Re: [PATCH 3/4] KVM: Register cpuhp and syscore callbacks when enabling hardware

[Chao Gao <chao.gao@intel.com>]

>+static int hardware_enable_all(void)
>+{
>+	int r;
>+
>+	guard(mutex)(&kvm_lock);
>+
>+	if (kvm_usage_count++)
>+		return 0;
>+
>+	r = cpuhp_setup_state(CPUHP_AP_KVM_ONLINE, "kvm/cpu:online",
>+			      kvm_online_cpu, kvm_offline_cpu);

A subtle change is: cpuhp_setup_state() calls kvm_online_cpu() serially
on all CPUs. Previously, hardware enabling is done with on_each_cpu().
I assume performance isn't a concern here. Right?

>+	if (r)
>+		return r;

decrease kvm_usage_count on error?

>+
>+	register_syscore_ops(&kvm_syscore_ops);
>+
>+	/*
>+	 * Undo virtualization enabling and bail if the system is going down.
>+	 * If userspace initiated a forced reboot, e.g. reboot -f, then it's
>+	 * possible for an in-flight operation to enable virtualization after
>+	 * syscore_shutdown() is called, i.e. without kvm_shutdown() being
>+	 * invoked.  Note, this relies on system_state being set _before_
>+	 * kvm_shutdown(), e.g. to ensure either kvm_shutdown() is invoked
>+	 * or this CPU observes the impending shutdown.  Which is why KVM uses
>+	 * a syscore ops hook instead of registering a dedicated reboot
>+	 * notifier (the latter runs before system_state is updated).
>+	 */
>+	if (system_state == SYSTEM_HALT || system_state == SYSTEM_POWER_OFF ||
>+	    system_state == SYSTEM_RESTART) {
>+		unregister_syscore_ops(&kvm_syscore_ops);
>+		cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
>+		return -EBUSY;

ditto

>+	}
>+
>+	return 0;
>+}