linux-kernel - Re: [PATCH v5 02/10] x86: Compile-time asm code validation

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Message-ID: <20150610221716.GF1125@treble.redhat.com>
Date:	Wed, 10 Jun 2015 17:17:16 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Michal Marek <mmarek@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	live-patching@...r.kernel.org, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v5 02/10] x86: Compile-time asm code validation

On Wed, Jun 10, 2015 at 01:58:45PM -0500, Josh Poimboeuf wrote:
> On Wed, Jun 10, 2015 at 11:15:19AM -0700, Andy Lutomirski wrote:
> > On Wed, Jun 10, 2015 at 10:53 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > > On Wed, Jun 10, 2015 at 10:21:36AM -0700, Andy Lutomirski wrote:
> > >> GCC can generate those, and the ia32_ptregs_common label is an example
> > >> of such a thing.
> > >>
> > >> I'd rather have the script understand tail calls and possibly require
> > >> that ia32_ptregs_common have a dummy frame pointer save in front
> > >> before the label if needed.
> > >
> > > Why do you prefer tail calls there?  See patch 3 for how I handled that
> > > for ia32_ptregs_common (I duplicated the code with macros).
> > >
> > > I think adding support for tail calls in the tooling would be tricky.
> > > So I'm just trying to figure out if there's a good reason to keep them.
> > 
> > To save code size by deduplicating common tails.  The code currently
> > does that, and it would be nice to avoid bloating the code to keep the
> > validator happy.
> 
> Well, I wonder whether it's really worth sacrificing code readability
> and consistency, and maybe some improved i-cache locality, to save a few
> hundred bytes of code size.

I should also mention that my proposed ia32_ptregs_common patch, which
duplicated the needed code, was more optimized for performance than code
size.

But if you're more worried about code size, we could turn
ia32_ptregs_common into a proper callable function, and then replace

   jmp ia32_ptregs_common

with:

   call ia32_ptregs_common
   ret

So it becomes a regular call instead of a tail call.  It only adds a few
instructions and the function is self-contained.  Would that be good
enough?

-- 
Josh
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