KVM: limit lapic periodic timer frequency

[Marcelo Tosatti <mtosatti@redhat.com>]

Otherwise its possible to starve the host by programming lapic timer
with a very high frequency.

Cc: stable@kernel.org
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: kvm/arch/x86/kvm/lapic.c
===================================================================
--- kvm.orig/arch/x86/kvm/lapic.c
+++ kvm/arch/x86/kvm/lapic.c
@@ -667,6 +667,15 @@ static void start_apic_timer(struct kvm_
 
 	if (!apic->lapic_timer.period)
 		return;
+	/*
+ 	 * Do not allow the guest to program periodic timers with small
+ 	 * interval, since the hrtimers are not throttled by the host
+	 * scheduler.
+	 */
+	if (apic_lvtt_period(apic)) {
+        	if (apic->lapic_timer.period < NSEC_PER_MSEC/2)
+                	apic->lapic_timer.period = NSEC_PER_MSEC/2;
+	}
 
 	hrtimer_start(&apic->lapic_timer.timer,
 		      ktime_add_ns(now, apic->lapic_timer.period),