Re: [PATCH v9 1/6] iommufd: Support a HWPT without an iommu driver for noiommu

[Pranjal Shrivastava]

On Thu, Jun 11, 2026 at 10:26:53AM -0700, Jacob Pan wrote:
> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> 
> Create just a little part of a real iommu driver, enough to
> slot in under the dev_iommu_ops() and allow iommufd to call
> domain_alloc_paging_flags() and fail everything else.
> 
> This allows explicitly creating a HWPT under an IOAS.
> 
> A new Kconfig option IOMMUFD_NOIOMMU is introduced to differentiate
> from the VFIO group/container based noiommu mode.
> 
> Reviewed-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Reviewed-by: Samiullah Khawaja <skhawaja@xxxxxxxxxx>
> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Jacob Pan <jacob.pan@xxxxxxxxxxxxxxxxxxx>
> ---
> V8:
>   - Guard vIOMMU and vDevice allocation paths for noiommu (Sashiko)
> v7:
>   - Drain no-IOMMU generic-PT freelist (Sashiko)
>   - Import generic-PT IOMMU namespace (Sashiko)
> v6: (Yi)
>   - Sort includes alphabetically (iommu.h after generic_pt/iommu.h)
>   - Fix comment: s/mock page table/SW-only page table/ to avoid confusion
>     with selftest mock
>   - Rewrite noiommu_amdv1_ops comment: explain why AMDV1 format is chosen
>     (multi-page size options), remove references to group-container mode distinction
> v5:
>    - Use the new IOMMUFD_NOIOMMU Kconfig instead of VFIO_NOIOMMU
>    - Use consistent wording referring to VFIO noiommu mode (Kevin)
>    - Copyright date fix (Kevin)
> v4:
>    - Make iommufd_noiommu_ops const
> v3:
>    - Add comment to explain the design difference over the
>      legacy noiommu VFIO code.
> ---
>  drivers/iommu/iommufd/Kconfig           |  12 +++
>  drivers/iommu/iommufd/Makefile          |   1 +
>  drivers/iommu/iommufd/hw_pagetable.c    |  19 ++++-
>  drivers/iommu/iommufd/hwpt_noiommu.c    | 105 ++++++++++++++++++++++++
>  drivers/iommu/iommufd/iommufd_private.h |  12 +++
>  drivers/iommu/iommufd/main.c            |   1 +
>  drivers/iommu/iommufd/viommu.c          |  14 +++-
>  7 files changed, 158 insertions(+), 6 deletions(-)
>  create mode 100644 drivers/iommu/iommufd/hwpt_noiommu.c
> 
> diff --git a/drivers/iommu/iommufd/Kconfig b/drivers/iommu/iommufd/Kconfig
> index 455bac0351f2..6c3bea83631b 100644
> --- a/drivers/iommu/iommufd/Kconfig
> +++ b/drivers/iommu/iommufd/Kconfig
> @@ -16,6 +16,18 @@ config IOMMUFD
>  	  If you don't know what to do here, say N.
>  
>  if IOMMUFD
> +config IOMMUFD_NOIOMMU
> +	bool
> +	depends on !GENERIC_ATOMIC64 # IOMMU_PT_AMDV1 requires cmpxchg64

Nit: Should we explicitly depend on 64BIT here?
I see further in the patch we assume 64-bit addresses:

+	cfg.common.hw_max_vasz_lg2 = 64;
+	cfg.common.hw_max_oasz_lg2 = 52;

> +	select GENERIC_PT
> +	select IOMMU_PT
> +	select IOMMU_PT_AMDV1
> +	help
> +	  Provides a SW-only IO page table for devices without hardware
> +	  IOMMU backing. This uses the AMDV1 page table format for
> +	  IOVA-to-PA lookups only, not for hardware DMA translation.
> +	  To be selected by VFIO_NOIOMMU when VFIO_DEVICE_CDEV is enabled.
> +

Apart from that,
Reviewed-by: Pranjal Shrivastava <praan@xxxxxxxxxx>

Thanks,
Praan