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

From: Eric W. Biederman
Date: Mon Oct 22 2012 - 16:31:26 EST

"H. Peter Anvin" <hpa@xxxxxxxxx> writes:

> On 10/22/2012 01:04 PM, Eric W. Biederman wrote:
>> HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx> writes:
>>> We disable BSP if boot cpu is AP.
>>> INIT-INIT-SIPI sequence, a protocal to initiate AP, cannot be used for
>>> BSP since it causes BSP jump to BIOS init code; typical visible
>>> behaviour is hang or immediate reset, depending on the BIOS init code.
>>> INIT can be used to reset AP in a fatal system error state as
>>> described in MP spec 3.7.3 Processor-specific INIT. In contrast, there
>>> is no processor-specific INIT for BSP to initilize from a fatal system
>>> error. It might be possible to do so by NMI plus any hand-crafted
>>> reset code that is carefully designed, but at least I have no idea in
>>> this direction now.
>> Has anyone looked at clearing bit 8 of the IA32_APIC_BASE_MSR (0x1B) on
>> the bootstrap processor? Bit 8 being the bit that indicates we are a
>> bootstrap processor.
>> If we can clear that bit INIT will always place the processor in
>> wait-for-startup-ipi mode and we won't have this problem.
>> That would also solve the hotunplug the bootstrap processor without
>> using an NMI as well.
> 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

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