We cannot complete the interrupt (i.e. write to the claims/complete
register until the interrupt handler has actually run. We don't run the
interrupt handler immediately from intr_isrc_dispatch(), we only
schedule it for later execution.
If we immediately complete it (i.e. before the interrupt handler proper
has run) the interrupt may be triggered again if the interrupt source
remains set. From RISC-V Instruction Set Manual: Volume II: Priviliged
Architecture, 7.4 Interrupt Gateways:
"If a level-sensitive interrupt source deasserts the interrupt after the
PLIC core accepts the request and before the interrupt is serviced, the
interrupt request remains present in the IP bit of the PLIC core and
will be serviced by a handler, which will then have to determine that
the interrupt device no longer requires service."
In other words, we may receive interrupts twice.
Avoid that by postponing the completion until after the interrupt
handler has run.
If the interrupt is handled by a filter rather than by scheduling an
interrupt thread we must also complete the interrupt, so set up a
post_filter handler (which is the same as the post_ithread handler).
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