linux-kernel - Re: [PATCH v1 2/2] x86, apic: Disable BSP if boot cpu is AP

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Date:	Mon, 22 Oct 2012 13:16:40 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	x86@...nel.org, mingo@...e.hu, tglx@...utronix.de,
	len.brown@...el.com, fenghua.yu@...el.com, vgoyal@...hat.com,
	grant.likely@...retlab.ca, rob.herring@...xeda.com
Subject: Re: [PATCH v1 2/2] x86, apic: Disable BSP if boot cpu is AP

On 10/22/2012 01:04 PM, Eric W. Biederman wrote:
> HATAYAMA Daisuke <d.hatayama@...fujitsu.com> writes:
> 
>> We disable BSP if boot cpu is AP.
>>
>> INIT-INIT-SIPI sequence, a protocal to initiate AP, cannot be used for
>> BSP since it causes BSP jump to BIOS init code; typical visible
>> behaviour is hang or immediate reset, depending on the BIOS init code.
>>
>> INIT can be used to reset AP in a fatal system error state as
>> described in MP spec 3.7.3 Processor-specific INIT. In contrast, there
>> is no processor-specific INIT for BSP to initilize from a fatal system
>> error. It might be possible to do so by NMI plus any hand-crafted
>> reset code that is carefully designed, but at least I have no idea in
>> this direction now.
> 
> Has anyone looked at clearing bit 8 of the IA32_APIC_BASE_MSR (0x1B) on
> the bootstrap processor?  Bit 8 being the bit that indicates we are a
> bootstrap processor.
> 
> If we can clear that bit INIT will always place the processor in
> wait-for-startup-ipi mode and we won't have this problem.
> 
> That would also solve the hotunplug the bootstrap processor without
> using an NMI as well.
> 

IIRC Fenghua experimented with that and it didn't work.  Not all BIOSes
use that bit to determine BSP-ness.

	-hpa

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