Re: [PATCH v5 06/11] hw/pci: Introduce pci_setup_iommu_per_bus() for per-bus IOMMU ops retrieval

[Jonathan Cameron via <qemu-devel@nongnu.org>]

On Mon, 23 Jun 2025 10:42:25 +0100
Shameer Kolothum <shameerali.kolothum.thodi@huawei.com> wrote:

> Currently, pci_setup_iommu() registers IOMMU ops for a given PCIBus.
> However, when retrieving IOMMU ops for a device using
> pci_device_get_iommu_bus_devfn(), the function checks the parent_dev
> and fetches IOMMU ops from the parent device, even if the current
> bus does not have any associated IOMMU ops.
> 
> This behavior works for now because QEMU's IOMMU implementations are
> globally scoped, and host bridges rely on the bypass_iommu property
> to skip IOMMU translation when needed.
> 
> However, this model will break with the soon to be introduced
> arm-smmuv3 device, which allows users to associate the IOMMU
> with a specific PCIe root complex (e.g., the default pcie.0
> or a pxb-pcie root complex).
> 
> For example, consider the following setup with multiple root
> complexes:
> 
> -device arm-smmuv3,primary-bus=pcie.0,id=smmuv3.0 \
> ...
> -device pxb-pcie,id=pcie.1,bus_nr=8,bus=pcie.0 \
> -device pcie-root-port,id=pcie.port1,bus=pcie.1 \
> -device virtio-net-pci,bus=pcie.port1
> 
> In Qemu, pxb-pcie acts as a special root complex whose parent is
> effectively the default root complex(pcie.0). Hence, though pcie.1
> has no associated SMMUv3 as per above, pci_device_get_iommu_bus_devfn()
> will incorrectly return the IOMMU ops from pcie.0 due to the fallback
> via parent_dev.
> 
> To fix this, introduce a new helper pci_setup_iommu_per_bus() that
> explicitly sets the new iommu_per_bus field in the PCIBus structure.

Maybe call out where this will later be called from?

Otherwise seems like a reasonable solution to me.

One trivial comment inline.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>


> Update pci_device_get_iommu_bus_devfn() to use this when determining
> the correct IOMMU ops, ensuring accurate behavior for per-bus IOMMUs.
> 
> Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> ---
> Please refer cover letter for more details on the issue that
> this is trying to fix.
> ---
>  hw/pci/pci.c             | 25 +++++++++++++++++++++++++
>  include/hw/pci/pci.h     |  2 ++
>  include/hw/pci/pci_bus.h |  1 +
>  3 files changed, 28 insertions(+)
> 
> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> index c70b5ceeba..e1940c05d9 100644
> --- a/hw/pci/pci.c
> +++ b/hw/pci/pci.c

> @@ -3169,6 +3182,18 @@ void pci_setup_iommu(PCIBus *bus, const PCIIOMMUOps *ops, void *opaque)
>      bus->iommu_opaque = opaque;
>  }
>  
> +/*
> + * This is same as pci_setup_iommu() except it sets the iommu_per_bus
> + * to true indicating the iommu is specific to this bus and

Trivial: Odd line wrap.  At least not can go up a line.

> + * not applicable to any parent or child.
> + */
> +void pci_setup_iommu_per_bus(PCIBus *bus, const PCIIOMMUOps *ops,
> +                             void *opaque)
> +{
> +    pci_setup_iommu(bus, ops, opaque);
> +    bus->iommu_per_bus = true;