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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