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

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Message-ID: <20180814120433.1f5e8da1@gandalf.local.home>
Date:   Tue, 14 Aug 2018 12:04:33 -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 Tue, 14 Aug 2018 09:33:52 +0100
Julien Thierry <julien.thierry@....com> wrote:

> On 14/08/18 03:03, Steven Rostedt wrote:
> > 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),)
> >>> +    $(warning Cannot use CONFIG_DYNAMIC_FTRACE_WITH_REGS: \
> >>> +             -fpatchable-function-entry not supported by compiler)  
> >>
> >> Shouldn't this be an error? The option -fpatchable-function-entry has
> >> been added to the CC_FLAGS_FTRACE, so any call to the compiler is gonna
> >> break anyway. Or am I missing something?  
> > 
> > I'm guessing this adds a more informative message on that error. One
> > will know why -fpatchable-function-entry was added to the CFLAGS. I'm
> > for more informative error messages being a victim of poor error
> > messages causing me to dig deep into the guts of the build
> > infrastructure to figure out simple issues.
> >   
> 
> Yes, I agree it is better to have this message. My point was that we 
> could have "$error" instead of "$warning" to stop the compilation right 
> away since we know everything is gonna break (and on parallel builds 
> this warning is gonna be drowned in compiler errors).
> 

OK, I see what you mean. If the resulting build wont boot, then yes
this should be an error and not a warning.

-- Steve