doc: add VFIO troubleshooting in Linux guide

There are common problems with VFIO that get asked over and over on the
mailing list. Document common problems with VFIO and how to fix them or
at least figure out what went wrong.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>

diff --git a/doc/guides/linux_gsg/linux_drivers.rst b/doc/guides/linux_gsg/linux_drivers.rst
index 9c2220d..90635a4 100644 (file)
--- a/doc/guides/linux_gsg/linux_drivers.rst
+++ b/doc/guides/linux_gsg/linux_drivers.rst
@@ -290,3 +290,49 @@ To restore device ``82:00.0`` to its original kernel binding:
 .. code-block:: console
 
     ./usertools/dpdk-devbind.py --bind=ixgbe 82:00.0
+
+Troubleshooting VFIO
+--------------------
+
+In certain situations, using ``dpdk-devbind.py`` script
+to bind a device to VFIO driver may fail.
+The first place to check is the kernel messages:
+
+.. code-block:: console
+
+   dmesg | tail
+   ...
+   [ 1297.875090] vfio-pci: probe of 0000:31:00.0 failed with error -22
+   ...
+
+In most cases, the ``error -22`` indicates that the VFIO subsystem
+could not be enabled because there is no IOMMU support.
+
+To check whether the kernel has been booted with correct parameters,
+one can check the kernel command-line:
+
+.. code-block:: console
+
+   cat /proc/cmdline
+
+Please refer to earlier sections on how to configure kernel parameters
+correctly for your system.
+
+If the kernel is configured correctly, one also has to make sure that
+the BIOS configuration has virtualization features (such as Intel® VT-d).
+There is no standard way to check if the platform is configured correctly,
+so please check with your platform documentation to see if it has such features,
+and how to enable them.
+
+In certain distributions, default kernel configuration is such that
+the no-IOMMU mode is disabled altogether at compile time.
+This can be checked in the boot configuration of your system:
+
+.. code-block:: console
+
+   cat /boot/config-$(uname -r) | grep NOIOMMU
+   # CONFIG_VFIO_NOIOMMU is not set
+
+If ``CONFIG_VFIO_NOIOMMU`` is not enabled in the kernel configuration,
+VFIO driver will not support the no-IOMMU mode,
+and other alternatives (such as UIO drivers) will have to be used.