KVM: document locking for kvm_io_device_ops

slots_lock is taken everywhere when device ops are called.
Document this as we will use this to rework locking for io.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

virt/kvm/iodev.h

@@ -20,6 +20,9 @@
 
 struct kvm_io_device;
 
+/**
+ * kvm_io_device_ops are called under kvm slots_lock.
+ **/
 struct kvm_io_device_ops {
 	void (*read)(struct kvm_io_device *this,
 		     gpa_t addr,