linux-kernel - Re: [PATCH 1/9] LoongArch/ftrace: Add basic support

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Message-ID: <20220819215240.3caf89e2@gandalf.local.home>
Date:   Fri, 19 Aug 2022 21:52:40 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jinyang He <hejinyang@...ngson.cn>
Cc:     Qing Zhang <zhangqing@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Jiaxun Yang <jiaxun.yang@...goat.com>
Subject: Re: [PATCH 1/9] LoongArch/ftrace: Add basic support

On Sat, 20 Aug 2022 09:38:21 +0800
Jinyang He <hejinyang@...ngson.cn> wrote:

> I think we have implemented CONFIG_FTRACE_WITH_ARGS in dynamic ftrace
> in the [Patch3/9]. 

Sorry, I must have missed it.

> But, for non dynamic ftrace, it is hardly to
> implement it. Because the LoongArch compiler gcc treats mount as a

Don't bother implementing it for non-dynamic. I would just add a:

config HAVE_FTRACE_WITH_ARGS if DYNAMIC_FTRACE

and be done with it.

> really call, like 'call _mcount(__builtin_return_address(0))'. That
> means, they decrease stack, save args to callee saved regs and may
> do some optimization before calling mcount. It is difficult to find the
> original args and apply changes from tracers.

Right, there's no point in implementing it for non dynamic. Like I said,
non-dynamic is just a stepping stone for getting dynamic working. Once you
have dynamic working, it's up to you to throw out the non-dynamic. It's not
useful for anything other than porting to a new architecture or for
academic purposes.

-- Steve