Re: [PATCH v7 23/23] docs/devel: Add IOMMUFD nesting documentation

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

Zhenzhong,

On 10/24/25 10:43 AM, Zhenzhong Duan wrote:
> Add documentation about using IOMMUFD backed VFIO device with intel_iommu with
> x-flts=on.
>
> Suggested-by: Yi Liu <yi.l.liu@intel.com>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
> ---
>  docs/devel/vfio-iommufd.rst | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/docs/devel/vfio-iommufd.rst b/docs/devel/vfio-iommufd.rst
> index 3d1c11f175..f1e4d940e6 100644
> --- a/docs/devel/vfio-iommufd.rst
> +++ b/docs/devel/vfio-iommufd.rst
> @@ -164,3 +164,28 @@ RAM discarding for mdev.
>  
>  ``vfio-ap`` and ``vfio-ccw`` devices don't have same issue as their backend
>  devices are always mdev and RAM discarding is force enabled.
> +
> +Usage with intel_iommu with x-flts=on
s/with/featuring to void repetition?
> +-------------------------------------
> +
> +Only IOMMUFD backed VFIO device is supported when intel_iommu is configured
> +with x-flts=on, for legacy container backed VFIO device, below error shows:
> +
> +.. code-block:: none
> +
> +    qemu-system-x86_64: -device vfio-pci,host=0000:02:00.0: vfio 0000:02:00.0: Failed to set vIOMMU: Need IOMMUFD backend when x-flts=on
> +
> +VFIO device under PCI bridge is unsupported, use PCIE bridge if necessary,
> +or else below error shows:
s/or else/otherwise
> +
> +.. code-block:: none
> +
> +    qemu-system-x86_64: -device vfio-pci,host=0000:02:00.0,bus=bridge1,iommufd=iommufd0: vfio 0000:02:00.0: Failed to set vIOMMU: Host device under PCI bridge is unsupported when x-flts=on
s/under PCI bridge/downstream to a PCI bridge
may have been better I think
> +
> +If host IOMMU has ERRATA_772415_SPR17, kexec or reboot from "intel_iommu=on,sm_on"
> +to "intel_iommu=on,sm_off" in guest is also unsupported. Configure scalable mode
> +off as below if it's not needed by guest.
> +
> +.. code-block:: bash
> +
> +    -device intel-iommu,x-scalable-mode=off
Besides
Reviewed-by: Eric Auger <eric.auger@redhat.com>

Eric