Re: [DPDK] Issue: virtio_user with VLAN offload not tagging VLAN despite vlan_tci and ol_flags being set

[Samar Yadav <samar.yadav@broadcom.com>]

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Hi all/Maxime/Chenbox,

Hope you're doing well.

Please let me know if you need any more details on the VLAN offload issue,
or if you require steps to reproduce it. I'm happy to provide further
information to assist in resolving this.

Thanks,
Samar



On Wed, Aug 6, 2025 at 6:34 PM Samar Yadav <samar.yadav@broadcom.com> wrote:

> Hi all/Maxime/Chenbox,
>
> I’m observing an issue with virtio_user (with vhost-net backend) where
> VLAN offload doesn't seem to insert the VLAN tag, even though both
> vlan_tci and ol_flags are set correctly in the mbuf. We are using DPDK
> Version: 24.11 (custom build) and virtio_user for exception path traffic.
>
> Context
>
>
>    -
>
>    *Scenario*: When a packet with vlan_tci set is transmitted via
>    virtio_user with RTE_ETH_TX_OFFLOAD_VLAN_INSERT enabled, the packet
>    received on the parent TAP interface lacks the VLAN tag. It subsequently
>    gets dropped, since it was meant for the TAP sub-interface corresponding to
>    the VLAN.
>    -
>
>    *Expected*: The packet should carry the VLAN tag when seen on the TAP
>    interface, allowing it to be delivered to the correct sub-interface.
>
> Observations
>
>
>    -
>
>    At the point of calling virtio_xmit_pkts():
>    -
>
>       vlan_tci is correctly set (e.g., 61).
>       -
>
>       ol_flags contains RTE_MBUF_F_TX_VLAN (bit 0) => value = 0x1.
>       -
>
>    We confirmed the following in GDB just before calling
>    virtqueue_enqueue_xmit():
>
> (gdb) p **tx_pkts
>
> $6 = { ..., vlan_tci = 61, ..., ol_flags = 65, ... }
>
> (gdb) p /t tx_pkts.ol_flags
>
> $8 = 1000001
>
>
> The value 65 (0b1000001) confirms that RTE_ETH_TX_OFFLOAD_VLAN_INSERT (bit
> 0) is set.
> Despite this, the packet captured on the TAP interface is a plain Ethernet
> frame, with no VLAN tag.
>
> Adding few more outputs more gdb
>
> (gdb) p rte_eth_devices[33]
>
> $4 = {rx_pkt_burst = 0x558180950c36 <virtio_recv_mergeable_pkts>,
> tx_pkt_burst = 0x558180952440 <virtio_xmit_pkts>, tx_pkt_prepare =
> 0x558180951fa3 <virtio_xmit_pkts_prepare>, rx_queue_count = 0x0,
> rx_descriptor_status = 0x0,
>
>   tx_queue_count = 0x0, tx_descriptor_status = 0x0, recycle_tx_mbufs_reuse
> = 0x0, recycle_rx_descriptors_refill = 0x0, data = 0x60005fd8e080,
> process_private = 0x0, dev_ops = 0x55818211e2a0 <virtio_eth_dev_ops>,
>
>   flow_fp_ops = 0x55818218d140 <rte_flow_fp_default_ops>, device =
> 0x558185a960b0, intr_handle = 0x558185a96160, link_intr_cbs = {tqh_first =
> 0x0, tqh_last = 0x5581825a42f8 <rte_eth_devices+547128>}, post_rx_burst_cbs
> = {
>
>     0x0 <repeats 1024 times>}, pre_tx_burst_cbs = {0x0 <repeats 1024
> times>}, state = RTE_ETH_DEV_ATTACHED, security_ctx = 0x0}
>
>
> (gdb) p *rte_eth_devices[33].data
>
> $6 = {name = "virtio_user0", '\000' <repeats 51 times>, rx_queues =
> 0x60005fc2ef80, tx_queues = 0x60005fc2cf00, nb_rx_queues = 1, nb_tx_queues
> = 1, sriov = {active = 0 '\000', nb_q_per_pool = 0 '\000', def_vmdq_idx =
> 0, def_pool_q_idx = 0},
>
>   dev_private = 0x60005fc60380, dev_link = {{val64 = 34359738367,
> {link_speed = 4294967295, link_duplex = 1, link_autoneg = 1, link_status =
> 1}}}, dev_conf = {link_speeds = 0, rxmode = {mq_mode = RTE_ETH_MQ_RX_NONE,
> mtu = 9000,
>
>       max_lro_pkt_size = 0, offloads = 8193, reserved_64s = {0, 0},
> reserved_ptrs = {0x0, 0x0}}, txmode = {mq_mode = RTE_ETH_MQ_TX_NONE,
> offloads = 32813, pvid = 0, hw_vlan_reject_tagged = 0 '\000',
> hw_vlan_reject_untagged = 0 '\000',
>
>       hw_vlan_insert_pvid = 0 '\000', reserved_64s = {0, 0}, reserved_ptrs
> = {0x0, 0x0}}, lpbk_mode = 0, rx_adv_conf = {rss_conf = {rss_key = 0x0,
> rss_key_len = 0 '\000', rss_hf = 0, algorithm =
> RTE_ETH_HASH_FUNCTION_DEFAULT},
>
>       vmdq_dcb_conf = {nb_queue_pools = 0, enable_default_pool = 0 '\000',
> default_pool = 0 '\000', nb_pool_maps = 0 '\000', pool_map = {{vlan_id = 0,
> pools = 0} <repeats 64 times>}, dcb_tc = "\000\000\000\000\000\000\000"},
> dcb_rx_conf = {
>
>         nb_tcs = 0, dcb_tc = "\000\000\000\000\000\000\000"}, vmdq_rx_conf
> = {nb_queue_pools = 0, enable_default_pool = 0 '\000', default_pool = 0
> '\000', enable_loop_back = 0 '\000', nb_pool_maps = 0 '\000', rx_mode = 0,
> pool_map = {{
>
>             vlan_id = 0, pools = 0} <repeats 64 times>}}}, tx_adv_conf =
> {vmdq_dcb_tx_conf = {nb_queue_pools = 0, dcb_tc =
> "\000\000\000\000\000\000\000"}, dcb_tx_conf = {nb_tcs = 0, dcb_tc =
> "\000\000\000\000\000\000\000"}, vmdq_tx_conf = {
>
>         nb_queue_pools = 0}}, dcb_capability_en = 0, intr_conf = {lsc = 0,
> rxq = 0, rmv = 0}}, mtu = 9000, min_rx_buf_size = 4294967295,
> rx_mbuf_alloc_failed = 0, mac_addrs = 0x60005ff404c0, mac_pool_sel = {0
> <repeats 128 times>},
>
>   hash_mac_addrs = 0x0, port_id = 33, promiscuous = 0 '\000', scattered_rx
> = 0 '\000', all_multicast = 0 '\000', dev_started = 1 '\001', lro = 0
> '\000', dev_configured = 1 '\001', flow_configured = 0 '\000',
>
>   rx_queue_state = "\001", '\000' <repeats 1022 times>, tx_queue_state =
> "\001", '\000' <repeats 1022 times>, dev_flags = 0, numa_node = -1,
> vlan_filter_conf = {ids = {0 <repeats 64 times>}}, owner = {id = 0,
>
>     name = '\000' <repeats 63 times>}, representor_id = 0, backer_port_id
> = 64, flow_ops_mutex = {__data = {__lock = 0, __count = 0, __owner = 0,
> __nusers = 0, __kind = 0, __spins = 0, __elision = 0, __list = {__prev =
> 0x0, __next = 0x0}},
>
>     __size = '\000' <repeats 39 times>, __align = 0}}
>
>
> (gdb) p *(struct virtio_user_dev*)rte_eth_devices[33].data.dev_private
>
> $3 = {hw = {vqs = 0x60005ff40400, guest_features = 4563441697,
> vtnet_hdr_size = 12, started = 1 '\001', weak_barriers = 1 '\001',
> vlan_strip = 1 '\001', rx_ol_scatter = true, has_tx_offload = 1 '\001',
> has_rx_offload = 0 '\000',
>
>     use_vec_rx = 0 '\000', use_vec_tx = 0 '\000', use_inorder_rx = 0
> '\000', use_inorder_tx = 0 '\000', opened = 1 '\001', port_id = 33,
> mac_addr = "\000PV\235#g", get_speed_via_feat = false, speed = 4294967295,
> duplex = 1 '\001',
>
>     intr_lsc = 1 '\001', max_mtu = 9698, max_rx_pkt_len = 9030, state_lock
> = {locked = 0}, inject_pkts = 0x0, max_queue_pairs = 1, rss_rx_queues = 0,
> rss_hash_types = 0, rss_reta = 0x0, rss_key = 0x0,
>
>     req_guest_features = 9223372445160806441, cvq = 0x0, use_va = true},
> backend_type = VIRTIO_USER_BACKEND_VHOST_KERNEL, is_server = false, callfds
> = 0x6000396000c0, kickfds = 0x60005ff40880, mac_specified = 1,
> max_queue_pairs = 1,
>
>   queue_pairs = 1, queue_size = 2048, features = 4563441697,
> device_features = 4563442051, frontend_features = 32, unsupported_features
> = 17293822186582074972, status = 15 '\017', net_status = 0, mac_addr =
> "\000PV\235#g",
>
>   path = "/dev/vhost-net", '\000' <repeats 4081 times>, ifname =
> 0x558185a04b70 "avi_eth1", vrings = {ptr = 0x60005ff40780, split =
> 0x60005ff40780, packed = 0x60005ff40780}, packed_queues = 0x0, qp_enabled =
> 0x60005ff406c0,
>
>   ops = 0x55818218cc80 <virtio_ops_kernel>, mutex = {__data = {__lock = 0,
> __count = 0, __owner = 0, __nusers = 0, __kind = 0, __spins = 0, __elision
> = 0, __list = {__prev = 0x0, __next = 0x0}}, __size = '\000' <repeats 39
> times>,
>
>     __align = 0}, started = true, hw_cvq = false, scvq = 0x0, backend_data
> = 0x558185a96100, notify_area = 0x0}
>
>
>
> Workaround
>
> If we explicitly insert the VLAN tag into the packet data (i.e., inline
> VLAN insertion), the TAP interface sees the tag as expected, and the packet
> is correctly forwarded to the intended sub-interface.
> Conclusion
>
> It appears that the VLAN offload flag and vlan_tci are not being honored
> or processed correctly somewhere in the virtio path with vhost-net
> backend. This leads to silent drops on the host kernel side due to missing
> VLAN tags.
> Request
>
> Could you please confirm if this is a known limitation or a bug? If it’s
> the latter, I’d be happy to provide more debug info or steps to reproduce.
>
>
> Thanks,
> Samar Yadav
> Broadcom
>
>

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