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

[Wu, Feng]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Saturday, June 13, 2015 3:04 AM
> To: Avi Kivity
> Cc: Wu, Feng; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> pbonzini@xxxxxxxxxx; mtosatti@xxxxxxxxxx; eric.auger@xxxxxxxxxx
> Subject: Re: [v4 08/16] KVM: kvm-vfio: User API for IRQ forwarding
> 
> On Fri, 2015-06-12 at 21:48 +0300, Avi Kivity wrote:
> > 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@xxxxxxxxx]
> > >>> Sent: Friday, June 12, 2015 3:59 AM
> > >>> To: Wu, Feng; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > >>> Cc: pbonzini@xxxxxxxxxx; mtosatti@xxxxxxxxxx;
> > >>> alex.williamson@xxxxxxxxxx; eric.auger@xxxxxxxxxx
> > >>> 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@xxxxxxxxxx>
> > >>>>
> > >>>> 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).
> 
> Yep, those all sound like good things and I believe that's similar in
> design to the way we had originally discussed this interaction at
> LPC/KVM Forum several years ago.  I'd be in favor of that approach.
> Thanks,

This seems a little complex compared to the current solution, since I am
not quite familiar with VFIO, Alex, can you help on this if we need to do this
that way, especially for the VFIO part?

Thanks,
Feng

> 
> Alex

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