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

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Date:   Thu, 7 Feb 2019 16:00:29 +0100
From:   Torsten Duwe <duwe@....de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Julien Thierry <julien.thierry@....com>,
        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, Feb 07, 2019 at 09:51:57AM -0500, Steven Rostedt wrote:
> 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()?

Yes, this was the fiendish plan ;-)

> 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.

Ok, so just substitute REC_IP_BRANCH_OFFSET arithmetic with
ftrace_call_adjust() and keep the MCOUNT_ADDR logic.
Thanks for the clarification.

	Torsten