linux-kernel - Re: Virtio interrupt remapping

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Message-ID: <6b661c62-c322-4f2b-8e4a-da1d5c5e48a1@gmail.com>
Date: Fri, 13 Jun 2025 14:50:01 -0400
From: Demi Marie Obenour <demiobenour@...il.com>
To: Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>, virtualization@...ts.linux.dev,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, devel@...ctrum-os.org,
 Alyssa Ross <hi@...ssa.is>, Thomas Gleixner <tglx@...utronix.de>,
 Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org
Subject: Re: Virtio interrupt remapping

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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