x86/reboot: Assert that IRQs are disabled when turning off virtualiza…

…tion

Assert that IRQs are disabled when turning off virtualization in an
emergency.  KVM enables hardware via on_each_cpu(), i.e. could re-enable
hardware if a pending IPI were delivered after disabling virtualization.

Remove a misleading comment from emergency_reboot_disable_virtualization()
about "just" needing to guarantee the CPU is stable (see above).

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kernel/reboot.c

@@ -532,7 +532,6 @@ static inline void nmi_shootdown_cpus_on_restart(void);
 
 static void emergency_reboot_disable_virtualization(void)
 {
-	/* Just make sure we won't change CPUs while doing this */
 	local_irq_disable();
 
 	/*
@@ -821,6 +820,13 @@ void cpu_emergency_disable_virtualization(void)
 {
 	cpu_emergency_virt_cb *callback;
 
+	/*
+	 * IRQs must be disabled as KVM enables virtualization in hardware via
+	 * function call IPIs, i.e. IRQs need to be disabled to guarantee
+	 * virtualization stays disabled.
+	 */
+	lockdep_assert_irqs_disabled();
+
 	rcu_read_lock();
 	callback = rcu_dereference(cpu_emergency_virt_callback);
 	if (callback)