Re: [PATCH v4 3/9] KVM: Clear assigned guest IRQ on release

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

On Tue, 2010-11-09 at 12:58 +0200, Avi Kivity wrote:
> On 11/08/2010 01:21 PM, Jan Kiszka wrote:
> > When we deassign a guest IRQ, clear the potentially asserted guest line.
> > There might be no chance for the guest to do this, specifically if we
> > switch from INTx to MSI mode.
> >
> > Signed-off-by: Jan Kiszka<jan.kiszka@siemens.com>
> > ---
> >   virt/kvm/assigned-dev.c |    3 +++
> >   1 files changed, 3 insertions(+), 0 deletions(-)
> >
> > diff --git a/virt/kvm/assigned-dev.c b/virt/kvm/assigned-dev.c
> > index 7c98928..ecc4419 100644
> > --- a/virt/kvm/assigned-dev.c
> > +++ b/virt/kvm/assigned-dev.c
> > @@ -141,6 +141,9 @@ static void deassign_guest_irq(struct kvm *kvm,
> >   	kvm_unregister_irq_ack_notifier(kvm,&assigned_dev->ack_notifier);
> >   	assigned_dev->ack_notifier.gsi = -1;
> >
> > +	kvm_set_irq(assigned_dev->kvm, assigned_dev->irq_source_id,
> > +		    assigned_dev->guest_irq, 0);
> > +
> >   	if (assigned_dev->irq_source_id != -1)
> >   		kvm_free_irq_source_id(kvm, assigned_dev->irq_source_id);
> >   	assigned_dev->irq_source_id = -1;
> 
> I guess this can't hurt.  Did you see it happen in practice?
> 
> Note: all this kvm_set_irq(..., [01]) is incorrect as it doesn't account 
> for polarity.  Currently the qemu-emulated chipset uses level high pci 
> interrupts, but that's not a given from kvm's point of view.
> 
> I think vfio fixes this by only routing msi interrupts via the kernel, 
> and routing level interrupts through userspace, which can adjust 
> polarity.  Alex/Michael?

The latest patches will route legacy interrupts via irqfd if available
too.  We do have the issue that KVM pulses interrupts injected this way,
but it seems to work nonetheless.  I was thinking about proposing a
level flag for set_irqfd to only set it, allowing the ack notifier to
deassert it.  Perhaps we also need a flag to toggle the polarity.

Alex