Re: [RFC] Notify IRQ sources of level interrupt ack/EOI

[David Woodhouse]

On Wed, 2023-01-11 at 11:25 -0500, Michael S. Tsirkin wrote:
> On Wed, Jan 11, 2023 at 02:41:58PM +0000, David Woodhouse wrote:
> > This allows drivers to register a callback on a qemu_irq, which is
> > invoked when a level-triggered IRQ is acked on the irqchip.
> > 
> > This allows us to simulate level-triggered interrupts more efficiently,
> > by resampling the state of the interrupt only when it actually matters.
> 
> I think we tried this with vfio and had to give up on this.
> I don't remember the details though. Alex probably does?

Hm, not sure why there would be any insurmountable problems.

I've seen this working at scale in a different VMM with split-irqchip
and PCI INTX + Xen event channel support.

And it's what the kernel does internally, and exposes through its dual-
eventfd APIs in both KVM IRQ routing and VFIO interrupts.

That said, I don't care *that* much. I can live with the way I've done
it for the Xen support, by polling the status on a vCPU0 vmexit.
Looking at the VFIO code made me throw up in my mouth a little bit, but
I just won't do that again... :)

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