linux-kernel - Re: [PATCH v8 0/6] kvm: level irqfd support

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Date:	Thu, 16 Aug 2012 10:45:55 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	mst@...hat.com, gleb@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 0/6] kvm: level irqfd support

On Thu, 2012-08-16 at 19:32 +0300, Avi Kivity wrote:
> On 08/11/2012 01:37 AM, Alex Williamson wrote:
> > v8:
> > 
> > Trying a new approach.  Nobody seems to like the internal IRQ
> > source ID object and the interactions it implies between irqfd
> > and eoifd, so let's get rid of it.  Instead, simply expose
> > IRQ source IDs to userspace.  This lets the user be in charge
> > of freeing them or hanging onto a source ID for later use.  They
> > can also detach and re-attach components at will.  It also opens
> > up the possibility that userspace might want to use each IRQ
> > source ID for more than one GSI (and avoids the kernel needing
> > to manage that).  Per suggestions, EOIFD is now IRQ_ACKFD.
> > 
> > I really wanted to add a de-assert-only option to irqfd so the
> > irq_ackfd could be fed directly into an irqfd, but I'm dependent
> > on the ordering of de-assert _then_ signal an eventfd.  Keeping
> > that ordering doesn't seem to be possible, especially since irqfd
> > uses a workqueue, if I attempt to make that connection.  Thanks,
> 
> I can't say I'm happy with exposing irq source IDs.  It's true that they
> correspond to a physical entity so they can't be said to be an
> implementation detail, but adding more ABIs has a cost and I can't say
> that I see another user for this.
> 
> Can you provide a link to the combined irqfd+ackfd implementation?  I'm
> inclined now to go for the simplest solution possible.

As soon as I write it :)  Keeping lists to handle the one-to-many
deassert-to-notify will notch up the complexity, but it'll be
interesting to see how it compares.  Thanks,

Alex

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