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

[Avi Kivity <avi@redhat.com>]

On 06/18/2012 01:55 PM, Michael S. Tsirkin wrote:
> On Mon, Jun 18, 2012 at 01:14:13PM +0300, Avi Kivity wrote:
>> On 06/18/2012 01:11 PM, Michael S. Tsirkin wrote:
>> 
>> >> (vhost,
>> >> msi-less ivshmem clone)?
>> > 
>> > I guess vhost can poll eventfd and reinject an interrupt.
>> > Of course to bypass qemu completely we also need to support reads over
>> > ioeventfd somehow.
>> > 
>> 
>> eventfd is not suitable for level triggered interrupts as far as I can
>> tell.  It's about passing edges, and level interrupts aren't.  We need
>> something like a pipe for communicating state (or just use KVM_IRQ_LINE).
> 
> Just using KVM_IRQ_LINE won't be enough.
> There's no software event when device deasserts the
> line, and no stadard way for drivers to do this.
> So kvm needs to trigger on some event to poll the real interrupt
> state, to check whether it's ok to clear for the guest,
> and forward it to userspace.

This in turn only applies to real devices.  Emulated devices of course
know the state of the emulated interrupt line.

Maybe we need different interfaces then.

-- 
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