Re: Virtio interrupt remapping

[Demi Marie Obenour <demiobenour@gmail.com>]

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On 6/13/25 14:13, Jean-Philippe Brucker wrote:
> Hi,
> 
> On Fri, Jun 13, 2025 at 01:08:07PM -0400, Demi Marie Obenour wrote:
>> I’m working on virtio-IOMMU interrupt remapping for Spectrum OS [1],
>> and am running into a problem.  All of the current interrupt remapping
>> drivers use __init code during initialization, and I’m not sure how to
>> plumb the struct virtio_device * into the IOMMU initialization code.
>>
>> What is the proper way to do this, where “proper” means that it doesn’t
>> do something disgusting like “stuff the virtio device in a global
>> variable”?
> 
> I'm not familiar at all with interrupt remapping, but I suspect a major
> hurdle will be device probing order: the PCI subsystem probes the
> virtio-pci transport device relatively late during boot, and the virtio
> driver probes the virtio-iommu device afterwards, at which point we can
> call viommu_probe() and inspect the device features and config.  This can
> be quite late in userspace if virtio and virtio-iommu get loaded as
> modules (which distros tend to do).> 
> The way we know to hold off initializing dependent devices before the
> IOMMU is ready is by reading the firmware tables. In devicetree the
> "msi-parent" and "msi-map" properties point to the interrupt remapping
> device, so by reading those Linux knows to wait for the probe of the
> remapping device before setting up those endpoints. The ACPI VIOT
> describes this topology as well, although at the moment it does not have
> separate graphs for MMU and interrupts, like devicetree does (could
> probably be added to the spec if needed, but I'm guessing the topologies
> may be the same for a VM).  If the interrupt infrastructure supports
> probe deferral, then that's probably the way to go.

I don't see any examples of probe deferral in the codebase.  Would it
instead be possible to require virtio-iommu (and thus virtio) to be
built-in rather than modules?

CCing the IRQ and PCI maintainers as well.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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