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

[Avi Kivity <avi@redhat.com>]

On 11/09/2010 08:36 PM, Alex Williamson wrote:
> >
> >  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.

That doesn't sound too good.

> 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.

Or maybe irqfd is not up to this task.

I'd rather stick with the ordinary ioctls for this until proven they're 
too slow.  By proven, I mean a userspace irq forwarding implementation 
that doesn't suck like we it does now (can be just a prototype).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.