linux-kernel - Re: [PATCH v7 2/3] arm64: implement ftrace with regs

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Date:   Thu, 7 Feb 2019 09:51:57 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Julien Thierry <julien.thierry@....com>
Cc:     Torsten Duwe <duwe@....de>, Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v7 2/3] arm64: implement ftrace with regs

On Thu, 7 Feb 2019 10:33:50 +0000
Julien Thierry <julien.thierry@....com> wrote:

> I don't see really much documentation on that function. As far as I can
> tell it is only called once for each site (and if it didn't, we'd always
> be placing the same instruction, but I agree it wouldn't be nice). It
> could depend on how far you can expand the notion of "adjusting" :) .
> 
> Steven, do you have an opinion on whether it would be acceptable to
> modify function entry code in ftrace_call_adjust() ?

Just to make sure I'm on the same page as you are. You want to modify
the function entry code at the time of the ftrace_call_adjust()?

I would update the rec->ip to the offset you want at
ftrace_call_adjust() but not do any modifications. It really isn't safe
to do it there. But right after that is called, you will have the arch
specific call of ftrace_make_nop() called with MCOUNT_ADDR as the
second parameter to let you know that this is the first time the call
is made at this address. This is where you can do that initial
modifications.

-- Steve