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capability. One of reason using interrupt taskqueue in re(4) was to reduce number of TX/RX interrupts under load because re(4) controllers have no good TX/RX interrupt moderation mechanism. Basic TX interrupt moderation is done by hardware for most controllers but RX interrupt moderation through undocumented register showed poor RX performance so it was disabled in r215025. Using taskqueue to handle RX interrupt greatly reduced number of interrupts but re(4) consumed all available CPU cycles to run the taskqueue under high TX/RX network load. This can happen even with RTL810x fast ethernet controller and I believe this is not acceptable for most systems. To mitigate the issue, use one-shot timer register to moderate RX interrupts. The timer register provides programmable one-shot timer and can be used to suppress interrupt generation. The timer runs at 125MHZ on PCIe controllers so the minimum time allowed for the timer is 8ns. Data sheet says the register is 32 bits but experimentation shows only lower 13 bits are valid so maximum time that can be programmed is 65.528us. This yields theoretical maximum number of RX interrupts that could be generated per second is about 15260. Combined with TX completion interrupts re(4) shall generate less than 20k interrupts. This number is still slightly high compared to other intelligent ethernet controllers but system is very responsive even under high network load. Introduce sysctl variable dev.re.%d.int_rx_mod that controls amount of time to delay RX interrupt processing in units of us. Value 0 completely disables RX interrupt moderation. To provide old behavior for controllers that have MSI/MSI-X capability, introduce a new tunable hw.re.intr_filter. If the tunable is set to non-zero value, driver will use interrupt taskqueue. The default value of the tunable is 0. This tunable has no effect on controllers that has no MSI/MSI-X capability or if MSI/MSI-X is explicitly disabled by administrator. While I'm here cleanup interrupt setup/teardown since re(4) uses single MSI/MSI-X message at this moment.
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