em(4) suffers from threads jumping in and out of context.
em_local_timer() is not rearmed enough and will jump in during normal processing of queues and transmit out of order data.
Reduce thread count by consolidating Multi-queue processing to a combined tx/rx model.
Remove restrictions on 82574 for EM_MULTIQUEUE. With needing only 3 MSI-X interrupts, this should allow other cards to be tested. I have not done this.
Do not defer to em_handle_que() taskqueue during transmit processing, ever.