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

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

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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.

Jan


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