The documentation still refers to PCIEM_EVENT_MMIO_READ but it's never referenced in the code on the main branch
I'll admit that I asked for a simple compilable example illustrating something simple like the read events because it looks like it's just reading from the shared memory, and maybe generating an event for any read or write access with the PCIEM_EVENT_MMIO_WRITE event type
PCIEM_EVENT_MMIO_READ is kept for reference on the while (head != tail) loop just to remind the user that there can be more than 1 event registered in terms of access type.
Let's say that you register your watchpoint in READ mode (I still have to change the IOCTL for that as currently is hardcoded for writes: attr.bp_type = HW_BREAKPOINT_W), then you'd be consuming PCIEM_EVENT_MMIO_READ events instead of PCIEM_EVENT_MMIO_WRITE.
The fact that the PCIEM_EVENT_MMIO_READ define is there is to help me remind me to incorporate that missing logic.