linux-kernel - Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

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Date:	Wed, 18 Feb 2009 18:10:48 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Yinghai Lu <yinghai@...nel.org>, Nick Piggin <npiggin@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
	single ipi fallback for smp_call_function_many())

ok, it's documented:

   Intel® 64 and IA-32 Architectures
   Software Developer’s Manual
                           Volume 3A:
       System Programming Guide, Part 1

 9.5.3        MSR Access in x2APIC Mode

 To allow for efficient access to the APIC registers in x2APIC 
 mode, the serializing semantics of WRMSR are relaxed when 
 writing to the APIC registers. Thus, system software should not 
 use “WRMSR to APIC registers in x2APIC mode” as a serializing 
 instruction. Read and write accesses to the APIC registers will 
 occur in program order. A WRMSR to an APIC register may 
 complete before all preceding stores are globally visible; 
 software can prevent this by inserting a serializing 
 instruction or MFENCE before the WRMSR.

	Ingo
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