linux-kernel - Re: [PATCH v4 1/3] arm64: implement ftrace with regs

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Message-ID: <20181031141819.lv2tuj4ne5nf3lh3@lakrids.cambridge.arm.com>
Date:   Wed, 31 Oct 2018 14:18:19 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Torsten Duwe <duwe@....de>, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Julien Thierry <julien.thierry@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        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>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, kristina.martsenko@....com
Subject: Re: [PATCH v4 1/3] arm64: implement ftrace with regs

On Wed, Oct 31, 2018 at 02:19:07PM +0100, Jiri Kosina wrote:
> On Wed, 31 Oct 2018, Mark Rutland wrote:
> 
> > I guess skipping the original function prologue would simplify the
> > implementation of the replacement function (and would mean that the regs
> > held the function arguments per the procedure call standard), but AFAICT
> > other architectures aren't relying on that, so it doesn't seem to be a
> > strict requirement.
> > 
> > What am I missing?
> > 
> > How does livepatching handle the pre-mcount function preambles on
> > architectures with existing support?
> 
> Other architectures do rely on that. That's exactly for example why on x86 
> we use '-pg -mfentry', to make sure we hook the function *before* 
> prologue.

Ah, I'd missed -mfentry for x86. I now see that's also the case with
__gnu_mcount_nc on arch/arm, so that covers my confusion.

Thanks for correcting me, and sorry for noise!

Mark.