[patch 13/30] KVM: limit lapic periodic timer frequency

[Greg KH <gregkh@suse.de>]

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2.6.30-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Marcelo Tosatti <mtosatti@redhat.com>

(cherry picked from commit 1444885a045fe3b1905a14ea1b52540bf556578b)

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

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 arch/x86/kvm/lapic.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -635,6 +635,15 @@ static void start_apic_timer(struct kvm_
 
 	if (!apic->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->timer.period < NSEC_PER_MSEC/2)
+			apic->timer.period = NSEC_PER_MSEC/2;
+	}
 
 	hrtimer_start(&apic->timer.dev,
 		      ktime_add_ns(now, apic->timer.period),