Re: [RFC PATCH] kvm: Extend irqfd to support level interrupts

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Jun 17, 2012 at 03:38:57PM -0600, Alex Williamson wrote:
> On Sun, 2012-06-17 at 21:44 +0300, Michael S. Tsirkin wrote:
> > On Sat, Jun 16, 2012 at 10:34:39AM -0600, Alex Williamson wrote:
> > > I'm looking for opinions on this approach.  For vfio device assignment
> > > we minimally need a way to get EOIs from the in-kernel irqchip out to
> > > userspace.  Getting that out via an eventfd would allow us to bounce
> > > all level interrupts out to userspace, where we would de-assert the
> > > device interrupt in qemu and unmask the physical device.  Ideally we
> > > could deassert the interrupt in KVM, which allows us to send the EOI
> > > directly to vfio.  To do that, we need to use a new IRQ source ID so
> > > the guest sees the logical OR of qemu requested state and external
> > > device state.
> > 
> > Given that yopu want to involve userspace anyway, why insist on irqfd
> > for this?  You can simply use KVM_IRQ_LINE_STATUS from qemu, no?
> 
> Well, actually I'd like to have a way to bypass userspace, which the
> combination of an irqfd + eventfd w/ deassert does.

Above you said "bounce all level interrupts out to userspace"?

>  I'm not quite sure
> I understand how KVM_IRQ_LINE_STATUS would work for this.  AIUI, that
> effectively gives us a way to post an interrupt AND let us know whether
> it was masked, coalesced, or delivered.  So I'd have to poll by posting
> a potentially spurious interrupt and if it was spurious unmask the
> physical device and wait for a real interrupt?  What am I missing,
> because that seems barely functional?  Thanks,
> 
> Alex

Sorry if I did not make this clear.  You still need to add a way to pass
EOI writes out to userspace. But that's all you need: once
userspace gets EOI events for specific GSIs, it can control IRQ level from userspace
in the same way it does it for emulated devices:
wake up on EOI, poll all devices, if no interrupt asserted -
clear IRQ line and unmask IRQ in assigned device.

Above is with plain exit to userspace rather than eventfd,
though eventfd is also possible but then you need to clear
irq level in kernel and re-assert from userspace later -
I think it's ok but need to think about possible races.
 

-- 
MST