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

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Message-ID: <nycvar.YFH.7.76.1810311417361.30233@cbobk.fhfr.pm>
Date:   Wed, 31 Oct 2018 14:19:07 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
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, 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.

Thanks,

-- 
Jiri Kosina
SUSE Labs