Re: [PATCH v7 08/23] vfio/iommufd: Force creating nesting parent HWPT

["Cédric Le Goater" <clg@redhat.com>]

On 10/24/25 10:43, Zhenzhong Duan wrote:
> Call pci_device_get_viommu_flags() to get if vIOMMU supports
> VIOMMU_FLAG_WANT_NESTING_PARENT.
> 
> If yes, create a nesting parent HWPT and add it to the container's hwpt_list,
> letting this parent HWPT cover the entire second stage mappings (GPA=>HPA).
> 
> This allows a VFIO passthrough device to directly attach to this default HWPT
> and then to use the system address space and its listener.
> 
> Introduce a vfio_device_get_viommu_flags_want_nesting() helper to facilitate
> this implementation.
> 
> It is safe to do so because a vIOMMU will be able to fail in set_iommu_device()
> call, if something else related to the VFIO device or vIOMMU isn't compatible.
> 
> Suggested-by: Nicolin Chen <nicolinc@nvidia.com>
> Suggested-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
> Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
> Reviewed-by: Eric Auger <eric.auger@redhat.com>
> Reviewed-by: Yi Liu <yi.l.liu@intel.com>
> ---
>   include/hw/vfio/vfio-device.h |  2 ++
>   hw/vfio/device.c              | 12 ++++++++++++
>   hw/vfio/iommufd.c             |  9 +++++++++
>   3 files changed, 23 insertions(+)
> 
> diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
> index a0b8fc2eb6..48d00c7bc4 100644
> --- a/include/hw/vfio/vfio-device.h
> +++ b/include/hw/vfio/vfio-device.h
> @@ -267,6 +267,8 @@ void vfio_device_prepare(VFIODevice *vbasedev, VFIOContainer *bcontainer,
>   
>   void vfio_device_unprepare(VFIODevice *vbasedev);
>   
> +bool vfio_device_get_viommu_flags_want_nesting(VFIODevice *vbasedev);
> +
>   int vfio_device_get_region_info(VFIODevice *vbasedev, int index,
>                                   struct vfio_region_info **info);
>   int vfio_device_get_region_info_type(VFIODevice *vbasedev, uint32_t type,
> diff --git a/hw/vfio/device.c b/hw/vfio/device.c
> index 5ed3103e72..be94947623 100644
> --- a/hw/vfio/device.c
> +++ b/hw/vfio/device.c
> @@ -23,6 +23,7 @@
>   
>   #include "hw/vfio/vfio-device.h"
>   #include "hw/vfio/pci.h"
> +#include "hw/iommu.h"
>   #include "hw/hw.h"
>   #include "trace.h"
>   #include "qapi/error.h"
> @@ -521,6 +522,17 @@ void vfio_device_unprepare(VFIODevice *vbasedev)
>       vbasedev->bcontainer = NULL;
>   }
>   
> +bool vfio_device_get_viommu_flags_want_nesting(VFIODevice *vbasedev)
> +{
> +    VFIOPCIDevice *vdev = vfio_pci_from_vfio_device(vbasedev);
> +
> +    if (vdev) {
> +        return !!(pci_device_get_viommu_flags(&vdev->parent_obj) &

Using PCI_DEVICE(vdev) would be more appropriate. It can come later.

Thanks,

C.



> +                  VIOMMU_FLAG_WANT_NESTING_PARENT);
> +    }
> +    return false;
> +}
> +
>   /*
>    * Traditional ioctl() based io
>    */
> diff --git a/hw/vfio/iommufd.c b/hw/vfio/iommufd.c
> index 8de765c769..f9d0926274 100644
> --- a/hw/vfio/iommufd.c
> +++ b/hw/vfio/iommufd.c
> @@ -404,6 +404,15 @@ static bool iommufd_cdev_autodomains_get(VFIODevice *vbasedev,
>           flags = IOMMU_HWPT_ALLOC_DIRTY_TRACKING;
>       }
>   
> +    /*
> +     * If vIOMMU requests VFIO's cooperation to create nesting parent HWPT,
> +     * force to create it so that it could be reused by vIOMMU to create
> +     * nested HWPT.
> +     */
> +    if (vfio_device_get_viommu_flags_want_nesting(vbasedev)) {
> +        flags |= IOMMU_HWPT_ALLOC_NEST_PARENT;
> +    }
> +
>       if (cpr_is_incoming()) {
>           hwpt_id = vbasedev->cpr.hwpt_id;
>           goto skip_alloc;