Re: [RFC PATCH] KVM: Add module for IRQ forwarding

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/05/20 16:34, Alex Williamson wrote:
>> Alternatively, if you assign the i2c controller, I don't understand why
>> the guest doesn't discover interrupts on its own.  Of course you need to
>> tell the guest about the devices in the ACPI tables, but why is this new
>> concept necessary?
>
> the interrupt for this sub-device is unrelated to the PCI
> controller device, it's an entirely arbitrary (from our perspective)
> relationship described via ACPI.

Ok, that's what I was missing.

> We could potentially use device specific interrupts to expose this via
> the controller device, but then vfio-pci needs to learn how to
> essentially become an i2c controller to enumerate the sub-devices and
> collect external dependencies.  This is not an approach I've embraced
> versus the alternative of the host i2c driver claiming the PCI
> controller, enumerating the sub-devices, and binding the resulting
> device, complete with built-in interrupt support via vfio-platform.

I agree that's the way to go.

Perhaps adding arbitrary non-PCI interrupts, i.e. neither INTX nor
MSI(-X), to vfio-pci could be acceptable.  However, the device claim
must claim them, and that seems hard to do when you rebind the PCI
device to pci-stub.

Paolo