The kernel masks an IRQ line when it delivers the interrupt to userspace
(trigger() -> pic_mask()/ioapic_mask()) and only re-enables it when the
driver writes the count back (kwrite -> acknowledge() -> pic_unmask()/
ioapic_unmask()). That write-back is therefore mandatory, not optional.
ahcid only performed it inside the `is > 0` branch. IRQ lines are shared
(on q35 the AHCI controller sits on IRQ 10 with other devices), so an
interrupt raised by a different device reaches the handler with the HBA
interrupt status register reading 0. In that case ahcid returned without
writing back, leaving the line masked permanently: every subsequent AHCI
completion was lost and all disk I/O blocked forever.
Move the write-back out of the branch so the line is re-armed whether or
not the interrupt turned out to be ours, and keep scheme.tick() gated on
`is > 0` since there is no completion to service otherwise. Device-level
status is still cleared before unmasking, so this cannot cause a storm.
Red Bear OS
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