linux-kernel - Re: [PATCH 1/2] ftrace: Introduce ftrace_need_init

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Date:   Sat, 31 Jul 2021 08:47:35 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Sven Schnelle <svens@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, linux-s390@...r.kernel.org
Subject: Re: [PATCH 1/2] ftrace: Introduce ftrace_need_init_nop()

On Fri, Jul 30, 2021 at 10:07:41PM -0400, Steven Rostedt wrote:
> On Wed, 28 Jul 2021 23:25:45 +0200
> Ilya Leoshkevich <iii@...ux.ibm.com> wrote:
> 
> > Implementing live patching on s390 requires each function's prologue to
> > contain a very special kind of nop, which gcc and clang don't generate.
> > However, the current code assumes that if CC_USING_NOP_MCOUNT is
> > defined, then whatever the compiler generates is good enough.
> > 
> > Move the CC_USING_NOP_MCOUNT check into the new ftrace_need_init_nop()
> > macro, that the architectures can override.
> > 
> > An alternative solution is to disable using -mnop-mcount in the
> > Makefile, however, this makes the build logic (even) more complicated
> > and forces the arch-specific code to deal with the useless __fentry__
> > symbol.
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@...ux.ibm.com>
> 
> I'm fine with this patch, and also you taking it in via the s390 tree.
> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Thanks a lot!