linux-kernel - Re: [PATCH] i386/audit: stop scribbling on the stack frame

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Date:	Tue, 28 Oct 2014 07:30:01 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Eric Paris <eparis@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Richard Guy Briggs <rgb@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, przanoni@...il.com,
	hpa@...ux.intel.com, linux-tip-commits@...r.kernel.org,
	linux-audit@...hat.com
Subject: Re: [PATCH] i386/audit: stop scribbling on the stack frame


* Eric Paris <eparis@...hat.com> wrote:

> On Mon, 2014-10-27 at 10:02 -0700, H. Peter Anvin wrote:
> > On 10/27/2014 06:55 AM, Eric Paris wrote:
> > > My patch was already committed to the -tip urgent branch.  I believe any
> > > optimization should be based on that branch, Richard.  If you are trying
> > > to wrangle every bit of speed out of this, should you
> > > 
> > > push %esi;
> > > push %edi;
> > > CFI_ADJUST_CFA_OFFSET 8
> > > call __audit_syscall_entry
> > > pop;
> > > pop;
> > > CFI_ADJUST_CFA_OFFSET -8
> > > 
> > > Instead of using the pushl_cfi and popl_cfi macros?
> > > 
> > > I wrote my patch to be obviously correct, but agree there are certainly
> > > some speedups possible.
> > > 
> > 
> > Uh... not only is that plain wrong (the CFI should be adjusted after
> > each instruction that changes the stack pointer),
> 
> Sure, things would be screwed up between the two push's
> 
> >  but what the heck is
> > wrong with using the macros?
> 
> I was asking if that would save an instruction or two by 
> consolidating the CFI update and if so would that tradeoff be 
> worth it, given the regularity of this code being run.

CFI updates have no effect on runtime behavior whatsoever (they 
don't emit any instructions), they only affect the debug info 
being constructed.

Thanks,

	Ingo
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