Re: [PATCH 2/2] Enter 2nd kernel with BSP

[HATAYAMA Daisuke]

From: Andi Kleen <andi@xxxxxxxxxxxxxx>
Subject: Re: [PATCH 2/2] Enter 2nd kernel with BSP
Date: Mon, 23 Apr 2012 03:49:15 -0700

>> Split logic into BSP's and AP's: BSP waits for AP halting.
>>
>> Don't remove variable crashing_cpu for debugging use; useful for
>> determining one what CPU crash happens.
> 
> Fenghua has this patchkit to allow offlining the BSP. What happens then?
> 
> It would be good to understand why initializing APs from other APs
> should not work.  Not aware of such a limitation in x86.
> 
> AFAIK when we online a random cpu after boot there is no effort to do it
> from BSP. Why does that work and not from the kexec boot?
> 

Hello Andi,

I have yet to test what I explain here, so I'm sorry I say these with
``would''.

APs to APs case would be entirely my misunderstanding. The problematic
case would be APs to BSP case only.

native_cpu_up avoids to wake up cpu with boot_cpu_physical_apicid, but
the boot_cpu_physical_apicid would be just a boot cpu now, which could
be non-BSP on the 2nd kernel; equal to crashing cpu on the 1st
kernel. It would need to check explicitly whether a given cpu is BSP
or not and this would be better fix here. This is suggested by Eric.

Thanks.
HATAYAMA, Daisuke

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