Re: [KVM PATCH v2 2/2] kvm: add support for irqfd via eventfd-notificationinterface

[Avi Kivity]

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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