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

[Gleb Natapov]

On Sun, Mar 07, 2010 at 02:44:50PM +0100, Jan Kiszka wrote:
> 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.
Current BIOS does not support cpu hotplug/unplug. If after reset
unplugged vcpu is brought up again then there is a bug there somewhere.

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

--
                        Gleb.