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

From: Julien Thierry
Date: Thu Feb 07 2019 - 09:59:04 EST

On 07/02/2019 14:51, Steven Rostedt wrote:
> On Thu, 7 Feb 2019 10:33:50 +0000
> Julien Thierry <julien.thierry@xxxxxxx> 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.


Julien Thierry