Re: [Qemu-devel] [PATCH v10 16/26] intel_iommu: add support for split irqchip

[Peter Xu <peterx@redhat.com>]

On Sun, Jun 26, 2016 at 03:27:50PM +0200, Jan Kiszka wrote:
> On 2016-06-26 03:48, Peter Xu wrote:
> > On Sat, Jun 25, 2016 at 05:18:40PM +0200, Jan Kiszka wrote:
> >> On 2016-06-25 15:18, Peter Xu wrote:
> >>> On Sat, Jun 25, 2016 at 10:08:10AM +0200, Jan Kiszka wrote:
> > 
> > [...]
> > 
> >>> I have a thought on how to implement the "sink" you have mentioned:
> >>>
> >>> First of all, in KVM, we provide a new KVM_IRQ_ROUTING_* type, maybe
> >>> called:
> >>>
> >>>   KVM_IRQ_ROUTING_EVENTFD
> >>
> >> Not really, because all sources are either using eventfds, which you can
> >> also terminate in user space (already done for vhost and vfio in certain
> >> scenarios - IIRC) or originate there anyway (IOAPIC).
> > 
> > But how should we handle the cases when the interrupt path are all in
> > kernel?
> 
> There are none which we can't redirect (only full in-kernel irqchip
> would have, but that's unsupported anyway).
> 
> > 
> > For vhost, data should be transfered all inside kernel when split
> > irqchip and irqfd are used: when vhost got data, it triggers irqfd to
> > deliver the interrupt to KVM. Along the way, we should all in kernel.
> > 
> > For vfio, we have vfio_msihandler() who handles the hardware IRQ and
> > then triggers irqfd as well to KVM. Again, it seems all in kernel
> > space, no chance to stop that as well.
> > 
> > Please correct me if I was wrong.
> 
> Look at what vhost is doing e.g.: when a virtqueue is masked, it
> installs an event notifier that records incoming events in a pending
> state field. When it's unmasked, the corresponding KVM irqfd is installed.

You are right. Thanks for the explaination.

-- peterx