linux-kernel - Re: [PATCH v7 2/3] arm64: implement ftrace with regs

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Message-ID: <5efd27b1-81b5-267a-9fe8-67bb084d56eb@arm.com>
Date:   Thu, 7 Feb 2019 14:58:55 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Torsten Duwe <duwe@....de>, Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        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 v7 2/3] arm64: implement ftrace with regs



On 07/02/2019 14:51, Steven Rostedt wrote:
> On Thu, 7 Feb 2019 10:33:50 +0000
> Julien Thierry <julien.thierry@....com> wrote:
> 
>> I don't see really much documentation on that function. As far as I can
>> tell it is only called once for each site (and if it didn't, we'd always
>> be placing the same instruction, but I agree it wouldn't be nice). It
>> could depend on how far you can expand the notion of "adjusting" :) .
>>
>> Steven, do you have an opinion on whether it would be acceptable to
>> modify function entry code in ftrace_call_adjust() ?
> 
> Just to make sure I'm on the same page as you are. You want to modify
> the function entry code at the time of the ftrace_call_adjust()?
> 

Yes, that was the intention, the reason being that we want to have an
instruction that is just patched once on each entry for initialization.

> I would update the rec->ip to the offset you want at
> ftrace_call_adjust() but not do any modifications. It really isn't safe
> to do it there. But right after that is called, you will have the arch
> specific call of ftrace_make_nop() called with MCOUNT_ADDR as the
> second parameter to let you know that this is the first time the call
> is made at this address. This is where you can do that initial
> modifications.

Yes, this is was the current version of this patch is doing, I was just
wondering whether we could clean things up a little (i.e. have
ftrace_make_nop() not generate a non-nop instruction :) ).

But we're not going to hack through the API if this is definitely not
what it's intended for. We'll keep it as is and just update the rec->ip
with ftrace_call_adjust().

If we really want to clean things up, we could look into patching this
at instruction at build time. But if it's not trivial I guess we can
keep that for a later time.

Thanks,

-- 
Julien Thierry