Re: [PATCH v7 07/12] hw/pci: Introduce pci_setup_iommu_per_bus() for per-bus IOMMU ops retrieval

[Nicolin Chen <nicolinc@nvidia.com>]

On Tue, Jul 08, 2025 at 04:40:50PM +0100, Shameer Kolothum wrote:
> @@ -2909,6 +2909,19 @@ static void pci_device_get_iommu_bus_devfn(PCIDevice *dev,
>              }
>          }
>  
> +        /*
> +         * When multiple PCI Express Root Buses are defined using pxb-pcie,
> +         * the IOMMU configuration may be specific to each root bus. However,
> +         * pxb-pcie acts as a special root complex whose parent is effectively
> +         * the default root complex(pcie.0). Ensure that we retrieve the
> +         * correct IOMMU ops(if any) in such cases.
> +         */
> +        if (pci_bus_is_express(iommu_bus) && pci_bus_is_root(iommu_bus)) {
> +            if (!iommu_bus->iommu_per_bus && parent_bus->iommu_per_bus) {
> +                break;
 
Mind elaborating why the current bus must unset iommu_per_bus while
its parent sets iommu_per_bus?

My understanding is that for a pxb-pcie we should set iommu_per_bus
but for its parent (the default root complex) we should unset its
iommu_per_bus?

Thanks
Nicolin