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

[Michael S. Tsirkin]

On Sun, Jul 11, 2010 at 09:30:59PM +0300, Avi Kivity wrote:
> On 07/11/2010 09:26 PM, Alex Williamson wrote:
> >On Sun, 2010-07-11 at 21:14 +0300, Avi Kivity wrote:
> >>On 07/11/2010 09:09 PM, Alex Williamson wrote:
> >>>For device assignment, we need to know when the VM writes an end
> >>>of interrupt to the APIC, which allows us to de-assert the interrupt
> >>>line and clear the DisINTx bit.  Add a new wrapper for ioapic
> >>>generated interrupts with a callback on eoi and create an interface
> >>>for drivers to be notified on eoi.
> >>>
> >>You aren't going to get this with kvm's in-kernel irqchip, so we need a
> >>new interface there.
> >Registering an eventfd for the eoi seems like a reasonable alternative.
> 
> I'm worried about that racing (with what?)

With device asserting the interrupt?
Need to make sure that all possible scenarious work well:

        device asserts interrupt
        driver clears interrupt
        device asserts interrupt
        eoi

        device asserts interrupt
        driver clears interrupt
        eoi
        device asserts interrupt

etc

Not that I see issues, these are things we need to check.

> >I also need to figure out how to avoid bouncing the vfio interrupt
> >events through qemu, but it's a functional start.  Thanks,
> 
> I thought the scheduler has/wants to have something that moves the
> irq to whatever thread it wakes up.  With irqfd, it would flow
> naturally.
> 
> -- 
> I have a truly marvellous patch that fixes the bug which this
> signature is too narrow to contain.