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>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
---
 docs/devel/vfio-iommufd.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/docs/devel/vfio-iommufd.rst b/docs/devel/vfio-iommufd.rst
index 3d1c11f175..6ebb4a89ba 100644
--- a/docs/devel/vfio-iommufd.rst
+++ b/docs/devel/vfio-iommufd.rst
@@ -164,3 +164,20 @@ 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 featuring x-flts=on
+------------------------------------------
+
+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,
+otherwise below error shows:
+
+.. 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 downstream to a PCI bridge is unsupported when x-flts=on
-- 
2.47.1