Re: [PATCH v1 2/4] KVM: arm/arm64: vgic: Improve comment on kvm_vgic_inject_irq

[Leo Yan <leo.yan@linaro.org>]

On Fri, Feb 22, 2019 at 09:39:23AM +0000, Marc Zyngier wrote:

[...]

> > > > diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> > > > index 7cfdfbc910e0..79fe64c15051 100644
> > > > --- a/virt/kvm/arm/vgic/vgic.c
> > > > +++ b/virt/kvm/arm/vgic/vgic.c
> > > > @@ -394,7 +394,7 @@ bool vgic_queue_irq_unlock(struct kvm *kvm, struct vgic_irq *irq,
> > > >  /**
> > > >   * kvm_vgic_inject_irq - Inject an IRQ from a device to the vgic
> > > >   * @kvm:     The VM structure pointer
> > > > - * @cpuid:   The CPU for PPIs
> > > > + * @cpuid:   The CPU for PPIs and SPIs
> > > >   * @intid:   The INTID to inject a new state to.
> > > >   * @level:   Edge-triggered:  true:  to trigger the interrupt
> > > >   *			      false: to ignore the call  
> > > 
> > > What does the CPU mean for SPIs? By definition, the routing of an SPI
> > > is defined by the distributor configuration.  
> > 
> > In the code, KVM injects PPIs by specifying CPU id, so that every PPI
> > is bound to specific target CPU.  But for SPIs, it always pass '0' for
> > cpuid, from my understanding this means VM will set interrupt affinity
> > to VCPU0 by default; in theory we also can set different cpuid for
> > SPIs so that the SPIs also can be handled by other secondary VCPUs;
> > this is why I think @cpuid also can be used by SPIs.
> 
> SPIs are not hardcoded to vcpu0. This would be a gross violation of the
> architecture. To convince yourself of this, just run a guest:
> 
> root@unassigned-hostname:~# cat /proc/interrupts 
>            CPU0       CPU1       
>   2:       7315       7353     GIC-0  27 Level     arch_timer
>   4:        158          0     GIC-0  33 Level     uart-pl011
>  42:          0          0     GIC-0  23 Level     arm-pmu
>  43:          0          0     pl061   3 Edge      ACPI:Event
>  44:          0          0       MSI 32768 Edge      virtio1-config
>  45:      10476          0       MSI 32769 Edge      virtio1-req.0
>  46:          0          0       MSI 16384 Edge      virtio0-config
>  47:          3         10       MSI 16385 Edge      virtio0-input.0
> [...]
> 
> On this last line, you can see an SPI being routed to both of these
> vcpus.
> 
> I urge you to read the code further, and understand that for any other
> interrupt class, the cpuid parameter is *ignored*. Yes, we pass zero in
> that case. We could also pass an approximation of PI with the same
> effect.

Very appreciate for the elaborated example; will read the code
furthermore.

> The interrupt affinity is either defined by the distributor
> configuration (SPIs) or the ITS configuration (LPIs).

Given to the up example, I am struggling to understand how you can set
the interrupt affinity for virtio device.

Do you set the physical interrupt affinity to CPU0/1 in host OS and
forward it to guest OS?  Or set interrupt affinity in guest OS (I
tried on Juno board to set irq affinity in guest OS from
'/proc/irq/xxx/smp_affinity' but failed)?   Or this is accomplished by
user space tool (lkvm or qemu)?

Sorry if I am asking a stupid question :)

Thanks,
Leo Yan

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