linux-kernel - Re: [KVM PATCH v2 2/2] kvm: add support for irqfd via eventfd-notification interface

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Date:	Tue, 28 Apr 2009 08:07:26 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Avi Kivity <avi@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	davidel@...ilserver.org
Subject: Re: [KVM PATCH v2 2/2] kvm: add support for irqfd via	eventfd-notification
 interface

Avi Kivity wrote:
> Gregory Haskins wrote:
>> Avi Kivity wrote:
>>  
>>> Avi Kivity wrote:
>>>    
>>>>> So what is your proposal for such interface?
>>>>>
>>>>>           
>>>> ->write().
>>>>
>>>>       
>>> Alternatively, a new fileop ->signal_event(), which would map to
>>> eventfd_signal() or irqfd_signal().  This would be defined to work in
>>> irq contexts.  It may also be useful for uio interrupts.
>>>
>>>     
>> Hmm...I'm not crazy about either of those.  write() has obvious
>> limitations both from a interrupt execution context, as well as the
>> awkwardness of dealing with creating+passing a viable "userspace"
>> pointer from kernel code.  On the other hand, a new fileop doesn't quite
>> seem appropriate either since it doesn't apply to the overall fileop
>> abstraction very well.
>>
>> We could potentially have a separate vtable interface just for
>> event-type fds, and make eventfd and irqfd the first implementations.
>> But I am not sure it is worth it.  What I suggest is that we work within
>> the existing eventfd interface.  It was designed specifically to signal
>> events, after all.
>>
>> If at some point in the future we need to ensure that the callbacks are
>> not invoked from a preempt-off/irq-off critical section, we can revist
>> the eventfd internals at that time.  Note that since we would like to
>> support signaling from interrupt context anyway, trying to get rid of
>> the wqh critical section that we have today may be a fools errand
>> (*). Instead, we should probably focus on making the injection path
>> support
>> non-preemptible contexts, as this will have the biggest benefits and
>> gains in the long run.
>>
>>   
> But again, you're forcing everyone who uses irqfd to require eventfd.
>
> Maybe we should change eventfd_signal() to fall back to ->write if the
> file happens not to be an eventfd.  It could also handle the
> nonpreemptible context as well.
>
>
Yep, that works.  And the nice thing from my perspective is: we don't
need a v4 ;)

-Greg


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