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

Alex Williamson
2010-07-11T20:04:08Z

On Sun, 2010-07-11 at 22:23 +0300, Michael S. Tsirkin wrote:
> On Sun, Jul 11, 2010 at 01:21:18PM -0600, Alex Williamson wrote:
> > 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
> >
>
> What about threaded interrupts? I think (correct me if I am wrong)
> that they work like this:
>
> device asserts interrupt
> guest disables interrupt

Is this the guest manipulating DisINTx itself? I suppose it could be a
device dependent disable as well.

> eoi
> guest enables interrupt
> driver clears interrupt

These two are hopefully reversed or else the driver is expecting to
clear and potentially reassert interrupts anyway.

> device asserts interrupt
>
> If so, your code will clear DisINTx immediately which
> will always get us another host interrupt:
> performance will be hurt. I am also not sure
> we'll not lose interrupts.

Level interrupts are lossy afaik, if it gets cleared but an interrupt
condition still exists, it should be reasserted.

> It seems we need to track interrupt disable/enable as well, and only
> clear DisINTx after eoi with interrupts enabled. Not sure what is the
> interface for this.

If a driver uses device dependent code to disable interrupts, there's no
issue, we'll clear DisINTx, but the device still won't generate an
interrupt until the dependent code is re-enabled by the guest (assuming
there's no cross talk between DisINTx and device dependent components).

For the case that a guest driver disables via DisINTx, it seems easy to
trap and track that. So we get:

device asserts interrupt
guest disables interrupt
(trapped, qemu-vfio sets intx.guest_disabled = 1)
eoi
(qemu-vfio deasserts qemu interrupts, but because of above doesn't clear DisINTx)
guest enables interrupt
(allowed to pass through, intx.guest_disabled = 0)
driver clears interrupt
device asserts interrupt

I've already got an intx.pending bit, so I think this just changes the eoi to:

vdev->intx.pending = 0;
qemu_set_irq(vdev->pdev.irq[vdev->intx.pin], 0);
if (!vdev->intx.guest_disabled) {
vfio_unmask_intx(vdev);
}

Writing the command register DisINTx bit then just gets some kind of:

if (cmd & PCI_COMMAND_INTX_DISABLE && intx.pending) {
intx.guest_disabled = 1;
cmd &= ~PCI_COMMAND_INTX_DISABLE;
} else if (!(cmd & PCI_COMMAND_INTX_DISABLE) && intx.guest_disabled) {
intx.guest_disabled = 0;
}
... allow write

That work? Thanks,

Alex