KVM: PIT: provide an option to disable interrupt reinjection

Certain clocks (such as TSC) in older 2.6 guests overaccount for lost
ticks, causing severe time drift. Interrupt reinjection magnifies the
problem.

Provide an option to disable it.

[avi: allow room for expansion in case we want to disable reinjection
      of other timers]

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

1 files changed, 4 insertions, 0 deletions

diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index 72bd275a9b5c..528daadeba49 100644
--- a/arch/x86/kvm/i8254.c
+++ b/arch/x86/kvm/i8254.c
@@ -201,6 +201,9 @@ static int __pit_timer_fn(struct kvm_kpit_state *ps)
         if (!atomic_inc_and_test(&pt->pending))
                 set_bit(KVM_REQ_PENDING_TIMER, &vcpu0->requests);
 
+        if (!pt->reinject)
+                atomic_set(&pt->pending, 1);
+
         if (vcpu0 && waitqueue_active(&vcpu0->wq))
                 wake_up_interruptible(&vcpu0->wq);
 
@@ -580,6 +583,7 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm)
         pit_state->irq_ack_notifier.gsi = 0;
         pit_state->irq_ack_notifier.irq_acked = kvm_pit_ack_irq;
         kvm_register_irq_ack_notifier(kvm, &pit_state->irq_ack_notifier);
+        pit_state->pit_timer.reinject = true;
         mutex_unlock(&pit->pit_state.lock);
 
         kvm_pit_reset(pit);