From: ebiederm@xmission.com (Eric W. Biederman)
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>, linux-kernel@vger.kernel.org,
        kexec@lists.infradead.org, x86@kernel.org, mingo@elte.hu,
        tglx@linutronix.de, len.brown@intel.com, fenghua.yu@intel.com,
        vgoyal@redhat.com, grant.likely@secretlab.ca, rob.herring@calxeda.com
References: <20121016043357.20003.5885.stgit@localhost6.localdomain6>
	<20121016043528.20003.601.stgit@localhost6.localdomain6>
	<873916i88t.fsf@xmission.com> <5085A9A8.5020004@zytor.com>
	<87fw56fduo.fsf@xmission.com> <5085AD7D.106@zytor.com>
	<87a9vedyqe.fsf@xmission.com> <5085B0D0.9020508@zytor.com>
Date: Mon, 22 Oct 2012 14:29:25 -0700
In-Reply-To: <5085B0D0.9020508@zytor.com> (H. Peter Anvin's message of "Mon,
	22 Oct 2012 13:47:12 -0700")
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Subject: Re: [PATCH v1 2/2] x86, apic: Disable BSP if boot cpu is AP
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"H. Peter Anvin" <hpa@zytor.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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