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

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Message-ID: <87mwze8abu.fsf@xmission.com>
Date:	Mon, 22 Oct 2012 14:29:25 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.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

"H. Peter Anvin" <hpa@...or.com> writes:

> On 10/22/2012 01:43 PM, Eric W. Biederman wrote:
>> 
>> The reason the BIOSen go wonky is the INIT cause the cpu to go to the
>> reset vector at 4G-16 bytes.  So it is very much expected that the
>> BIOSen start acting like you just came out of reset.
>> 
>> If you can clear bit 8 of IA32_APIC_BASE_MSR and inform the cpu to not
>> send the cpu to 4G-16 bytes and instead send the cpu into it's magic
>> startup-ipi-wait mode then the BIOSen will not be involved on that path.
>> 
>> It is a simple question of does the cpu support clearing bit 8
>> meaningfully.
>> 
>> If the cpu allows bit 8 to be cleared and sends the cpu to the reset
>> vector on receipt of the INIT IPI I would call that a deviation from the
>> x86 cpu specification.
>> 
>> So clearing bit 8 is not a question about BIOSen it is a question of can
>> we avoid the BIOSen, by using an obscure under-documented cpu feature.
>> 
>
> 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.

Eric

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