KVM: x86: Don't disable IRQs when unregistering user-return notifier

Remove the code to disable IRQs when unregistering KVM's user-return
notifier now that KVM doesn't invoke kvm_on_user_return() when disabling
virtualization via IPI function call, i.e. now that there's no need to
guard against re-entrancy via IPI callback.

Note, disabling IRQs has largely been unnecessary since commit
a377ac1cd9d7b ("x86/entry: Move user return notifier out of loop") moved
fire_user_return_notifiers() into the section with IRQs disabled. In doing
so, the commit somewhat inadvertently fixed the underlying issue that
was papered over by commit 1650b4ebc99d ("KVM: Disable irq while
unregistering user notifier"). I.e. in practice, the code and comment
has been stale since commit a377ac1cd9d7b.

Signed-off-by: Hou Wenlong <[e-mail]>
[sean: rewrite changelog after rebasing, drop lockdep assert]
Reviewed-by: Kai Huang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

arch/x86/kvm/x86.c

···
 	struct kvm_user_return_msrs *msrs
 		= container_of(urn, struct kvm_user_return_msrs, urn);
 	struct kvm_user_return_msr_values *values;
-	unsigned long flags;
 
-	/*
-	 * Disabling irqs at this point since the following code could be
-	 * interrupted and executed through kvm_arch_disable_virtualization_cpu()
-	 */
-	local_irq_save(flags);
-	if (msrs->registered) {
-		msrs->registered = false;
-		user_return_notifier_unregister(urn);
-	}
-	local_irq_restore(flags);
+	msrs->registered = false;
+	user_return_notifier_unregister(urn);
+
 	for (slot = 0; slot < kvm_nr_uret_msrs; ++slot) {
 		values = &msrs->values[slot];
 		if (values->host != values->curr) {