This is way to catch some cases of decrementer overflow, when the
decrementer has underflowed an odd number of times, while MSR[EE] was
disabled.

With a typical small decrementer, a timer that fires when MSR[EE] is
disabled will be "lost" if MSR[EE] remains disabled for between 4.3 and
8.6 seconds after the timer expires. In any case, the decrementer
interrupt would be taken at 8.6 seconds and the timer would be found at
that point.

So this check is for catching extreme latency events, and it prevents
those latencies from being a further few seconds long.  It's not obvious
this is a good tradeoff. This is already a watchdog magnitude event and
that situation is not improved a significantly with this check. For
large decrementers, it's useless.

Therefore remove this check, which avoids a mftb when enabling hard
disabled interrupts (e.g., when enabling after coming from hardware
interrupt handlers). Perhaps more importantly, it also removes the
clunky MSR[EE] vs PACA_IRQ_HARD_DIS incoherency in soft-interrupt replay
which simplifies the code.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201107014336.2337337-1-npiggin@gmail.com