_op(a2)
>    ...
> 2: auipc    ra, ftrace_stub
>    jalr     ftrace_stub(ra)
> 
> The text was patched by ftrace in 1 and 2.
> 
> ...and now:
> f:
>    auipc    t0, ftrace_caller
> A: nop
>    ...
>    ...
> 
> ftrace_caller:
>    ...
>    auipc    a2, function_trace_op
>    ld       a2, function_trace_op(a2)
>    ...
>    auipc    ra, ftrace_call_dest
>    ld       ra, ftrace_call_dest(ra)
>    jalr     ra 
> 
> The text is only patched in A, and the tracer func is loaded via
> ftrace_call_dest.

Previously the operation was no-op, right?

Confused...

-- 
An old man doll... just what I always wanted! - Clara

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