KVM: document KVM_REINJECT_CONTROL ioctl

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -3092,6 +3092,30 @@ of IOMMU pages.
 
 The rest of functionality is identical to KVM_CREATE_SPAPR_TCE.
 
+4.98 KVM_REINJECT_CONTROL
+
+Capability: KVM_CAP_REINJECT_CONTROL
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_reinject_control (in)
+Returns: 0 on success,
+         -EFAULT if struct kvm_reinject_control cannot be read,
+         -ENXIO if KVM_CREATE_PIT or KVM_CREATE_PIT2 didn't succeed earlier.
+
+i8254 (PIT) has two modes, reinject and !reinject.  The default is reinject,
+where KVM queues elapsed i8254 ticks and monitors completion of interrupt from
+vector(s) that i8254 injects.  Reinject mode dequeues a tick and injects its
+interrupt whenever there isn't a pending interrupt from i8254.
+!reinject mode injects an interrupt as soon as a tick arrives.
+
+struct kvm_reinject_control {
+	__u8 pit_reinject;
+	__u8 reserved[31];
+};
+
+pit_reinject = 0 (!reinject mode) is recommended, unless running an old
+operating system that uses the PIT for timing (e.g. Linux 2.4.x).
+
 5. The kvm_run structure
 ------------------------