From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v1 2/2] x86, apic: Disable BSP if boot cpu is AP
Date: Mon, 22 Oct 2012 17:35:47 -0700

> On 10/22/2012 02:29 PM, Eric W. Biederman wrote:
>>>
>>> As I said, I thought Fenghua tried that but it didn't work,
>>> experimentally.
>>
>> Fair enough.  You described the problem with clearing bit 8 in a weird
>> way.
>>
>> If the best we can muster are fuzzy memories it may be worth
>> revisiting.
>> Perhaps it works on enough cpu models to be interesting.
>>
> 
> It isn't fuzzy memories... this was done as late as 1-2 months ago.  I
> just don't know the details.
> 
> Fenghua, could you help fill us in?
> 

I overlooked completely the fact that BSP flag is rewritable.

I tried Eric's suggestion using attached test programs and saw it
worked fine at least on the three cpus around me below:

- Intel(R) Xeon(R) CPU E7- 4820  @ 2.00GHz
- Intel(R) Xeon(R) CPU E7- 8870  @ 2.40GHz
- Intel(R) Xeon(TM) CPU 1.80GHz
  - 32 bits CPU

Next I found the description about this in 8.4.2, IASDM Vol.3:

  The MP initialization protocol imposes the following requirements
  and restrictions on the system:

  * The MP protocol is executed only after a power-up or RESET. If the
    MP protocol has completed and a BSP is chosen, subsequent INITs
    (either to a specific processor or system wide) do not cause the
    MP protocol to be repeated. Instead, each logical processor
    examines its BSP flag (in the IA32_APIC_BASE MSR) to determine
    whether it should execute the BIOS boot-strap code (if it is the
    BSP) or enter a wait-for-SIPI state (if it is an AP).

So this is no longer undocumented behaviour for recent cpus, I think.

Considering these, I'll make a patch to clear BSP flag at appropreate
position in kernel boot-up code. OTOH, according to the discussion, it
was reported that clearing BSP flag affected some BIOSes. To deal with
this, I'll prepare a kernel option to decide whether to clear BSP flag
or not.

Does anyone have any comments now? Or please comment after I submit a
new patch.

Thanks.
HATAYAMA, Daisuke