linux-kernel - Re: [PATCH v6] arm64: implement ftrace with regs

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Message-ID: <20190114121359.GB26056@350D>
Date:   Mon, 14 Jan 2019 23:13:59 +1100
From:   Balbir Singh <bsingharora@...il.com>
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>,
        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 v6] arm64: implement ftrace with regs

On Fri, Jan 04, 2019 at 05:50:18PM +0000, Mark Rutland wrote:
> Hi Torsten,
> 
> On Fri, Jan 04, 2019 at 03:10:53PM +0100, Torsten Duwe wrote:
> > Use -fpatchable-function-entry (gcc8) to add 2 NOPs at the beginning
> > of each function. Replace the first NOP thus generated with a quick LR
> > saver (move it to scratch reg x9), so the 2nd replacement insn, the call
> > to ftrace, does not clobber the value. Ftrace will then generate the
> > standard stack frames.
> >

Do we know what the overhead would be, if this was a link time change
for the first instruction?

Also, I was under the impression that some arch's do ftrace_call_replace
under stop_machine(), is that a possibility here?

Balbir Singh