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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Mon, 4 Feb 2019 14:43:42 +0100
Message-ID: <CAKv+Gu-3vzo4YJxN=qQ5yRDJnWjCt7L96HkoG3y2AUJk_zX3ag@mail.gmail.com>
Subject: Re: [PATCH v7 2/3] arm64: implement ftrace with regs
To:     Torsten Duwe <duwe@lst.de>
Cc:     Julien Thierry <julien.thierry@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        AKASHI Takahiro <takahiro.akashi@linaro.org>,
        Amit Daniel Kachhap <amit.kachhap@arm.com>,
        "Singh, Balbir" <bsingharora@gmail.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        live-patching@vger.kernel.org
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On Mon, 4 Feb 2019 at 13:03, Torsten Duwe <duwe@lst.de> wrote:
>
> On Tue, Jan 22, 2019 at 02:55:12PM +0100, Ard Biesheuvel wrote:
> > On Tue, 22 Jan 2019 at 14:28, Torsten Duwe <duwe@lst.de> wrote:
> > >
> > > On Tue, Jan 22, 2019 at 10:18:17AM +0000, Julien Thierry wrote:
> > > > Hi Torsten,
> > > >
> > > > A few suggestions below.
> > > >
> > > > > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > > > > +#define ARCH_SUPPORTS_FTRACE_OPS 1
> > > > > +#define REC_IP_BRANCH_OFFSET AARCH64_INSN_SIZE
> > > > > +/* All we need is some magic value. Simply use "_mCount:" */
> > > >
> > > > Nit: Should the casing be "_mcount" ?
> > >
> > > No! The string makes it clear what it's supposed to be and the peculiar
> > > casing makes it unique and leaves no doubt were it came from. So whenever
> > > you see this register value in a crash dump you don't have to wonder about
> > > weird linkage errors, as it surely did not originate from a symtab.
> > >
> >
> > In that case, do you need to deal with endianness here?
> >
> > > > > +#define MCOUNT_ADDR                (0x5f6d436f756e743a)
>
> Strictly speaking, yes. OTOH "wrong-andian" machines always show a difference
> when memory is dumped in bigger units than bytes, so when you see the register
> value in hex...
>
> Since all that's needed is a somewhat unique constant, let's not over-engineer
> this ok?
>

Ah ok, if it is only for visual recognition, then I guess it doesn't matter.

> If there are no other comments I'll send out v8 with the discussed changes.
>
>         Torsten
>