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Date:   Mon, 14 Jan 2019 12:26:16 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Balbir Singh <bsingharora@gmail.com>
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>,
        Amit Daniel Kachhap <amit.kachhap@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org
Subject: Re: [PATCH v6] arm64: implement ftrace with regs
Message-ID: <20190114122616.GD10258@lakrids.cambridge.arm.com>
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 <20190104175017.GA7157@lakrids.cambridge.arm.com>
 <20190114121359.GB26056@350D>
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On Mon, Jan 14, 2019 at 11:13:59PM +1100, Balbir Singh wrote:
> 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?

No, but it should be possible to benchamrk that for a given workload,
which is what I'd like to see.

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

Something like that is a possibility.

I think we need numbers either way.

Thanks,
Mark.