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

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Date:	Fri, 17 Jul 2015 14:01:33 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	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>,
	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 Fri, Jul 17, 2015 at 1:59 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Fri, Jul 17, 2015 at 01:46:39PM -0700, Andy Lutomirski wrote:
>> On Fri, Jul 17, 2015 at 1:44 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>> > On Fri, Jul 17, 2015 at 01:39:09PM -0700, Andy Lutomirski wrote:
>> >> On Fri, Jul 17, 2015 at 1:37 PM, 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.
>> >> >
>> >>
>> >> This really is about frame pointers, right?  How about
>> >> ENTRY_FRAMEPTR_xyz where xyz can be prologue, epilogue, return,
>> >> whatever?
>> >
>> > Wouldn't the "ENTRY" in ENTRY_FRAMEPTR_RETURN be confusing at the end of
>> > a function?
>>
>> I meant ENTRY_FRAMEPTR_xyz and the beginning and ENDPROC_FRAMEPTR_xyz
>> (ENTRY is debatable, but that's what we currently have).  ENDPROC
>> could easily be replaced with anything else.
>
> So do you mean ENTRY_FRAMEPTR_PROLOGUE and ENDPROC_FRAMEPTR_EPILOGUE?
> Or something else?
>

I like it.  I think this bikeshed might be well painted now!

--Andy

> --
> Josh



-- 
Andy Lutomirski
AMA Capital Management, LLC
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