netdev-dpdk: Fix requested MTU size validation.

This commit replaces MTU_TO_FRAME_LEN(mtu) with MTU_TO_MAX_FRAME_LEN(mtu)
in netdev_dpdk_set_mtu(), in order to determine if the total length of
the L2 frame with an MTU of ’mtu’ exceeds NETDEV_DPDK_MAX_PKT_LEN.

When setting an MTU we first check if the requested total frame length
(which includes associated L2 overhead) will exceed the maximum
frame length supported in netdev_dpdk_set_mtu(). The frame length is
calculated by MTU_TO_FRAME_LEN  as MTU + ETHER_HEADER + ETHER_CRC. The MTU
for the device will be set at a later stage in dpdk_eth_dev_init() using
rte_eth_dev_set_mtu(mtu).

However when using rte_eth_dev_set_mtu(mtu) the calculation used to check
that the frame does not exceed the max frame length for that device varies
between DPDK device drivers. For example ixgbe driver calculates the
frame length for a given MTU as

mtu + ETHER_HDR_LEN + ETHER_CRC_LEN

i40e driver calculates it as

mtu + ETHER_HDR_LEN + ETHER_CRC_LEN + I40E_VLAN_TAG_SIZE * 2

em driver calculates it as

mtu + ETHER_HDR_LEN + ETHER_CRC_LEN + VLAN_TAG_SIZE

Currently it is possible to set an MTU for a netdev_dpdk device that exceeds
the upper limit MTU for that devices DPDK driver. This leads to a segfault.
This is because the frame length comparison as is, does not take into account
the addition of the vlan tag overhead expected in the drivers. The
netdev_dpdk_set_mtu() call will incorrectly succeed but the subsequent
dpdk_eth_dev_init() will fail before the queues have been created for the
DPDK device. This coupled with assumptions regarding reconfiguration
requirements for the netdev will lead to a segfault when the rxq is polled
for this device.

A simple way to avoid this is by using MTU_TO_MAX_FRAME_LEN(mtu) when
validating a requested MTU in netdev_dpdk_set_mtu().
MTU_TO_MAX_FRAME_LEN(mtu) is equivalent to the following:

mtu + ETHER_HDR_LEN + ETHER_CRC_LEN + (2 * VLAN_HEADER_LEN)

By using MTU_TO_MAX_FRAME_LEN at the netdev_dpdk_set_mtu() stage, OvS
now takes into account the maximum L2 overhead that a DPDK driver could
allow for in its frame size calculation. This allows OVS to flag an error
rather than the DPDK driver if the frame length exceeds the max DPDK frame
length. OVS can fail gracefully at this point and use the default MTU of
1500 to continue to configure the port.

Note: this fix is a work around, a better approach would be if DPDK devices
could report the maximum MTU value that can be requested on a per device
basis. This capability however is not currently available. A downside of
this patch is that the MTU upper limit will be reduced by 8 bytes for
DPDK devices that do not need to account for vlan tags in the frame length
driver calculations e.g. ixgbe devices upper MTU limit is reduced from
the OVS point of view from 9710 to 9702.

CC: Mark Kavanagh <mark.b.kavanagh@intel.com>
Fixes: 0072e93 ("netdev-dpdk: add support for jumbo frames")
Signed-off-by: Ian Stokes <ian.stokes@intel.com>
Co-authored-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>

Documentation/intro/install/dpdk.rst

@@ -643,6 +643,18 @@ Limitations
 
 .. _DPDK release notes: http://dpdk.org/doc/guides/rel_notes/release_17_11.html
 
+- Upper bound MTU: DPDK device drivers differ in how the L2 frame for a
+  given MTU value is calculated e.g. i40e driver includes 2 x vlan headers in
+  MTU overhead, em driver includes 1 x vlan header, ixgbe driver does not
+  include a vlan  header in overhead. Currently it is not possible for OVS
+  DPDK to know what upper bound MTU value is supported for a given device.
+  As such OVS DPDK must provision for the case where the L2 frame for a given
+  MTU includes 2 x vlan headers. This reduces the upper bound MTU value for
+  devices that do not include vlan headers in their L2 frames by 8 bytes e.g.
+  ixgbe devices upper bound MTU is reduced from 9710 to 9702. This work
+  around is temporary and is expected to be removed once a method is provided
+  by DPDK to query the upper bound MTU value for a given device.
+
 Reporting Bugs
 --------------
 

lib/netdev-dpdk.c

@@ -2134,7 +2134,18 @@ netdev_dpdk_set_mtu(struct netdev *netdev, int mtu)
 {
     struct netdev_dpdk *dev = netdev_dpdk_cast(netdev);
 
-    if (MTU_TO_FRAME_LEN(mtu) > NETDEV_DPDK_MAX_PKT_LEN
+    /* XXX: Ensure that the overall frame length of the requested MTU does not
+     * surpass the NETDEV_DPDK_MAX_PKT_LEN. DPDK device drivers differ in how
+     * the L2 frame length is calculated for a given MTU when
+     * rte_eth_dev_set_mtu(mtu) is called e.g. i40e driver includes 2 x vlan
+     * headers, the em driver includes 1 x vlan header, the ixgbe driver does
+     * not include vlan headers. As such we should use
+     * MTU_TO_MAX_FRAME_LEN(mtu) which includes an additional 2 x vlan headers
+     * (8 bytes) for comparison. This avoids a failure later with
+     * rte_eth_dev_set_mtu(). This approach should be used until DPDK provides
+     * a method to retrieve the upper bound MTU for a given device.
+     */
+    if (MTU_TO_MAX_FRAME_LEN(mtu) > NETDEV_DPDK_MAX_PKT_LEN
         || mtu < ETHER_MIN_MTU) {
         VLOG_WARN("%s: unsupported MTU %d\n", dev->up.name, mtu);
         return EINVAL;