Re: [v4 08/16] KVM: kvm-vfio: User API for IRQ forwarding

[Avi Kivity <avi.kivity@gmail.com>]

On 06/12/2015 06:41 PM, Alex Williamson wrote:
> On Fri, 2015-06-12 at 00:23 +0000, Wu, Feng wrote:
>>> -----Original Message-----
>>> From: Avi Kivity [mailto:avi.kivity@gmail.com]
>>> Sent: Friday, June 12, 2015 3:59 AM
>>> To: Wu, Feng; kvm@vger.kernel.org; linux-kernel@vger.kernel.org
>>> Cc: pbonzini@redhat.com; mtosatti@redhat.com;
>>> alex.williamson@redhat.com; eric.auger@linaro.org
>>> Subject: Re: [v4 08/16] KVM: kvm-vfio: User API for IRQ forwarding
>>>
>>> On 06/11/2015 01:51 PM, Feng Wu wrote:
>>>> From: Eric Auger <eric.auger@linaro.org>
>>>>
>>>> This patch adds and documents a new KVM_DEV_VFIO_DEVICE group
>>>> and 2 device attributes: KVM_DEV_VFIO_DEVICE_FORWARD_IRQ,
>>>> KVM_DEV_VFIO_DEVICE_UNFORWARD_IRQ. The purpose is to be able
>>>> to set a VFIO device IRQ as forwarded or not forwarded.
>>>> the command takes as argument a handle to a new struct named
>>>> kvm_vfio_dev_irq.
>>> Is there no way to do this automatically?  After all, vfio knows that a
>>> device interrupt is forwarded to some eventfd, and kvm knows that some
>>> eventfd is forwarded to a guest interrupt.  If they compare notes
>>> through a central registry, they can figure out that the interrupt needs
>>> to be forwarded.
>> Oh, just like Eric mentioned in his reply, this description is out of context of
>> this series, I will remove them in the next version.
>
> I suspect Avi's question was more general.  While forward/unforward is
> out of context for this series, it's very similar in nature to
> enabling/disabling posted interrupts.  So I think the question remains
> whether we really need userspace to participate in creating this
> shortcut or if kvm and vfio can some how orchestrate figuring it out
> automatically.
>
> Personally I don't know how we could do it automatically.  We've always
> relied on userspace to independently setup vfio and kvm such that
> neither have any idea that the other is there and update each side
> independently when anything changes.  So it seems consistent to continue
> that here.  It doesn't seem like there's much to gain performance-wise
> either, updates should be a relatively rare event I'd expect.
>
> There's really no metadata associated with an eventfd, so "comparing
> notes" automatically might imply some central registration entity.  That
> immediately sounds like a much more complex solution, but maybe Avi has
> some ideas to manage it.  Thanks,
>

The idea is to have a central registry maintained by a posted interrupts 
manager.  Both vfio and kvm pass the filp (along with extra information) 
to the posted interrupts manager, which, when it detects a filp match, 
tells each of them what to do.

The advantages are:
- old userspace gains the optimization without change
- a userspace API is more expensive to maintain than internal kernel 
interfaces (CVEs, documentation, maintaining backwards compatibility)
- if you can do it without a new interface, this indicates that all the 
information in the new interface is redundant.  That means you have to 
check it for consistency with the existing information, so it's extra 
work (likely, it's exactly what the posted interrupt manager would be 
doing anyway).