Nothing, that's (one of the reasons) why cpu hotplug is not
implemented on x86.
[ Disclosure: I tried to rewrite the x86 boot code to actually bring
CPUs up one at a time, rather than the minimal wedge seen in this
patch, and failed. Me no x86-pert, and the boot code is hairy. ]
I'm still considering a big reader lock around the actual bringing up
of CPUs, pending me completing my audit of what code actually needs
this (surprisingly, very little generic code actually cares how many
CPUs we have, so it's mainly a few corner cases). This would be
cheap, and allow code that cared to lock out cpu changes.
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