linux-kernel - Re: [PATCH v6 04/12] x86/smpboot.c: Don't offline CPU0 if any irq can not be migrated out of it and remove CPU0 check in smp

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Date:	Thu, 17 May 2012 15:52:51 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Tony Luck <tony.luck@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Asit K Mallick <asit.k.mallick@...el.com>,
	Arjan Dan De Ven <arjan@...ux.intel.com>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Len Brown <len.brown@...el.com>,
	"Srivatssa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Chen Gong <gong.chen@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...r.kernel.org>, x86 <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 04/12] x86/smpboot.c: Don't offline CPU0 if any irq
 can not be migrated out of it and remove CPU0 check in smp_callin()

On 05/14/2012 05:17 AM, Ingo Molnar wrote:
> 
> * Tony Luck <tony.luck@...el.com> wrote:
> 
>> Biggest code impact of that is the extra code to bring cpu0 
>> back online using NMI instead of INIT. We can't use INIT 
>> because if cpu0 gets one, it just resets the whole machine :-( 
>> But obviously we'd like to avoid special cases where there is 
>> a sane way to do so.
> 
> Could we just standardize on NMI bringup during regular bootup?
> 

The first instance has to be SIPI because you can't give the CPU an NMI
without the IDT being setup.  We *could* assume that the processor is
parked in real mode with the valid real-mode IDT set up (in which case
the vector at physical address 8 applies) but that seems to assume a lot
from the BIOS for little gain.  In particular I suspect that UEFI-based
BIOSes may very well park the CPU in protected or long mode.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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