[Bug 284057] vmxnet3/iflib: crash in vmxnet3_isc_txd_credits_update

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=284057

--- Comment #12 from Andriy Gapon <avg@FreeBSD.org> ---
Another thing.
There is a curious comment in vmxnet3_msix_intr_assign which I cannot really
understand:
/*
 * Don't provide the corresponding rxq irq for reference -
 * we want the transmit task to be attached to a task queue
 * that is different from the one used by the corresponding
 * rxq irq.  That is because the TX doorbell writes are very
 * expensive as virtualized MMIO operations, so we want to
 * be able to defer them to another core when possible so
 * that they don't steal receive processing cycles during
 * stack turnarounds like TCP ACK generation.  The other
 * piece to this approach is enabling the iflib abdicate
 * option (currently via an interface-specific
 * tunable/sysctl).
 */

Just out of curiosity I set dev.vmx.0.iflib.tx_abdicate=1.
Since doing that I do not see any concurrent calls to
vmxnet3_isc_txd_credits_update for the same queue.
That is, each queue is always handled on its own CPU.

Used this for checking:
dtrace -n 'vmxnet3_isc_txd_credits_update:entry /args[1] !=
curthread->td_oncpu/ { @[stack()] = count(); }'

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