dpdk网卡收包分析

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;
}