These functions are commonly found in device drivers being ported from uniprocessors. Such drivers rely on the use of splhi() to gain exclusive use of a global resource.
The spl functions are supported by IRIX and they are effective in a uniprocessor driver. However, in a multiprocessor, the functions affect only the interrupt handling of the current CPU. Other CPUs in the system continue to handle interrupts, including interrupts initiated by the driver that called splhi().
A driver that is not multiprocessor-aware (one that does not have D_MP in its pfxdevflag constant; see "Driver Flag Constant") runs only in CPU 0 of a multiprocessor, so in this case the spl functions are still effective. Since they set the interrupt level on CPU 0 where the driver runs, and since the driver's interrupts can only be handled on CPU 0, the use of splhi() gives the driver exclusive use of its resources.
A driver that is multiprocessor-aware uses basic locks, synchronization variables, and other tools to control access to resources, and never uses an spl function. This improves performance in a multiprocessor, does not harm performance in a uniprocessor, and reduces the latency of all interrupts.