Re: [PATCH v5 02/21] hw/pci: Introduce pci_device_get_viommu_cap()

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Aug 22, 2025 at 02:40:40AM -0400, Zhenzhong Duan wrote:
> Introduce a new PCIIOMMUOps optional callback, get_viommu_cap() which
> allows to retrieve capabilities exposed by a vIOMMU. The first planned
> vIOMMU device capability is VIOMMU_CAP_HW_NESTED that advertises the
> support of HW nested stage translation scheme. pci_device_get_viommu_cap
> is a wrapper that can be called on a PCI device potentially protected by
> a vIOMMU.
> 
> get_viommu_cap() is designed to return 64bit bitmap of purely emulated
> capabilities which are only determined by user's configuration, no host
> capabilities involved. Reasons are:
> 
> 1. host may has heterogeneous IOMMUs, each with different capabilities
> 2. this is migration friendly, return value is consistent between source
>    and target.
> 3. host IOMMU capabilities are passed to vIOMMU through set_iommu_device()
>    interface which have to be after attach_device(), when get_viommu_cap()
>    is called in attach_device(), there is no way for vIOMMU to get host
>    IOMMU capabilities yet, so only emulated capabilities can be returned.
>    See below sequence:
> 
>      vfio_device_attach():
>          iommufd_cdev_attach():
>              pci_device_get_viommu_cap() for HW nesting cap
>              create a nesting parent hwpt
>              attach device to the hwpt
>              vfio_device_hiod_create_and_realize() creating hiod
>      ...
>      pci_device_set_iommu_device(hiod)
> 
> 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>