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

[Alex Williamson <alex.williamson@redhat.com>]

On Sun, 2010-07-11 at 21:54 +0300, Michael S. Tsirkin wrote:
> 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 think those are all protected by host and qemu vfio drivers managing
DisINTx.  The way I understand it to work now is:

	device asserts interrupt
	interrupt lands in host vfio driver
	host vfio sets DisINTx on the device
	host vfio sends eventfd
	eventfd lands in qemu vfio, does a qemu_set_irq
        ... guest processes
	guest writes eoi to apic, lands back in qemu vfio driver
	qemu vfio deasserts qemu interrupt
	qemu vfio clears DisINTx

So I don't think there's a race as long as ordering is sane for toggling
DisINTx.  Thanks,

Alex