linux-kernel - Re: Virtio interrupt remapping

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Message-ID: <877c1ed9o7.fsf@alyssa.is>
Date: Sat, 14 Jun 2025 10:11:52 +0200
From: Alyssa Ross <hi@...ssa.is>
To: Demi Marie Obenour <demiobenour@...il.com>, 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, Thomas Gleixner <tglx@...utronix.de>, Bjorn Helgaas
 <bhelgaas@...gle.com>, linux-pci@...r.kernel.org
Subject: Re: Virtio interrupt remapping

Demi Marie Obenour <demiobenour@...il.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.

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