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

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Message-ID: <87a9vedyqe.fsf@xmission.com>
Date:	Mon, 22 Oct 2012 13:43:21 -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 01:31 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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