Re: Extensions for KVM MSI related ioctls

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/07/2015 12:25, Andre Przywara wrote:
> 
> For using MSIs in a guest when running on an ARM(64) system using a
> GICv3 interrupt controller we need to have a device ID available. On
> real hardware this information is sampled from the bus by the ITS part
> of the interrupt controller.
> To make this work for guests, we need to extend two ioctls which deal
> with MSIs: KVM_SIGNAL_MSI and KVM_SET_GSI_ROUTING.
> 
> The idea that we sketched so far in [1] and [2] is to use a new
> capability (KVM_CAP_MSI_DEVID) to advertise both a flag bit for
> KVM_SIGNAL_MSI and a new type for KVM_SET_GSI_ROUTING.
> 
> Since current kernels bail out on any flag value != 0 in KVM_SIGNAL_MSI,
> we need the new capability to tell userland about it in a reliable and
> portable way (to avoid hacks like #ifdef ARM && USES_IRQ_ROUTING in
> userland).
> 
> For KVM_SET_GSI_ROUTING there was the idea of using the very same flag
> value in it's own flag field, but I find it saner to use a new routing
> type instead (KVM_IRQ_ROUTING_EXTENDED_MSI). Both approaches would
> piggy-back on the existing struct kvm_irq_routing_msi and re-purpose the
> pad field in there.

I think I prefer the flag.  Offhand it sounds easier to add support for
it to non-ARM architectures, compared to KVM_IRQ_ROUTING_EXTENDED_MSI.

> Summarized:
> 1) Add a new KVM_CAP_MSI_DEVID to advertise the device ID extension.
> 2) Use a KVM_MSI_VALID_DEVID flag in KVM_SIGNAL_MSI to re-purpose part
>    of struct kvm_msi.

These two sounds good.

Paolo

> 3) Add a new routing type KVM_IRQ_ROUTING_EXTENDED_MSI for
>    KVM_SET_GSI_ROUTING to add device IDs in struct kvm_irq_routing_msi.
> 
> Is that a sensible way to extend the KVM API?