linux-kernel - Re: [PATCH 1/3] arm64: implement ftrace with regs

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Date:   Mon, 13 Aug 2018 22:03:15 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Julien Thierry <julien.thierry@....com>
Cc:     Torsten Duwe <duwe@....de>, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH 1/3] arm64: implement ftrace with regs

On Mon, 13 Aug 2018 11:54:06 +0100
Julien Thierry <julien.thierry@....com> wrote:

> > --- a/arch/arm64/Makefile
> > +++ b/arch/arm64/Makefile
> > @@ -78,6 +78,15 @@ ifeq ($(CONFIG_ARM64_MODULE_PLTS),y)
> >   KBUILD_LDFLAGS_MODULE	+= -T $(srctree)/arch/arm64/kernel/module.lds
> >   endif
> >   
> > +ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +  CC_FLAGS_FTRACE := -fpatchable-function-entry=2
> > +  KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
> > +  ifeq ($(call cc-option,-fpatchable-function-entry=2),)
> > +    $(warning Cannot use CONFIG_DYNAMIC_FTRACE_WITH_REGS: \
> > +             -fpatchable-function-entry not supported by compiler)  
> 
> Shouldn't this be an error? The option -fpatchable-function-entry has 
> been added to the CC_FLAGS_FTRACE, so any call to the compiler is gonna 
> break anyway. Or am I missing something?

I'm guessing this adds a more informative message on that error. One
will know why -fpatchable-function-entry was added to the CFLAGS. I'm
for more informative error messages being a victim of poor error
messages causing me to dig deep into the guts of the build
infrastructure to figure out simple issues.

-- Steve