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

From: ebiederm@xmission.com (Eric W. Biederman)
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: len.brown@intel.com, fenghua.yu@intel.com, x86@kernel.org,
	kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
	rob.herring@calxeda.com, grant.likely@secretlab.ca,
	HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>,
	tglx@linutronix.de, mingo@elte.hu, vgoyal@redhat.com
Subject: Re: [PATCH v1 2/2] x86, apic: Disable BSP if boot cpu is AP
Date: Mon, 22 Oct 2012 13:43:21 -0700	[thread overview]
Message-ID: <87a9vedyqe.fsf@xmission.com> (raw)
In-Reply-To: <5085AD7D.106@zytor.com> (H. Peter Anvin's message of "Mon, 22 Oct 2012 13:33:01 -0700")

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

> On 10/22/2012 01:31 PM, Eric W. Biederman wrote:
>>>
>>> IIRC Fenghua experimented with that and it didn't work.  Not all BIOSes
>>> use that bit to determine BSP-ness.
>> 
>> What does a BIOS have to do with anything?
>> 
>> The practical issue here is does an INIT IPI cause the cpu to go into
>> startup-ipi-wait or to start booting at 4G-16 bytes.
>> 
>> For dealing with BIOSen we may still need to use the bootstrap processor
>> for firmware calls, cpu suspend, and other firmware weirdness, but that
>> should all be completely orthogonal to the behavior to what happens
>> when an INIT IPI is sent to the cpu.
>> 
>> The only firmware problem I can imagine having is cpu virtualization
>> bug.
>> 
>
> The whole problem is that some BIOSes go wonky after receiving an INIT
> (as in INIT-SIPI-SIPI) to the BSP.

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.

Eric

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