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 14:48:11 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.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

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.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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