[PATCH v1 1/1] iavf: document limitation on MTU

[Anatoly Burakov <anatoly.burakov@intel.com>]

When configuring a port, the configured MTU will
not include VLAN tag size, but the physical
function driver will add it automatically if the
port has VLAN filtering configured, which may
result in seemingly valid MTU to be rejected by
the PF.

Document the limitation.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
---
 doc/guides/nics/intel_vf.rst | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/doc/guides/nics/intel_vf.rst b/doc/guides/nics/intel_vf.rst
index ce2bd88cbe..78fd25bad4 100644
--- a/doc/guides/nics/intel_vf.rst
+++ b/doc/guides/nics/intel_vf.rst
@@ -730,3 +730,13 @@ and has this issue.
 
 Set the parameter `--force-max-simd-bitwidth` as 64/128/256
 to avoid selecting AVX-512 Tx path.
+
+ice: VLAN tag length not included in MTU
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When configuring MTU for a VF, MTU must not include VLAN tag length. In
+practice, when kernel driver configures VLAN filtering for a VF, the VLAN
+header tag length will be automatically added to MTU when configuring queues.
+As a consequence, when attempting to configure a VF port with MTU that, together
+with a VLAN tag header, exceeds maximum supported MTU, port configuration will
+fail if kernel driver has configured VLAN filtering on that VF.
-- 
2.43.0