[patch 41/45] KVM: limit lapic periodic timer frequency

[Greg KH]

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

------------------
From: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

commit 1444885a045fe3b1905a14ea1b52540bf556578b upstream.

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

Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

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

--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -573,6 +573,15 @@ 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),


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