Virtio interrupt remapping
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”? -- Sincerely, Demi Marie Obenour (she/her/hers)
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. Thanks, Jean
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)
Demi Marie Obenour <demiobenour@gmail.com> writes:
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?
It's certainly possible to have an optional feature in the kernel that depends on a module being built in where it otherwise wouldn't have to be.
[+Eric] On Sat, Jun 14, 2025 at 10:11:52AM +0200, Alyssa Ross wrote:
Demi Marie Obenour <demiobenour@gmail.com> writes:
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.
I think the flow with VIOT is roughly: // Scan an endpoint pci_bus_add_device() device_attach() driver_probe_device() really_probe() dev->bus->dma_configure() pci_dma_configure() acpi_dma_configure() acpi_iommu_configure_id() viot_iommu_configure() viot_dev_iommu_init() acpi_iommu_fwspec_init() iommu_fwspec_init() driver_deferred_probe_check_state() iommu ready ? 0 : -EPROBE_DEFER So if the IOMMU isn't available at this point, base/dd.c adds the device to the deferred probe list, to be retried later. Later: // Scan the IOMMU ... virtio_dev_probe() viommu_probe() iommu_device_register() add to iommu_device_list iommu->ready = true I believe after this probe completes, base/dd.c checks if dependent devices can now be probed: driver_bound() driver_deferred_probe_trigger() That should all be working and you don't need to add anything. The question is whether the interrupt core supports starting the setup of interrupt remapping in viommu_probe(), or if it needs to know of the IOMMU's config and features earlier during boot. Even if the viommu driver is builtin, those info may not be available early enough since they require PCI and virtio probe.
Would it instead be possible to require virtio-iommu (and thus virtio) to be built-in rather than modules?
It's certainly possible to have an optional feature in the kernel that depends on a module being built in where it otherwise wouldn't have to be.
Agree, no problem requiring this as a first step, but the IOMMU probe might still not be early enough. Thanks, Jean
participants (3)
-
Alyssa Ross -
Demi Marie Obenour -
Jean-Philippe Brucker