preserve the ABI and API for applications. It was removed in the port
to amd64, but was remained as garbage giving a micro-pessimization and
spurious single-step traps on i386.
pcb_psl was intended to be used just to do a context switch of PSL_I,
but this context switch was null in most or all versions, and
mis-switching of PSL_T was done instead.
Some history:
- in 386BSD-0.0, cpu_switch() ran at splhigh() and splhigh() did too
much interrupt disabling, so interrupts were hard-disabled across
cpu_switch() and too many other places
- in 386BSD-0.0-patchkit through FreeBSD-4 and FreeBSD-5 before
SMPng, splhigh() did soft interrupt masking, and cpu_switch() was
excessively cautious and did a cli at the start and a sti at the
end to hard-disable interrupts across the switch
- SMPng replaced the spl's and cli's by spinlocks (just sched_lock?),
so interrupts were hard-disabled across cpu_switch() and too many
other places again
- initial attempts to fix this intended to restore some soft
interrupt disabling, but to support variations in this cpu_switch()
used pushfl/popfl into pcb_psl to avoid hard-coding the assumption
that the initial and final states have PSL_I enabled. But the
version with soft interrupt disabling wasn't used for long, or was
never committed, (except I always used my different version of it
for UP) so the pushfl/popl and pcb_psl to hold them have been doing
less than nothing for about 14 years.
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