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

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Message-ID: <20150718025116.GB13059@gmail.com>
Date:	Sat, 18 Jul 2015 04:51:16 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
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


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Fri, Jul 17, 2015 at 12:44:42PM -0700, Andy Lutomirski wrote:
> > On Fri, Jul 17, 2015 at 12:43 PM, Ingo Molnar <mingo@...nel.org> wrote:
> > >
> > > * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > >
> > >>  ENTRY(aesni_set_key)
> > >> +     FRAME
> > >>  #ifndef __x86_64__
> > >>       pushl KEYP
> > >>       movl 8(%esp), KEYP              # ctx
> > >> @@ -1905,6 +1907,7 @@ ENTRY(aesni_set_key)
> > >>  #ifndef __x86_64__
> > >>       popl KEYP
> > >>  #endif
> > >> +     ENDFRAME
> > >>       ret
> > >>  ENDPROC(aesni_set_key)
> > >
> > > So cannot we make this a bit more compact and less fragile?
> > >
> > > Instead of:
> > >
> > >         ENTRY(aesni_set_key)
> > >                 FRAME
> > >         ...
> > >                 ENDFRAME
> > >                 ret
> > >         ENDPROC(aesni_set_key)
> > >
> > >
> > > How about writing this as:
> > >
> > >         FUNCTION_ENTRY(aesni_set_key)
> > >         ...
> > >         FUNCTION_RETURN(aesni_set_key)
> > >
> > > which does the same thing in a short, symmetric construct?
> > >
> > > One potential problem with this approach would be that what 'looks' like an entry
> > > declaration, but it will now generate real code.
> > >
> > > OTOH if people find this intuitive enough then it's a lot harder to mess it up,
> > > and I think 'RETURN' makes it clear enough that there's a real instruction
> > > generated there.
> > >
> > 
> > How about FUNCTION_PROLOGUE and FUNCTION_EPILOGUE?
> 
> Perhaps the macro name should describe what the epilogue does, since
> frame pointers aren't required for _all_ functions, only those which
> don't have call instructions.
> 
> What do you think about ENTRY_FRAME and ENDPROC_FRAME_RETURN?  The
> ending macro is kind of long, but at least it a) matches the existing
> ENTRY/ENDPROC convention for asm functions; b) gives a clue that frame
> pointers are involved; and c) lets you know that the return is there.

So the thing I like about these:

	FUNCTION_ENTRY(aesni_set_key)
	...
	FUNCTION_RETURN(aesni_set_key)

is the symmetry - it's a lot harder to misplace/miswrite these than two completely 
separately named things:

	ENTRY_FRAME(aesni_set_key)
	...
	ENDPROC_FRAME_RETURN(aesni_set_key)

Also, the 'FRAME' part will be pointless and somewhat misleading once we do 
dwarves, right?

Another valid variants would be:

	FUNCTION_ENTER(aesni_set_key)
	...
	FUNCTION_RET(aesni_set_key)

or:

	FUNCTION_START(aesni_set_key)
	...
	FUNCTION_RET(aesni_set_key)

or:

	ASM_FUNCTION_START(aesni_set_key)
	...
	ASM_FUNCTION_RET(aesni_set_key)

Note that the name has two parts:

 - The symmetric 'FUNCTION_' prefix tells us that this is a callable function that 
   we are defining. That is a very significant property of this construct, and 
   should be present in both the 'start' and the 'end' markers.

 - The '_RET' stresses the fact that it always generates a 'ret' instruction.

Note what the names _don't_ contain: that we generate debug info! That fact is not 
present in the naming, and that's very much intentional, because the precise form 
of debug info is conditional:

  - if CONFIG_FRAME_POINTERS=y then we push/pop a stack frame

  - if (later on) we do CFI annotations we don't push/pop a stack frame but emit 
    CFI debuginfo

In that sense 'FRAME' should never be in these names I think, nor 'PROC' (which is 
not symmetric).

Plus all 3 variants I suggested are very easy to remember, why I'd always have to 
look up any non-symmetric macro name called 'PROC'...

Thanks,

	Ingo
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