Re: [PATCH v4 06/27] hw/arm/smmuv3-accel: Restrict accelerated SMMUv3 to vfio-pci endpoints with iommufd

[Eric Auger <eric.auger@redhat.com>]

On 9/29/25 3:36 PM, Shameer Kolothum wrote:
> Accelerated SMMUv3 is only useful when the device can take advantage of
> the host's SMMUv3 in nested mode. To keep things simple and correct, we
> only allow this feature for vfio-pci endpoint devices that use the iommufd
> backend. We also allow non-endpoint emulated devices like PCI bridges and
> root ports, so that users can plug in these vfio-pci devices. We can only
> enforce this if devices are cold plugged. For hotplug cases, give appropriate
> warnings.
>
> Another reason for this limit is to avoid problems with IOTLB
> invalidations. Some commands (e.g., CMD_TLBI_NH_ASID) lack an associated
> SID, making it difficult to trace the originating device. If we allowed
> emulated endpoint devices, QEMU would have to invalidate both its own
> software IOTLB and the host's hardware IOTLB, which could slow things
> down.
>
> Since vfio-pci devices in nested mode rely on the host SMMUv3's nested
> translation (S1+S2), their get_address_space() callback must return the
> system address space so that VFIO core can setup correct S2 mappings
> for guest RAM.
>
> So in short:
>  - vfio-pci devices(with iommufd as backend) return the system address
>    space.
>  - bridges and root ports return the IOMMU address space.
>
> Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
> ---
>  hw/arm/smmuv3-accel.c               | 68 ++++++++++++++++++++++++++++-
>  hw/pci-bridge/pci_expander_bridge.c |  1 -
>  include/hw/pci/pci_bridge.h         |  1 +
>  3 files changed, 68 insertions(+), 2 deletions(-)
>
> diff --git a/hw/arm/smmuv3-accel.c b/hw/arm/smmuv3-accel.c
> index 79f1713be6..44410cfb2a 100644
> --- a/hw/arm/smmuv3-accel.c
> +++ b/hw/arm/smmuv3-accel.c
> @@ -7,8 +7,13 @@
>   */
>  
>  #include "qemu/osdep.h"
> +#include "qemu/error-report.h"
>  
>  #include "hw/arm/smmuv3.h"
> +#include "hw/pci/pci_bridge.h"
> +#include "hw/pci-host/gpex.h"
> +#include "hw/vfio/pci.h"
> +
>  #include "smmuv3-accel.h"
>  
>  static SMMUv3AccelDevice *smmuv3_accel_get_dev(SMMUState *bs, SMMUPciBus *sbus,
> @@ -29,15 +34,76 @@ static SMMUv3AccelDevice *smmuv3_accel_get_dev(SMMUState *bs, SMMUPciBus *sbus,
>      return accel_dev;
>  }
>  
> +static bool smmuv3_accel_pdev_allowed(PCIDevice *pdev, bool *vfio_pci)
> +{
> +
> +    if (object_dynamic_cast(OBJECT(pdev), TYPE_PCI_BRIDGE) ||
> +        object_dynamic_cast(OBJECT(pdev), TYPE_PXB_PCIE_DEV) ||
> +        object_dynamic_cast(OBJECT(pdev), TYPE_GPEX_ROOT_DEVICE)) {
> +        return true;
> +    } else if ((object_dynamic_cast(OBJECT(pdev), TYPE_VFIO_PCI))) {
> +        *vfio_pci = true;
> +        if (object_property_get_link(OBJECT(pdev), "iommufd", NULL)) {
> +            return true;
> +        }
> +    }
> +    return false;
> +}
> +
>  static AddressSpace *smmuv3_accel_find_add_as(PCIBus *bus, void *opaque,
>                                                int devfn)
>  {
> +    PCIDevice *pdev = pci_find_device(bus, pci_bus_num(bus), devfn);
>      SMMUState *bs = opaque;
>      SMMUPciBus *sbus = smmu_get_sbus(bs, bus);
>      SMMUv3AccelDevice *accel_dev = smmuv3_accel_get_dev(bs, sbus, bus, devfn);
>      SMMUDevice *sdev = &accel_dev->sdev;
> +    bool vfio_pci = false;
> +
> +    if (pdev && !smmuv3_accel_pdev_allowed(pdev, &vfio_pci)) {
> +        if (DEVICE(pdev)->hotplugged) {
> +            if (vfio_pci) {
> +                warn_report("Hot plugging a vfio-pci device (%s) without "
> +                            "iommufd as backend is not supported", pdev->name);
> +            } else {
> +                warn_report("Hot plugging an emulated device %s with "
> +                            "accelerated SMMUv3. This will bring down "
> +                            "performace", pdev->name);
> +            }
> +            /*
> +             * Both cases, we will return IOMMU address space. For hotplugged
> +             * vfio-pci dev without iommufd as backend, it will fail later in
> +             * smmuv3_notify_flag_changed() with "requires iommu MAP notifier"
> +             * error message.
> +             */
> +             return &sdev->as;
> +        } else {
> +            error_report("Device(%s) not allowed. Only PCIe root complex "
> +                         "devices or PCI bridge devices or vfio-pci endpoint "
> +                         "devices with iommufd as backend is allowed with "
> +                         "arm-smmuv3,accel=on", pdev->name);
> +            exit(1);
> +        }
> +    }
>  
> -    return &sdev->as;
> +    /*
> +     * We return the system address for vfio-pci devices(with iommufd as
> +     * backend) so that the VFIO core can set up Stage-2 (S2) mappings for
> +     * guest RAM. This is needed because, in the accelerated SMMUv3 case,
> +     * the host SMMUv3 runs in nested (S1 + S2)  mode where the guest
> +     * manages its own S1 page tables while the host manages S2.
> +     *
> +     * We are using the global &address_space_memory here, as this will ensure
> +     * same system address space pointer for all devices behind the accelerated
> +     * SMMUv3s in a VM. That way VFIO/iommufd can reuse a single IOAS ID in
> +     * iommufd_cdev_attach(), allowing the Stage-2 page tables to be shared
> +     * within the VM instead of duplicating them for every SMMUv3 instance.
> +     */
> +    if (vfio_pci) {
> +        return &address_space_memory;
From that comment one understands the need of a single and common AS.
However it is not obvious why it shall be

&address_space_memory and not an AS created on purpose.

Eric

> +    } else {
> +        return &sdev->as;
> +    }
>  }
>  
>  static const PCIIOMMUOps smmuv3_accel_ops = {
> diff --git a/hw/pci-bridge/pci_expander_bridge.c b/hw/pci-bridge/pci_expander_bridge.c
> index 1bcceddbc4..a8eb2d2426 100644
> --- a/hw/pci-bridge/pci_expander_bridge.c
> +++ b/hw/pci-bridge/pci_expander_bridge.c
> @@ -48,7 +48,6 @@ struct PXBBus {
>      char bus_path[8];
>  };
>  
> -#define TYPE_PXB_PCIE_DEV "pxb-pcie"
>  OBJECT_DECLARE_SIMPLE_TYPE(PXBPCIEDev, PXB_PCIE_DEV)
>  
>  static GList *pxb_dev_list;
> diff --git a/include/hw/pci/pci_bridge.h b/include/hw/pci/pci_bridge.h
> index a055fd8d32..b61360b900 100644
> --- a/include/hw/pci/pci_bridge.h
> +++ b/include/hw/pci/pci_bridge.h
> @@ -106,6 +106,7 @@ typedef struct PXBPCIEDev {
>  
>  #define TYPE_PXB_PCIE_BUS "pxb-pcie-bus"
>  #define TYPE_PXB_CXL_BUS "pxb-cxl-bus"
> +#define TYPE_PXB_PCIE_DEV "pxb-pcie"
>  #define TYPE_PXB_DEV "pxb"
>  OBJECT_DECLARE_SIMPLE_TYPE(PXBDev, PXB_DEV)
>