If you need interface counters on subinterfaces or virtual interfaces, you can emulate them with an empty policy-map, for example:

policy-map Count
 class class-default
!
interface Serial0/0/0.100 point-to-point
 service-policy input Count
 service-policy output Count

The service policy counters are then inspected with the show policy-map interface name command:

a1#show policy-map interface Serial 0/0/0.100

 Serial0/0/0.100

  Service-policy input: Count

    Class-map: class-default (match-any)
      10 packets, 840 bytes
      1 minute offered rate 0 bps, drop rate 0 bps
      Match: any

  Service-policy output: Count

    Class-map: class-default (match-any)
      61 packets, 7084 bytes
      1 minute offered rate 0 bps, drop rate 0 bps
      Match: any