linux-kernel - Re: [PATCH/RESEND] recordmcount: arm: Implement make

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Date:   Tue, 15 Nov 2016 19:25:12 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Stephen Boyd <sboyd@...eaurora.org>
Cc:     "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH/RESEND] recordmcount: arm: Implement make_nop

On 15 November 2016 at 19:18, Stephen Boyd <sboyd@...eaurora.org> wrote:
> On 11/15, Ard Biesheuvel wrote:
>> On 19 October 2016 at 00:42, Stephen Boyd <sboyd@...eaurora.org> wrote:
>> > In similar spirit to x86 and arm64 support, add a make_nop_arm()
>> > to replace calls to mcount with a nop in sections that aren't
>> > traced.
>> >
>> > Cc: Russell King <linux@....linux.org.uk>
>> > Acked-by: Rabin Vincent <rabin@....in>
>> > Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
>> > ---
>> >  scripts/recordmcount.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++
>> >  1 file changed, 65 insertions(+)
>> >
>> > diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
>> > index 5423a58d1b06..aeb34223167c 100644
>> > --- a/scripts/recordmcount.c
>> > +++ b/scripts/recordmcount.c
>> > @@ -213,6 +213,59 @@ static int make_nop_x86(void *map, size_t const offset)
>> >         return 0;
>> >  }
>> >
>> > +static unsigned char ideal_nop4_arm_le[4] = { 0x00, 0x00, 0xa0, 0xe1 }; /* mov r0, r0 */
>> > +static unsigned char ideal_nop4_arm_be[4] = { 0xe1, 0xa0, 0x00, 0x00 }; /* mov r0, r0 */
>>
>> Shouldn't you be taking the difference between BE8 and BE32 into
>> account here? IIRC, BE8 uses little endian encoding for instructions.
>
> I admit I haven't tested on a pre-armv6 CPU so I haven't come
> across the case of a BE32 CPU. But from what I can tell that
> doesn't matter.
>
> According to scripts/Makefile.build, cmd_record_mcount only runs
> the recordmcount program if CONFIG_FTRACE_MCOUNT_RECORD=y. That
> config is defined as:
>
>         config FTRACE_MCOUNT_RECORD
>                 def_bool y
>                 depends on DYNAMIC_FTRACE
>                 depends on HAVE_FTRACE_MCOUNT_RECORD
>
>
> And in arch/arm/Kconfig we see that DYNAMIC_FTRACE is selected:
>
>         select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
>
> which means that FTRACE_MCOUNT_RECORD can't be set when
> CPU_ENDIAN_BE32 is set.
>
> Do you agree that BE32 is not a concern here?
>

Yes. But that implies then that you should not be using big-endian
instruction encodings at all, and simply use the _le variants for both
LE and BE8