linux-kernel - Re: [PATCH v5 14/27] iommu/arm-smmu-v3: Support probing KVM emulated devices

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Message-ID: <20251217140046.GF31492@ziepe.ca>
Date: Wed, 17 Dec 2025 10:00:46 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvmarm@...ts.linux.dev, iommu@...ts.linux.dev,
	catalin.marinas@....com, will@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
	yuzenghui@...wei.com, joro@...tes.org, jean-philippe@...aro.org,
	praan@...gle.com, danielmentz@...gle.com, mark.rutland@....com,
	qperret@...gle.com, tabba@...gle.com
Subject: Re: [PATCH v5 14/27] iommu/arm-smmu-v3: Support probing KVM emulated
 devices

On Fri, Dec 12, 2025 at 03:53:13PM +0000, Mostafa Saleh wrote:
> > It is OK I guess, I wouldn't insist you change it, but I think it is
> > kind of gross. Registering the iommu driver against the platform
> > device instead of the aux is pretty ugly and denies userspace the
> > ability to see that the hypervisor is sitting in there through the
> > sysfs topology.
> 
> Yes, that’s why I was wondering if it’s better to keep this as a platform
> driver and create the aux devices for the parent(KVM) but that was really
> complicated to handle the probe ordering.

That sounds worse and inverts the required probe ordering.

Attach it to the aux device :\

> > Not sure why the commentary about built-in though, what does that have
> > to do with anything? If the aux driver is not built in then it will
> > just module load later and everything should be fine?
> 
> As at the moment the KVM driver doesn’t use drvdata(nor any device
> resources) and the main driver(aux) does, but if that was a module, we
> can’t know which version does what (if that changes in the future,
> although unlikely).

Kernel is monolithic, you don't need to worry about people doing silly
games with modules.

Jason