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Date:   Wed, 31 Oct 2018 14:18:19 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Jiri Kosina <jikos@kernel.org>
Cc:     Torsten Duwe <duwe@lst.de>, Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        AKASHI Takahiro <takahiro.akashi@linaro.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org, kristina.martsenko@arm.com
Subject: Re: [PATCH v4 1/3] arm64: implement ftrace with regs
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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.