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

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Message-ID: <48A4980C.8030805@zytor.com>
Date:	Thu, 14 Aug 2008 13:39:40 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Harvey Harrison <harvey.harrison@...il.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

Mathieu Desnoyers wrote:
> 
> 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 ?
> 

They do not signal faults, there just aren't any base addresses behind 
them.  Some AMD chips allow limits to be set on these segments -- 
apparently added on behalf of some hypervisor makers; I suspect that 
VMX/SVM is making that quickly obsolete.

So it should be just fine.

Acked-by: H. Peter Anvin <hpa@...or.com>

	-hpa

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