By the change, bpf_mtap can run without any locks as long as its bpf filter
doesn't match a target packet. Pushing data to a bpf buffer still needs
a lock. Removing the lock requires big changes and it's a future work.
Another known issue is that we need to remain some obsolete variables to
avoid breaking kvm(3) users such as netstat and fstat. One problem for
MP-ification is that in order to keep statistic counters of bpf_d we need
to use atomic operations for them. Once we retire the kvm(3) users, we
should make the counters per-CPU and remove the atomic operations.
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