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

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Message-ID: <20150722115204.GA23235@treble.redhat.com>
Date:	Wed, 22 Jul 2015 06:52:04 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	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 01:00:06PM -0500, Josh Poimboeuf wrote:
> On Mon, Jul 20, 2015 at 07:21:24PM +0200, Ingo Molnar wrote:
> > * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > Ok, I see how the naming scheme I proposed won't work with all that very well, but 
> > I'd still suggest using consistently named patterns.
> > 
> > Let me suggest yet another approach. How about open-coding something like this:
> > 
> >  FUNCTION_START(func)
> > 
> > 	push_bp
> > 	mov_sp_bp
> > 
> > 	...
> > 
> > 	pop_bp
> > 	ret
> > 
> >  FUNCTION_END(func)
> > 
> > This is just two easy things:
> > 
> >  - a redefine of the FUNCTION_ENTRY and ENDPROC names
> > 
> >  - the introduction of three quasi-mnemonics: push_bp, mov_sp_bp, pop_bp - which 
> >    all look very similar to a real frame setup sequence, except that we can easily 
> >    make them go away in the !CONFIG_FRAME_POINTERS case.
> > 
> > The advantage of this approach would be:
> > 
> >  - it looks pretty 'natural' and very close to how the real disassembly looks
> >    like in CONFIG_FRAME_POINTERS=y kernels. So while it's not as compact as some 
> >    of the other variants, it's close to what the real instruction sequence looks 
> >    like and that is a positive quality in itself.
> > 
> >  - it also makes it apparent 'on sight' that it's probably a bug to have
> >    unbalanced push/pop sequences in a regular function, to any reasonably alert 
> >    assembly coder.
> > 
> >  - if we ever unsupport framepointer kernels in the (far far) future, we can get
> >    rid of all lines with those 3 mnemonics and be done with it.
> > 
> >  - it's finegrained enough so that we can express all the special function/tail
> >    variants you listed above.
> > 
> > What do you think?
> 
> I agree that the edge cases make FUNCTION_ENTRY and FUNCTION_RETURN less
> attractive.  Slowly we are circling around to where we started :-)
> 
> Personally, I prefer FRAME/ENDFRAME instead of push_bp/mov_sp_bp/pop_bp,
> because it more communicates *what* it's doing rather than how.  IMO,
> it's easier to grok with a quick glance.

Ingo, any chance this last paragraph was a convincing argument to
continue to use FRAME/ENDFRAME over push_bp/mov_sp_bp/pop_bp?

(I think this is the last outstanding issue from the reviews, so I'm all
set to send out a new version of the patches once there's agreement on
this issue.)

-- 
Josh
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