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

[Wu, Feng]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Thursday, June 25, 2015 3:49 AM
> To: Eric Auger
> Cc: Joerg Roedel; Avi Kivity; Wu, Feng; kvm@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; pbonzini@xxxxxxxxxx; mtosatti@xxxxxxxxxx
> Subject: Re: [v4 08/16] KVM: kvm-vfio: User API for IRQ forwarding
> 
> On Wed, 2015-06-24 at 18:25 +0200, Eric Auger wrote:
> > Hi Joerg,
> >
> > On 06/24/2015 05:50 PM, Joerg Roedel wrote:
> > > On Mon, Jun 15, 2015 at 06:17:03PM +0200, Eric Auger wrote:
> > >> I guess this discussion also is relevant wrt "[RFC v6 00/16] KVM-VFIO
> > >> IRQ forward control" series? Or is that "central registry maintained by
> > >> a posted interrupts manager" something more specific to x86?
> > >
> > > From what I understood so far, the feature you implemented for ARM is a
> > > bit different from the ones that get introduced to x86.
> > >
> > > Can you please share some details on how the ARM version works? I am
> > > interested in how the GICv2 is configured for IRQ forwarding. The
> > > question is whether the forwarding information needs to be updated from
> > > KVM and what information about the IRQ KVM needs for this.
> >
> > The principle is that when you inject a virtual IRQ to a guest, you
> > program a register in the GIC, known as a list register. There you put
> > both the virtual IRQ you want to inject but also the physical IRQ it is
> > linked with (HWbit mode set = forwarding set). When the guest completes
> > the virtual IRQ the GIC HW automatically deactivates the physical IRQ
> > found in the list register. In that mode the physical IRQ deactivation
> > is under the ownership of the guest (actually automatically done by the HW).
> >
> > If HWbit mode is *not* set (forwarding not set), you do not specify the
> > HW IRQ in the list register. The host deactivates the physical IRQ &
> > masks it before triggering the virtual IRQ. Only the virtual IRQ ID is
> > programmed in the list register. When the guest completes the virtual
> > IRQ, a physical maintenance IRQ is triggered. The hyp mode is entered
> > and eventually the host unmasks the IRQ.
> >
> > Some illustrations can be found in
> > http://www.linux-kvm.org/images/a/a8/01x04-ARMdevice.pdf
> 
> 
> I think an important aspect for our design is that in the case of Posted
> Interrupts, they're only used for edge triggered interrupts so VFIO is
> only an information provider for KVM to configure it. 

Exactly! For PI, KVM only needs some information from VFIO when the
guests set the irq affinity.

Thanks,
Feng

 VFIO will
> hopefully just see fewer interrupts as they magically appear directly in
> the guest.  IRQ Forwarding however affects the de-assertion of level
> triggered interrupts.  VFIO needs to switch to something more like an
> edge handler when IRQ Forwarding is enabled.  So in that model, VFIO
> needs to provide information as well as consume it to change behavior.
> Thanks,
> 
> Alex

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