[Qemu-devel] Re: [RFC PATCH 4/5] APIC/IOAPIC EOI callback

[Alex Williamson]

On Sun, 2010-07-11 at 23:12 +0300, Michael S. Tsirkin wrote:
> On Sun, Jul 11, 2010 at 02:03:34PM -0600, Alex Williamson wrote:
> > > What about threaded interrupts?
> 
> Just to make the point, imagine a nested virt situation
> which uses current kvm device assignment in guest.
> Look at the interrupt handler we have there.

Is the problem you're worried about this:

        guest masks qemu ioapic rte
        device interrupt
        host vfio DisINTx+
        qemu vfio calls qemu_set_irq
        ...

In that case, the qemu ioapic irr bit is only toggled by qemu_set_irq
for level triggered interrupts, so the interrupt will be asserted in the
guest when it gets unmasked and we'll get the eoi.

I can't figure out where your other scenario can leave the DisINTx+:

        device asserts interrupt
        a) DisINTx+ via host vfio
        guest disables interrupt
        b) DisINTx+ via guest, already set
        eoi
        c) DisINTx- via qemu vfio
        driver clears interrupt        
        guest enables interrupt
        d) DisINTx- via guest
        device asserts interrupt

So between c) & d) we're potentially getting more interrupts than we
want, but I can't see anywhere that we drop DisINTx.  If you have a
scenario, let me know.  Thanks,

Alex