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Date: Wed, 22 Mar 2017 10:48:16 -0500
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: x86@kernel.org, Andy Lutomirski <luto@amacapital.net>,
        Pavel Machek <pavel@ucw.cz>,
        kernel list <linux-kernel@vger.kernel.org>,
        Andrew Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Peter Anvin <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2] x86: mostly disable '-maccumulate-outgoing-args'
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On Wed, Mar 22, 2017 at 08:51:53AM +0100, Ingo Molnar wrote:
> > +#if defined(CONFIG_FUNCTION_GRAPH_TRACER) && \
> > +	!defined(CC_USING_FENTRY) && \
> > +	!defined(CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE)
> > +# error Your compiler does not support function graph tracing
> > +#endif
> 
> Might make sense to add the compiler option that is missing, i.e. something like:
> 
>   # error Your compiler does not support function graph tracing (-mfentry)
> 
> (or whatever compiler feature is missing.)

I left it vague because otherwise it would need a paragraph :-)

After Steven's latest patches which port fentry to x86-32, I think the
precise version would be:

  # error The following combination is not supported: ((compiler missing -mfentry) || (CONFIG_X86_32 and !CONFIG_DYNAMIC_FTRACE)) && CONFIG_FUNCTION_GRAPH_TRACER && CONFIG_CC_OPTIMIZE_FOR_SIZE.

-- 
Josh