一个网络报文从网卡接收到被应用处理，中间主要需要经历两个阶段：
阶段一：网卡通过其DMA硬件将收到的报文写入到收包队列中（入队）
阶段二：应用从收包队列中读取报文（出队）
由于目前正在使用vpp/dpdk 优化waf引擎的工作,所以就看看ixgbe网卡在dpdk框架下是怎么工作的。
下面分别介绍一下 收包队列结构 初始化（使能） 收包流程
收发包的配置和初始化，主要是配置收发队列等。
收发包的配置最主要的工作就是配置网卡的收发队列，设置DMA拷贝数据包的地址等。使用数据包时，只要去对应队列取出指定地址的数据即可；主题配置函数见 rte_eth_dev_configure ；当收发队列配置完成后，就调用设备的配置函数，进行最后的配置。(*dev->dev_ops->dev_configure)(dev)，-----进入ixgbe_dev_configure()来分析其过程,主要是调用了ixgbe_check_mq_mode()来检查队列的模式。然后设置允许接收批量和向量的模式
2.数据包的获取和发送，主要是从队列中获取到数据包或者把数据包放到队列中。
收包队列的构造主要是通过网卡队列设置函数 rte_eth_rx_queue_setup设置相关参数；最后，调用到队列的setup函数做最后的初始化。ret = (*dev->dev_ops->rx_queue_setup)(dev, rx_queue_id, nb_rx_desc,
socket_id, rx_conf, mp);
对于ixgbe设备，rx_queue_setup就是函数ixgbe_dev_rx_queue_setup()
说一说主要的结构体：
/* Receive Descriptor - Advanced pkt_addr：报文数据的物理地址，网卡DMA将报文数据通过该物理地址写入 对应的内存空间。 hdr_addr：报文的头信息，hdr_addr的最后一个bit为DD位，因为是union结构， 即status_error的最后一个bit也对应DD位。 网卡每次来了新的数据包，就检查rx_ring当前这个buf的DD位是否为0， 如果为0那么表示当前buf可以使用，就让DMA将数据包copy到这个buf中， 然后设置DD为1。如果为1，那么网卡就认为rx_ring队列满了， 直接会将这个包给丢弃掉，记录一次imiss。（0->1）*/ union ixgbe_adv_rx_desc { struct { __le64 pkt_addr; /* Packet buffer address */ __le64 hdr_addr; /* Header buffer address */ } read; struct { struct { union { __le32 data; struct { __le16 pkt_info; /* RSS, Pkt type */ __le16 hdr_info; /* Splithdr, hdrlen */ } hs_rss; } lo_dword; union { __le32 rss; /* RSS Hash */ struct { __le16 ip_id; /* IP id */ __le16 csum; /* Packet Checksum */ } csum_ip; } hi_dword; } lower; struct { __le32 status_error; /* ext status/error */ __le16 length; /* Packet length */ __le16 vlan; /* VLAN tag */ } upper; } wb; /* writeback */ }; /** * Structure associated with each descriptor of the RX ring of a RX queue. sw_ring是由一个动态申请的数组构建的环形队列，队列的元素是ixgbe_rx_entry类型， 队列的大小可配，一般最大可配4096 mbuf：报文mbuf结构指针，mbuf用于管理一个报文，主要包含报文相关信息和报文数据。 */ struct ixgbe_rx_entry { struct rte_mbuf *mbuf; /**< mbuf associated with RX descriptor. */ }; /** * Structure associated with each RX queue. */ struct ixgbe_rx_queue { struct rte_mempool *mb_pool; /**< mbuf pool to populate RX ring. */ /*rx_ring主要存储报文数据的物理地址，物理地址供网卡DMA使用， 也称为DMA地址（硬件使用物理地址，将报文copy到报文物理位置上）。*/ volatile union ixgbe_adv_rx_desc *rx_ring; /**< RX ring virtual address. */ uint64_t rx_ring_phys_addr; /**< RX ring DMA address. */ volatile uint32_t *rdt_reg_addr; /**< RDT register address. */ volatile uint32_t *rdh_reg_addr; /**< RDH register address. */ /*sw_ring主要存储报文数据的虚拟地址，虚拟地址供应用使用 （软件使用虚拟地址，读取报文）报文数据的物理地址可以由报文数据的虚拟地址转化得到。*/ struct ixgbe_rx_entry *sw_ring; /**< address of RX software ring. */ struct ixgbe_scattered_rx_entry *sw_sc_ring; /**< address of scattered Rx software ring. */ struct rte_mbuf *pkt_first_seg; /**< First segment of current packet. */ struct rte_mbuf *pkt_last_seg; /**< Last segment of current packet. */ uint64_t mbuf_initializer; /**< value to init mbufs */ uint16_t nb_rx_desc; /**< number of RX descriptors. */ uint16_t rx_tail; /**< current value of RDT register. */ uint16_t nb_rx_hold; /**< number of held free RX desc. */ uint16_t rx_nb_avail; /**< nr of staged pkts ready to ret to app */ uint16_t rx_next_avail; /**< idx of next staged pkt to ret to app */ uint16_t rx_free_trigger; /**< triggers rx buffer allocation */ uint8_t rx_using_sse; /**< indicates that vector RX is in use */ #ifdef RTE_LIBRTE_SECURITY uint8_t using_ipsec; /**< indicates that IPsec RX feature is in use */ #endif #ifdef RTE_IXGBE_INC_VECTOR uint16_t rxrearm_nb; /**< number of remaining to be re-armed */ uint16_t rxrearm_start; /**< the idx we start the re-arming from */ #endif uint16_t rx_free_thresh; /**< max free RX desc to hold. */ uint16_t queue_id; /**< RX queue index. */ uint16_t reg_idx; /**< RX queue register index. */ uint16_t pkt_type_mask; /**< Packet type mask for different NICs. */ uint16_t port_id; /**< Device port identifier. */ uint8_t crc_len; /**< 0 if CRC stripped, 4 otherwise. */ uint8_t drop_en; /**< If not 0, set SRRCTL.Drop_En. */ uint8_t rx_deferred_start; /**< not in global dev start. */ /** flags to set in mbuf when a vlan is detected. */ uint64_t vlan_flags; uint64_t offloads; /**< Rx offloads with DEV_RX_OFFLOAD_* */ /** need to alloc dummy mbuf, for wraparound when scanning hw ring */ struct rte_mbuf fake_mbuf; /** hold packets to return to application */ struct rte_mbuf *rx_stage[RTE_PMD_IXGBE_RX_MAX_BURST*2]; };`
收包队列的启动主要是通过调用rte_eth_dev_start
DPDK是零拷贝的，那么分配的mem_pool中的对象怎么和队列以及驱动联系起来呢????
设备的启动是从rte_eth_dev_start()中开始,会调用
diag = (*dev->dev_ops->dev_start)(dev);
找到设备启动的真正启动函数：ixgbe_dev_start
其中队列初始化流程函数为：
ixgbe_alloc_rx_queue_mbufs(struct ixgbe_rx_queue *rxq) { struct ixgbe_rx_entry *rxe = rxq->sw_ring; uint64_t dma_addr; unsigned int i; /* Initialize software ring entries 队列所属内存池的ring中循环取出了nb_rx_desc个mbuf指针， 填充rxq->sw_ring。每个指针都指向内存池里的一个数据包空间 然后就先填充了新分配的mbuf结构，最最重要的是填充计算了dma_addr 初始化queue ring，即rxd的信息，标明了驱动把数据包放在dma_addr处。 最后把分配的mbuf“放入”queue 的sw_ring中， 这样，驱动收过来的包，就直接放在了sw_ring中。 */ for (i = 0; i < rxq->nb_rx_desc; i++) { volatile union ixgbe_adv_rx_desc *rxd; struct rte_mbuf *mbuf = rte_mbuf_raw_alloc(rxq->mb_pool); if (mbuf == NULL) { PMD_INIT_LOG(ERR, "RX mbuf alloc failed queue_id=%u", (unsigned) rxq->queue_id); return -ENOMEM; } mbuf->data_off = RTE_PKTMBUF_HEADROOM; mbuf->port = rxq->port_id; dma_addr = rte_cpu_to_le_64(rte_mbuf_data_iova_default(mbuf)); rxd = &rxq->rx_ring[i]; rxd->read.hdr_addr = 0; rxd->read.pkt_addr = dma_addr; rxe[i].mbuf = mbuf; } return 0; }
数据包的获取
网卡收到报文后，先存于网卡本地的buffer-Rx（Rx FIFO）中，然后由DMA通过PCI总线将报文数据写入操作系统的内存中，即数据报文完成入队操作，那么数据包的获取就是指上层应用从队列中去取出这些数据包
业务层面获取数据包是从rte_eth_rx_burst()开始：
int16_t nb_rx = (*dev->rx_pkt_burst)(dev->data->rx_queues[queue_id], rx_pkts, nb_pkts);
这里的dev->rx_pkt_burst在驱动初始化的时候已经注册过了，对于ixgbe设备，就是ixgbe_recv_pkts()函数
uint16_t ixgbe_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts) { struct ixgbe_rx_queue *rxq; volatile union ixgbe_adv_rx_desc *rx_ring; volatile union ixgbe_adv_rx_desc *rxdp; struct ixgbe_rx_entry *sw_ring; struct ixgbe_rx_entry *rxe; struct rte_mbuf *rxm; struct rte_mbuf *nmb; union ixgbe_adv_rx_desc rxd; uint64_t dma_addr; uint32_t staterr; uint32_t pkt_info; uint16_t pkt_len; uint16_t rx_id; uint16_t nb_rx; uint16_t nb_hold; uint64_t pkt_flags; uint64_t vlan_flags; nb_rx = 0; nb_hold = 0; rxq = rx_queue; rx_id = rxq->rx_tail;//从队列的tail位置开始取包 rx_ring = rxq->rx_ring; sw_ring = rxq->sw_ring; vlan_flags = rxq->vlan_flags; while (nb_rx < nb_pkts) {//循环获取nb_pkts个包 /* * The order of operations here is important as the DD status * bit must not be read after any other descriptor fields. * rx_ring and rxdp are pointing to volatile data so the order * of accesses cannot be reordered by the compiler. If they were * not volatile, they could be reordered which could lead to * using invalid descriptor fields when read from rxd. */ rxdp = &rx_ring[rx_id]; staterr = rxdp->wb.upper.status_error; //检查DD位是否为1，是1则说明该位置已放入数据包，否则表示没有报文，退出 if (!(staterr & rte_cpu_to_le_32(IXGBE_RXDADV_STAT_DD))) break; rxd = *rxdp; /* * End of packet. * * If the IXGBE_RXDADV_STAT_EOP flag is not set, the RX packet * is likely to be invalid and to be dropped by the various * validation checks performed by the network stack. * * Allocate a new mbuf to replenish the RX ring descriptor. * If the allocation fails: * - arrange for that RX descriptor to be the first one * being parsed the next time the receive function is * invoked [on the same queue]. * * - Stop parsing the RX ring and return immediately. * * This policy do not drop the packet received in the RX * descriptor for which the allocation of a new mbuf failed. * Thus, it allows that packet to be later retrieved if * mbuf have been freed in the mean time. * As a side effect, holding RX descriptors instead of * systematically giving them back to the NIC may lead to * RX ring exhaustion situations. * However, the NIC can gracefully prevent such situations * to happen by sending specific "back-pressure" flow control * frames to its peer(s). */ PMD_RX_LOG(DEBUG, "port_id=%u queue_id=%u rx_id=%u " "ext_err_stat=0x%08x pkt_len=%u", (unsigned) rxq->port_id, (unsigned) rxq->queue_id, (unsigned) rx_id, (unsigned) staterr, (unsigned) rte_le_to_cpu_16(rxd.wb.upper.length)); //申请一个mbuf（nmb），用于交换 nmb = rte_mbuf_raw_alloc(rxq->mb_pool); if (nmb == NULL) { PMD_RX_LOG(DEBUG, "RX mbuf alloc failed port_id=%u " "queue_id=%u", (unsigned) rxq->port_id, (unsigned) rxq->queue_id); rte_eth_devices[rxq->port_id].data->rx_mbuf_alloc_failed++; break; } nb_hold++; rxe = &sw_ring[rx_id]; rx_id++; if (rx_id == rxq->nb_rx_desc) rx_id = 0; /* Prefetch next mbuf while processing current one. */ rte_ixgbe_prefetch(sw_ring[rx_id].mbuf); /* * When next RX descriptor is on a cache-line boundary, * prefetch the next 4 RX descriptors and the next 8 pointers * to mbufs. */ if ((rx_id & 0x3) == 0) { rte_ixgbe_prefetch(&rx_ring[rx_id]); rte_ixgbe_prefetch(&sw_ring[rx_id]); } //从sw_ring中读取一个报文mbuf（存入rxm） rxm = rxe->mbuf; //往sw_ring中填空一个新报文mbuf（nmb） rxe->mbuf = nmb; //新mbuf对应的报文数据物理地址填入rx_ring对应位置，并将hdr_addr置0（DD位置0） dma_addr = rte_cpu_to_le_64(rte_mbuf_data_iova_default(nmb)); rxdp->read.hdr_addr = 0; rxdp->read.pkt_addr = dma_addr; /* * Initialize the returned mbuf. * 1) setup generic mbuf fields: * - number of segments, * - next segment, * - packet length, * - RX port identifier. * 2) integrate hardware offload data, if any: * - RSS flag & hash, * - IP checksum flag, * - VLAN TCI, if any, * - error flags. */ pkt_len = (uint16_t) (rte_le_to_cpu_16(rxd.wb.upper.length) - rxq->crc_len); //对读取mbuf的报文信息进行初始化 rxm->data_off = RTE_PKTMBUF_HEADROOM; rte_packet_prefetch((char *)rxm->buf_addr + rxm->data_off); rxm->nb_segs = 1; rxm->next = NULL; rxm->pkt_len = pkt_len; rxm->data_len = pkt_len; rxm->port = rxq->port_id; pkt_info = rte_le_to_cpu_32(rxd.wb.lower.lo_dword.data); /* Only valid if PKT_RX_VLAN set in pkt_flags */ rxm->vlan_tci = rte_le_to_cpu_16(rxd.wb.upper.vlan); pkt_flags = rx_desc_status_to_pkt_flags(staterr, vlan_flags); pkt_flags = pkt_flags | rx_desc_error_to_pkt_flags(staterr); pkt_flags = pkt_flags | ixgbe_rxd_pkt_info_to_pkt_flags((uint16_t)pkt_info); rxm->ol_flags = pkt_flags; rxm->packet_type = ixgbe_rxd_pkt_info_to_pkt_type(pkt_info, rxq->pkt_type_mask); if (likely(pkt_flags & PKT_RX_RSS_HASH)) rxm->hash.rss = rte_le_to_cpu_32( rxd.wb.lower.hi_dword.rss); else if (pkt_flags & PKT_RX_FDIR) { rxm->hash.fdir.hash = rte_le_to_cpu_16( rxd.wb.lower.hi_dword.csum_ip.csum) & IXGBE_ATR_HASH_MASK; rxm->hash.fdir.id = rte_le_to_cpu_16( rxd.wb.lower.hi_dword.csum_ip.ip_id); } /* * Store the mbuf address into the next entry of the array * of returned packets. *///读取的报文mbuf存入rx_pkts rx_pkts[nb_rx++] = rxm; } rxq->rx_tail = rx_id; /* * If the number of free RX descriptors is greater than the RX free * threshold of the queue, advance the Receive Descriptor Tail (RDT) * register. * Update the RDT with the value of the last processed RX descriptor * minus 1, to guarantee that the RDT register is never equal to the * RDH register, which creates a "full" ring situtation from the * hardware point of view... */ nb_hold = (uint16_t) (nb_hold + rxq->nb_rx_hold); if (nb_hold > rxq->rx_free_thresh) { PMD_RX_LOG(DEBUG, "port_id=%u queue_id=%u rx_tail=%u " "nb_hold=%u nb_rx=%u", (unsigned) rxq->port_id, (unsigned) rxq->queue_id, (unsigned) rx_id, (unsigned) nb_hold, (unsigned) nb_rx); rx_id = (uint16_t) ((rx_id == 0) ? (rxq->nb_rx_desc - 1) : (rx_id - 1)); IXGBE_PCI_REG_WRITE(rxq->rdt_reg_addr, rx_id); nb_hold = 0; } rxq->nb_rx_hold = nb_hold; return nb_rx; }