KVM: limit lapic periodic timer frequency

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>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 66122bfc74a9..5b9d1ae09cad 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -670,6 +670,15 @@ static void start_apic_timer(struct kvm_lapic *apic)
 
         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),