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>

3 files changed, 26 insertions, 0 deletions

diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index 72bd275..528daad 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);
diff --git a/arch/x86/kvm/i8254.h b/arch/x86/kvm/i8254.h
index 4178022..76959c4 100644
--- a/arch/x86/kvm/i8254.h
+++ b/arch/x86/kvm/i8254.h
@@ -9,6 +9,7 @@ struct kvm_kpit_timer {
 	s64 period; /* unit: ns */
 	s64 scheduled;
 	atomic_t pending;
+	bool reinject;
 };
 
 struct kvm_kpit_channel_state {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c3fbe8c..a1f1461 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -993,6 +993,7 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_NOP_IO_DELAY:
 	case KVM_CAP_MP_STATE:
 	case KVM_CAP_SYNC_MMU:
+	case KVM_CAP_REINJECT_CONTROL:
 		r = 1;
 		break;
 	case KVM_CAP_COALESCED_MMIO:
@@ -1728,6 +1729,15 @@ static int kvm_vm_ioctl_set_pit(struct kvm *kvm, struct kvm_pit_state *ps)
 	return r;
 }
 
+static int kvm_vm_ioctl_reinject(struct kvm *kvm,
+				 struct kvm_reinject_control *control)
+{
+	if (!kvm->arch.vpit)
+		return -ENXIO;
+	kvm->arch.vpit->pit_state.pit_timer.reinject = control->pit_reinject;
+	return 0;
+}
+
 /*
  * Get (and clear) the dirty memory log for a memory slot.
  */
@@ -1925,6 +1935,17 @@ long kvm_arch_vm_ioctl(struct file *filp,
 		r = 0;
 		break;
 	}
+	case KVM_REINJECT_CONTROL: {
+		struct kvm_reinject_control control;
+		r =  -EFAULT;
+		if (copy_from_user(&control, argp, sizeof(control)))
+			goto out;
+		r = kvm_vm_ioctl_reinject(kvm, &control);
+		if (r)
+			goto out;
+		r = 0;
+		break;
+	}
 	default:
 		;
 	}