linux-kernel - Re: [PATCH v3 29/29] iommu/arm-smmu-v3-kvm: Add IOMMU ops

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Message-ID: <20250812121056.GB599331@ziepe.ca>
Date: Tue, 12 Aug 2025 09:10:56 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	maz@...nel.org, oliver.upton@...ux.dev, joey.gouly@....com,
	suzuki.poulose@....com, yuzenghui@...wei.com,
	catalin.marinas@....com, will@...nel.org, robin.murphy@....com,
	jean-philippe@...aro.org, qperret@...gle.com, tabba@...gle.com,
	mark.rutland@....com, praan@...gle.com
Subject: Re: [PATCH v3 29/29] iommu/arm-smmu-v3-kvm: Add IOMMU ops

On Tue, Aug 12, 2025 at 10:29:38AM +0000, Mostafa Saleh wrote:
> On Mon, Aug 11, 2025 at 03:55:23PM -0300, Jason Gunthorpe wrote:
> > On Wed, Aug 06, 2025 at 02:10:35PM +0000, Mostafa Saleh wrote:
> > > I am not sure I understand, the SMMU driver will register its IOMMU
> > > ops to probe the devices
> > 
> > You couldn't do this. But why do you need the iommu subsystem to help
> > you do probing for the pKVM driver? Today SMMU starts all devices in
> > ABORT mode except for some it scans manually from the fw tables.
> > 
> > They switch to identity when the iommu subsystem attaches devices, you
> > can continue to do that by having the paravirt driver tell pkvm when
> > it attaches.
> > 
> > What is wrong with this approach?
> > 
> 
> My confusion is that in this proposal we have 2 drivers:
> - arm-smmu-v3-kvm: Register arm_smmu_ops? binds to the SMMUs

No, I don't mean two iommu subsystem drivers. You have only the
pkvm-iommu driver. Whatever you bind to the arm-smmu does not register
with the iommu subsystem.

> I am almost done with v4, which relies on a single driver, I don’t think
> it’s that complicated, it adds a few impl_ops + some few re-works.
> 
> I think that is much simpler than having 3 drivers.
> Also better for the current SMMUv3 driver maintainability to have the KVM driver
> as mode, where all the KVM logic is implemented in a new file which relies on few
> ops, similar to “tegra241-cmdqv.c”

I don't understand how you can do this, it is fundamentally not an
iommu subsystem driver if pkvm is in control of the HW.

And I still strongly do not want to see a para virt iommu driver hidden
inside the smmu driver. It should be its own thing.

The tegra ops were for customizing the iommu subsystem behavior of the
arm iommu driver, not to turn it into a wrapper for a different
paravirt driver!!

Jason