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

From: Eric W. Biederman
Date: Mon Oct 22 2012 - 16:43:23 EST

"H. Peter Anvin" <hpa@xxxxxxxxx> 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

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.

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