Date: Mon, 20 Jul 2015 11:36:46 -0500
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Michal Marek <mmarek@suse.cz>, Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <andi@firstfloor.org>, Pedro Alves <palves@redhat.com>,
        X86 ML <x86@kernel.org>, live-patching@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 13/21] x86/asm/crypto: Fix frame pointer usage in
 aesni-intel_asm.S
Message-ID: <20150720163646.GA28075@treble.redhat.com>
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On Mon, Jul 20, 2015 at 08:30:52AM -0700, Andy Lutomirski wrote:
> On Fri, Jul 17, 2015 at 8:56 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > The reason I suggested to put FRAME in the macro name is to try to
> > prevent it from being accidentally used for leaf functions, where it
> > isn't needed.
> >
> 
> Could someone remind me why it isn't needed for leaf functions?

If a function doesn't call any other functions, then it won't ever show
up in a stack trace unless:

a) the function itself walks the stack, in which case the frame pointer
   isn't necessary; or

b) The function gets hit by an interrupt/exception, in which case frame
   pointers can't be 100% relied upon anyway.

I've noticed that gcc *does* seem to create stack frames for leaf
functions.  But it's inconsistent, because the early exit path of some
functions will skip the stack frame creation and go straight to the
return.

We could probably get a good performance boost with the
-momit-leaf-frame-pointer flag.  Though it would make stack traces less
reliable when a leaf function gets interrupted.

-- 
Josh
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