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

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Date:	Thu, 23 Apr 2009 12:46:48 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	avi@...hat.com
Subject: Re: [KVM PATCH 2/3] eventfd: add a notifier mechanism

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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