Re: [RFC PATCH] iommu/arm-smmu-v3: Allow nested attach for PCI bridges without vDEVICE

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Aug 14, 2026 at 10:07:09AM +0000, Dmitry Malkin wrote:
> IOMMU groups can contain passed-through endpoints and host PCI bridges.
> QEMU creates vDEVICEs for the endpoints, but not for bridges that are
> not visible to the guest.

But why exactly do you have pci bridges in your groups? That isn't
entirely normal?

> Since f0ea207ed781 ("iommu/arm-smmu-v3: Introduce struct
> arm_smmu_vmaster"), group-wide translated-domain attachment therefore
> fails when the first bridge is visited:
> 
>   pcieport 0009:00:00.0:
>     missing vDEVICE association for translated nested domain:
>     cfg=5 ret=-2
> 
>   vfio-pci 0009:03:00.0:
>     VFIO_DEVICE_ATTACH_IOMMUFD_PT attach failed for pt_id 19: -ENOENT
> 
> The same configuration worked with Linux 6.14.

Hmm, but without a viommu we shouldn't have this vmaster stuff
activating in the first place, I thought? 

+               if (new_domain->type == IOMMU_DOMAIN_NESTED) {
+                       ret = arm_smmu_attach_prepare_vmaster(
+                               state, to_smmu_nested_domain(new_domain));

That should only be happening if the VMM is setting up vIOMMU.

And vIOMMU wasn't supported in 6.14, so if it "worked" the VMM was
doing something quite different - not seeing this as a regression

If viommu should support multi-device groups, I'm not sure. I want to
know why you even have multi-device groups in the first place because
that is not normal at all.

Jason