A bus interrupt level that maps to a CPU interrupt priority level above the scheduler priority level is called a high-level interrupt. High-level interrupts must be handled without using system services that manipulate threads. In particular, the only kernel routines that high-level interrupt handlers are allowed to call are:
mutex_enter(9F) and mutex_exit(9F) on a mutex initialized with an interrupt block cookie associated with the high-level interrupt
A bus-interrupt level by itself does not determine whether a device interrupts at high level: a given bus-interrupt level may map to a high-level interrupt on one platform, but map to an ordinary interrupt on another platform.
The driver can choose whether to support devices that have high-level interrupts, but it always has to check--it cannot assume that its interrupts are not high level. The function ddi_intr_hilevel(9F), given an interrupt number, returns a value indicating whether the interrupt is high level. For information on checking for high-level interrupts see "Registering Interrupts".