<div dir="ltr"><div>Hi all/Maxime/Chenbox,</div><div><br></div><div>Hope you're doing well.</div><div><br></div><div>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.</div><div><br></div><div>Thanks,</div><div>Samar</div><div><br></div><br></div><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Wed, Aug 6, 2025 at 6:34 PM Samar Yadav <<a href="mailto:samar.yadav@broadcom.com">samar.yadav@broadcom.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><p>Hi all/Maxime/Chenbox,</p> <p>I’m observing an issue with <code>virtio_user</code> (with <code>vhost-net</code> backend) where VLAN offload doesn't seem to insert the VLAN tag, even though both <code>vlan_tci</code> and <code>ol_flags</code> are set correctly in the mbuf. We are using DPDK Version: 24.11 (custom build) and virtio_user for exception path traffic.</p> <blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><h3>Context</h3></blockquote> <ul> <li> <p><strong>Scenario</strong>: When a packet with <code>vlan_tci</code> set is transmitted via <code>virtio_user</code> with <code>RTE_ETH_TX_OFFLOAD_VLAN_INSERT</code> 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.</p> </li> <li> <p><strong>Expected</strong>: The packet should carry the VLAN tag when seen on the TAP interface, allowing it to be delivered to the correct sub-interface.</p> </li> </ul> <blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><h3>Observations</h3></blockquote> <ul> <li> <p>At the point of calling <code>virtio_xmit_pkts()</code>:</p> <ul> <li> <p><code>vlan_tci</code> is correctly set (e.g., 61).</p> </li> <li> <p><code>ol_flags</code> contains <code>RTE_MBUF_F_TX_VLAN</code> (bit 0) => value = 0x1.</p> </li> </ul> </li> <li> <p>We confirmed the following in GDB just before calling <code>virtqueue_enqueue_xmit()</code>:</p></li></ul><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>(gdb) p **tx_pkts</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>$6 = { ..., vlan_tci = 61, ..., ol_flags = 65, ... }</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>(gdb) p /t tx_pkts.ol_flags</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>$8 = 1000001</div></blockquote></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div><br></div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>The value <code>65</code> (<code>0b1000001</code>) confirms that <code>RTE_ETH_TX_OFFLOAD_VLAN_INSERT (bit 0)</code> is set.<br>Despite this, the packet captured on the TAP interface is a plain Ethernet frame, with no VLAN tag.<br></div><div><br></div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>Adding few more outputs more gdb</div><div><br></div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>(gdb) p rte_eth_devices[33]</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>$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,</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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>,</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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 = {</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 0x0 <repeats 1024 times>}, pre_tx_burst_cbs = {0x0 <repeats 1024 times>}, state = RTE_ETH_DEV_ATTACHED, security_ctx = 0x0}</div></blockquote></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div><br></div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>(gdb) p *rte_eth_devices[33].data</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>$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},</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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,</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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',</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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},</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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 = {</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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 = {{</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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 = {</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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>},</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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',</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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,</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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}},</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> __size = '\000' <repeats 39 times>, __align = 0}}</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div><br></div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>(gdb) p *(struct virtio_user_dev*)rte_eth_devices[33].data.dev_private</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div>$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',</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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',</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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,</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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,</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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",</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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,</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> 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>,</div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><div> __align = 0}, started = true, hw_cvq = false, scvq = 0x0, backend_data = 0x558185a96100, notify_area = 0x0}</div></blockquote></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px"><ul> </ul> <h3>Workaround</h3> <p>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.</p> <h3>Conclusion</h3> <p>It appears that the VLAN offload flag and <code>vlan_tci</code> are not being honored or processed correctly somewhere in the virtio path with <code>vhost-net</code> backend. This leads to silent drops on the host kernel side due to missing VLAN tags.</p> <h3>Request</h3> <p>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.</p><p><br></p><p>Thanks,<br> Samar Yadav<br> Broadcom<br></p></blockquote></div> </blockquote></div>