Re: [KVM PATCH 2/3] eventfd: add a notifier mechanism

Gregory Haskins
2009-04-23T16:47:16Z

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Davide Libenzi wrote:
> On Thu, 23 Apr 2009, Gregory Haskins wrote:
>
>
>> This allows synchronous notifications to register with the eventfd
>> infrastructure. Unlike traditional vfs based eventfd readers, notifiees
>> do not implictly clear the counter on reception. However, the clearing
>> is primarily important to allowing threads to block waiting for events
>> anyway, so its an acceptable trade-off since blocking doesn't apply to
>> notifiers.
>>
>
> Do you really need to add a notifier? Eventfd already has a wait queue,
> and we support callback-based wakeups, so is there any reason we shouldn't
> use those and rely on the already existing wakeups?
>
Well, IIUC the issue is that a wait queue implies that you are in fact
waiting...which we may not. :)

The target in this particular application with kvm-irqfd is a vcpu
context, which *may* be sleeping in something like a HLT, but it also
could be in a number of other states such as non-root (guest) mode, it
could be running in the kernel, it could be up in userspace, etc.

That said: I am not married to the concept that this has to be a
notifier callback, but I do want to be able to meet the target
application. So if there is some way to do that within the existing
wait-queue contstruct, I am open to suggestions.

Thanks Davide,
-Greg

>
>
> - Davide
>
>
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