Re: Question about edge-triggered interrupt

[Peter Maydell]

On Thu, 11 Mar 2021 at 02:59, LIU Zhiwei <zhiwei_liu@c-sky.com> wrote:
> Currently, I am writing an interrupt controller (CLIC) for RISC-V.  I can't 
> find a good way to process edge-triggered interrupt.
>
> According to edge-triggered definition, if I select an edge-triggered  
> interrupt to serve , it will clean its pending status. However after serving 
> the interrupt,  there is no chance to select other pending interrupts.
>
> I have two methods.
>
> One is to add a timer for interrupt controller, so that every pending 
> interrupt can be served at some point.
> The other is that  always pull a pending interrupt to serve at the interrupt 
> return instruction.

You should do what the hardware you're modelling does. I'm
pretty sure it won't be using a timer, at any rate. Whether
it does something on interrupt-return insns will depend on
the architecture and how tightly the interrupt controller
is integrated with the CPU.

In general edge-triggered interrupts for an interrupt
controller just mean "when it sees a 0->1 transition
on its input line it does something, eg mark the interrupt
pending", and also usually "when the guest OS acknowledges
the interrupt (eg via a register write to the interrupt
controller), mark the interrupt as no longer pending", and
a "keep telling the OS that there is an available interrupt
of some kind until all the interrupts are no longer pending".

The details of how this works depend entirely on the
interrupt controller -- hopefully it has a specification
document which will tell you the actual behaviour.

thanks
-- PMM