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ddi_intr_hilevel() returns non-zero if the specified interrupt is a "high level" interrupt.
High level interrupts must be handled without using system services that manipulate thread or process states, because these interrupts are not blocked by the scheduler.
In addition, high level interrupt handlers must take care to do a minimum of work because they are not preemptable.
A typical high level interrupt handler would put data into a circular buffer and schedule a soft interrupt by calling ddi_trigger_softintr(). The circular buffer could be protected
by using a mutex that was properly initialized for the interrupt handler.
ddi_intr_hilevel() can be used before calling ddi_add_intr() to decide which type of interrupt handler should be used. Most device drivers are designed with
the knowledge that the devices they support will always generate low level interrupts, however some devices, for example those using SBus or VME bus level 6 or 7 interrupts must use this test because on
some machines those interrupts are high level (above the scheduler level) and on other machines they are not.
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