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

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Message-ID: <49F58FD6.907@novell.com>
Date:	Mon, 27 Apr 2009 06:58:30 -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

Gregory Haskins wrote:
> Avi Kivity wrote:
>   
>>
>> One day we'll have lockless injection and we'll want to drop this.  I
>> guess if we create the fd ourselves we can make it work, but I don't
>> see how we can do this with eventfd.
>>
>>     
>
> Hmm...this is a good point.  There probably is no way to use eventfd
> "off the shelf" in a way that doesn't cause this callback to be in a
> critical section.  Should we just worry about switching away from
> eventfd when this occurs, or should I implement a custom anon-fd now?
>   

Something else to consider:  eventfd supports calling eventfd_signal()
from interrupt context, so
even if the callback were not invoked from a preempt/irq off CS due to
the wqh->lock, the context may still be a CS anyway.  Now, userspace and
vbus based injection do not need to worry about this, but I wonder if
this is a  desirable attribute for some other source (device-assignment
perhaps)?

If so, is there a way to design the lockless-injection code such that
its still friendly to irq-context, or is this a mode that we would never
want to support anyway?  I think this would be a good thing to have at
least to maintain compatibility with the existing eventfd interface,
which has its own advantages.

-Greg

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