KVM: x86: Prevent starting PIT timers in the absence of irqchip support

commit 0924ab2c upstream

User space may create the PIT and forgets about setting up the irqchips.
In that case, firing PIT IRQs will crash the host:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000128
IP: [<ffffffffa10f6280>] kvm_set_irq+0x30/0x170 [kvm]
...
Call Trace:
 [<ffffffffa11228c1>] pit_do_work+0x51/0xd0 [kvm]
 [<ffffffff81071431>] process_one_work+0x111/0x4d0
 [<ffffffff81071bb2>] worker_thread+0x152/0x340
 [<ffffffff81075c8e>] kthread+0x7e/0x90
 [<ffffffff815a4474>] kernel_thread_helper+0x4/0x10

Prevent this by checking the irqchip mode before starting a timer. We
can't deny creating the PIT if the irqchips aren't set up yet as
current user land expects this order to work.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
[dannf: backported to Debian's 2.6.32]
Signed-off-by: Willy Tarreau <w@1wt.eu>

arch/x86/kvm/i8254.c

@@ -277,11 +277,15 @@ static struct kvm_timer_ops kpit_ops = {
 	.is_periodic = kpit_is_periodic,
 };
 
-static void create_pit_timer(struct kvm_kpit_state *ps, u32 val, int is_period)
+static void create_pit_timer(struct kvm *kvm, u32 val, int is_period)
 {
+	struct kvm_kpit_state *ps = &kvm->arch.vpit->pit_state;
 	struct kvm_timer *pt = &ps->pit_timer;
 	s64 interval;
 
+	if (!irqchip_in_kernel(kvm))
+		return;
+
 	interval = muldiv64(val, NSEC_PER_SEC, KVM_PIT_FREQ);
 
 	pr_debug("pit: create pit timer, interval is %llu nsec\n", interval);
@@ -333,13 +337,13 @@ static void pit_load_count(struct kvm *kvm, int channel, u32 val)
         /* FIXME: enhance mode 4 precision */
 	case 4:
 		if (!(ps->flags & KVM_PIT_FLAGS_HPET_LEGACY)) {
-			create_pit_timer(ps, val, 0);
+			create_pit_timer(kvm, val, 0);
 		}
 		break;
 	case 2:
 	case 3:
 		if (!(ps->flags & KVM_PIT_FLAGS_HPET_LEGACY)){
-			create_pit_timer(ps, val, 1);
+			create_pit_timer(kvm, val, 1);
 		}
 		break;
 	default: