linux-kernel - Re: [PATCH] ftrace: use only 5 byte nops for x86

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Message-ID: <alpine.LFD.1.10.0808151542330.3324@nehalem.linux-foundation.org>
Date:	Fri, 15 Aug 2008 15:44:10 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Ingo Molnar <mingo@...e.hu>, Jeremy Fitzhardinge <jeremy@...p.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Roland McGrath <roland@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Clark Williams <williams@...hat.com>, srostedt@...hat.com
Subject: Re: [PATCH] ftrace: use only 5 byte nops for x86



On Fri, 15 Aug 2008, H. Peter Anvin wrote:

> Ingo Molnar wrote:
> > > > 
> > > > TODO: Examine the cpuid to determine the nop to use.
> > > >   
> > > Don't think that will help in general.  qemu claims its a Pentium II.
> > 
> > ok. I've applied Steve's patch as it's a good bugfix. The extra paranoia
> > about faulting on 0x90 seems excessive but doesnt hurt ;-)
> > 
> 
> I think what we need to do is to test the NOPL instruction and create a
> Linux-specific CPUID bit for it.  I'll do that and submit to tip:x86/cpu.

I also suspect that we'd really be much better off just fixing the generic 
NOP tables for the 5-byte nop. As far as I could tell, from all the 
numbers that have been posted, absolutely _none_ show that there is any 
point at all to the 2-instruction 3/2-byte sequence.

So instead of having a magic special ftrace-only thing, why not just do it 
right, and fix the generic 5-byte nop sequence?

		Linus
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