linux-kernel - Re: [RFC PATCH 13/21] x86/asm/crypto: Fix frame pointer usage in aesni-intel

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Message-ID: <20150720171910.GB28075@treble.redhat.com>
Date:	Mon, 20 Jul 2015 12:19:10 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>, X86 ML <x86@...nel.org>,
	live-patching@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 13/21] x86/asm/crypto: Fix frame pointer usage in
 aesni-intel_asm.S

On Mon, Jul 20, 2015 at 06:52:32PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 20, 2015 at 11:36:46AM -0500, Josh Poimboeuf wrote:
> > 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.
> 
> In case the interrupt happens whilst setting up the frame, right?

Right.

> > 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.
> 
> So the information we'd loose in that case would be the location in the
> calling function, right?

Right.

> Which isn't a problem, if the current function (as obtained
> through RIP) is only ever called once. However if there's multiple call
> sites this might be a wee bit confusing.

Agreed, though the stack dump code always prints '?' for any kernel
address it finds on the stack.  So there would still be a good clue.


-- 
Josh
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