linux-kernel - Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with preemptible kernel and CPU hotplug

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Message-ID: <20080814203122.GC7896@Krystal>
Date:	Thu, 14 Aug 2008 16:31:22 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Harvey Harrison <harvey.harrison@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <srostedt@...hat.com>,
	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>,
	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>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86 alternatives : fix LOCK_PREFIX race with
	preemptible kernel and CPU hotplug

* Jeremy Fitzhardinge (jeremy@...p.org) wrote:
> Mathieu Desnoyers wrote:
>> So should I wait a bit for more comments or straightforwardly submit
>> this as a patch rather than RFC ?
>>   
>
> Looks like all the relevant people have reviewed it now, so I don't think 
> there's much more to say.
>
>    J

I'm just worried about this comment from Harvey Harrison :

arch/x86/mm/fault.c : is_prefetch()

                         * Values 0x26,0x2E,0x36,0x3E are valid x86 prefixes.
                         * In X86_64 long mode, the CPU will signal invalid
                         * opcode if some of these prefixes are present so
                         * X86_64 will never get here anyway
                         */

This comment refers to :

0x26 : ES segment override prefix
0x2E : CS segment override prefix
0x36 : SS segment override prefix
0x3E : DS segment override prefix

AMD documentation seems to indicate that these prefix will be null, not
that the cpu would signal "invalid opcodes" :

"AMD 64-Bit Technology" A.7
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/x86-64_overview.pdf

"In 64-bit mode, the DS, ES, SS and CS segment-override prefixes have no effect.
These four prefixes are no longer treated as segment-override prefixes in the
context of multipleprefix rules. Instead, they are treated as null prefixes."

Intel does not seem to state anything particular about these prefixes
for the 64-bit mode.

So, is this comment misleading, or is it using the term "invalid opcode"
in a way that does not imply generating a fault ?

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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