[Qemu-devel] Re: [PATCH v4 03/10] x86: Extend validity of cpu_is_bsp

[Jan Kiszka <jan.kiszka@web.de>]

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Jan Kiszka wrote:
> Gleb Natapov wrote:
>> On Thu, Mar 04, 2010 at 02:03:54PM +0200, Gleb Natapov wrote:
>>>> BTW, do real systems allow to hot plug BSP as well? Or how is the case
>>>> handled when you unplug the BSP and then reboot the box?
>>>>
>>> Did you mean hot unplug BSP? OS determines what CPU is BSP by checking
>>> BSP bit in APIC base register. My guess is that there is some pin on CPU
>>> which value is mirrored as BSP bit in APIC base register. Board may have
>>> some logic to check what sockets are populated and chose one of them as
>>> BSP by pulling its pin up. But this is only guess.
>>>
>> Actually this is much more simple:
>> SDM 8.4.1:
>>  The MP initialization protocol defines two classes of processors: the
>>  bootstrap processor (BSP) and the application processors (APs). Following
>>  a power-up or RESET of an MP system, system hardware dynamically selects
>>  one of the processors on the system bus as the BSP. The remaining
>>  processors are designated as APs.
>> And by "hardware" they mean CPUs themselves over apic BUS.
> 
> That should be straightforward, will have a look. I just want to ensure
> that cpu_is_bsp delivers a valid result all the time. Otherwise we will
> run into ordering issues again once some new user of this services shows up.
> 

...but you have to explain to me how CPU hotplugging in current qemu-kvm
is supposed to work. 'cpu_set n offline' does not have any visible
effect: Linux still allows to bring such a CPU online again, and it is
also brought up again after reset.

Besides this, I would probably need some other OS to test offlining the
BSP - Linux does not appear to support it.

Jan


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