netmap源码分析（三）内核态收包过程

netmap 收发报文是采用 poll，主要函数是netmap_poll，我们这里只分析收包过程，发包过程类似。

一、netmap_kring

先来看一个重要的结构netmap_kring：

struct netmap_kring {    struct netmap_ring  *ring;    uint32_t    nr_hwcur;    uint32_t    nr_hwtail;    // 对应 netmap_ring 的 head, cur 和 tail    uint32_t    rhead;    uint32_t    rcur;    uint32_t    rtail;    uint32_t    nkr_num_slots;  // 槽位个数    int32_t     nkr_hwofs;      // 网卡和netmap槽位相差的偏移    NM_SELINFO_T    si;     /* 等待队列 */    NM_LOCK_T   q_lock;     /* 队列的锁 */    NM_ATOMIC_T nr_busy;    /* 防止并发访问 */    struct netmap_adapter *na;    // 相关的回调函数    int (*nm_sync)(struct netmap_kring *kring, int flags);    int (*nm_notify)(struct netmap_kring *kring, int flags);};

二、Ring 内部布局

TX/RX 内部布局图

            RxRING                         TxRING      +-----------------+            +-----------------+      |                 |            |                 |      |XXX free slot XXX|            |XXX free slot XXX|      +-----------------+            +-----------------+head->| owned by user   |<-hwcur     | not sent to nic |<-hwcur      |                 |            | yet             |      +-----------------+            |                 | cur->| available to    |            |                 |      | user, not read  |            +-----------------+      | yet             |       cur->| (being          |      |                 |            |  prepared)      |      |                 |            |                 |      +-----------------+            +     ------      +tail->|                 |<-hwtail    |                 |<-hwlease      | (being          | ...        |                 | ...      |  prepared)      | ...        |                 | ...      +-----------------+ ...        |                 | ...      |                 |<-hwlease   +-----------------+      |                 |      tail->|                 |<-hwtail      |                 |            |                 |      |                 |            |                 |      |                 |            |                 |      +-----------------+            +-----------------+

三、netmap_poll 代码分析

int netmap_poll(struct netmap_priv_d *priv, int events, NM_SELRECORD_T *sr){    struct netmap_adapter *na;    struct netmap_kring *kring;    struct netmap_ring *ring;#define want_rx want[NR_RX]    struct mbq q;    enum txrx t;    mbq_init(&q);    mb();    if (want_rx) {        want_rx = 0;        t = NR_RX;        // 遍历所有收包队列，一旦有队列是空的，那就可以收包了        for (i = priv->np_qfirst[t]; i < priv->np_qlast[t]; i++) {            kring = &NMR(na, t)[i]; /* 取出一个收包队列 */            // 如果队列的 cur 和 tail 指针在同一位置，说明这个队列还没有数据，want_rx 置1，准备收包            if (kring->ring->cur == kring->ring->tail || kring->rhead != kring->ring->head) {                want_rx = 1;            }        }        if (!want_rx)  // 队列中还有数据            revents |= events & (POLLIN | POLLRDNORM);    }    if (want_rx) {do_retry_rx:        for (i = priv->np_qfirst[NR_RX]; i < priv->np_qlast[NR_RX]; i++) {            int found = 0;            kring = &na->rx_rings[i];            ring = kring->ring;            if (unlikely(nm_kr_tryget(kring, 1, &revents)))  // 看一下这个队列是否可以操作                continue;            kring->nr_kflags &= ~NR_FORWARD;            // nm_sync 指向 na->nm_rxsync，对于 ixgbe 是 ixgbe_netmap_rxsync 函数            if (kring->nm_sync(kring, 0))                revents |= POLLERR;            else                nm_sync_finalize(kring);            send_down |= (kring->nr_kflags & NR_FORWARD);            // rcur 和 rtail 如果不在同一位置说明队列中有数据            found = kring->rcur != kring->rtail;            nm_kr_put(kring);            if (found) {                revents |= want_rx;                retry_rx = 0;  // 如果队列中有数据，则置标志位为 0                kring->nm_notify(kring, 0);  // 还记得前一篇文章中提到的这个函数么，用于唤醒等待队列            }        }        if (retry_rx && sr) {            // nm_os_selrecord 调用的是 poll_wait() 函数            nm_os_selrecord(sr, check_all_rx ?                &na->si[NR_RX] : &na->rx_rings[priv->np_qfirst[NR_RX]].si);        }        if (send_down > 0 || retry_rx) {            retry_rx = 0;            if (send_down)                goto flush_tx;            else                goto do_retry_rx;  // 如果 retry_rx 标志位非 0，说明接收队列都还满着，继续遍历队列        }    }}

四、ixgbe_netmap_rxsync 分析

我们来看看收包的函数 ixgbe_netmap_rxsync

static int ixgbe_netmap_rxsync(struct netmap_kring *kring, int flags){    struct netmap_adapter *na = kring->na;    struct ifnet *ifp = na->ifp;    struct netmap_ring *ring = kring->ring;    u_int ring_nr = kring->ring_id;    u_int nm_i;   // netmap队列中槽位的索引    u_int nic_i;  // 网卡队列中槽位的索引    u_int n;    u_int const lim = kring->nkr_num_slots - 1;    u_int const head = kring->rhead;  // 队列的有效槽位指针    struct SOFTC_T *adapter = netdev_priv(ifp);  // 网卡适配器    struct ixgbe_ring *rxr = NM_IXGBE_RX_RING(adapter, ring_nr);  // 网卡接收队列    rmb();  // 内存屏障    if (netmap_no_pendintr || force_update) {        uint16_t slot_flags = kring->nkr_slot_flags;        nic_i = rxr->next_to_clean;        // 通过网卡队列槽位索引计算netmap队列槽位索引        nm_i = netmap_idx_n2k(kring, nic_i);        for (n = 0; ; n++) {            union ixgbe_adv_rx_desc *curr = NM_IXGBE_RX_DESC(rxr, nic_i); // ixgbe 接收队列信息描述结构            uint32_t staterr = le32toh(curr->wb.upper.status_error);            if ((staterr & IXGBE_RXD_STAT_DD) == 0)  // 查看 ixgbe 接收队列是否准备好                break;            ring->slot[nm_i].len = le16toh(curr->wb.upper.length);            ring->slot[nm_i].flags = (!(staterr & IXGBE_RXD_STAT_EOP) ? NS_MOREFRAG |                                        slot_flags:slot_flags);            nm_i = nm_next(nm_i, lim);    // 指向下一个netmap槽位索引            nic_i = nm_next(nic_i, lim);  // 指向下一个网卡槽位索引        }        if (n) {  // 更新指针位置            rxr->next_to_clean = nic_i;            kring->nr_hwtail = nm_i;        }    }    nm_i = kring->nr_hwcur;    if (nm_i != head) {  // 如果槽位索引不相同        nic_i = netmap_idx_k2n(kring, nm_i);        for (n = 0; nm_i != head; n++) {            struct netmap_slot *slot = &ring->slot[nm_i];            uint64_t paddr;            void *addr = PNMB(na, slot, &paddr);  // 计算netmap队列槽位的地址            union ixgbe_adv_rx_desc *curr = NM_IXGBE_RX_DESC(rxr, nic_i);            curr->read.pkt_addr = htole64(paddr);  // 网卡报文地址与netmap槽位地址关联            nm_i = nm_next(nm_i, lim);            nic_i = nm_next(nic_i, lim);        }        kring->nr_hwcur = head;        rxr->next_to_use = nic_i;        wmb();        // 保证最后一个槽位为空的，内核态使用        nic_i = nm_prev(nic_i, lim);        IXGBE_WRITE_REG(&adapter->hw, IXGBE_RDT(rxr->reg_idx), nic_i);    }    return 0;}

现在队列里面已经有数据了，那么我们来看看 netmap 是如何映射到用户态的吧

五、linux_netmap_mmap

static int linux_netmap_mmap(struct file *f, struct vm_area_struct *vma){    unsigned long off;    u_int memsize, memflags;    struct netmap_priv_d *priv = f->private_data;    struct netmap_adapter *na = priv->np_na;    mb();    // na->nm_mem 指向全局的 nm_mem    error = netmap_mem_get_info(na->nm_mem, &memsize, &memflags, NULL);    if (error)        return -error;    // 中间部分我去掉了一些关于有效性的判断    off = vma->vm_pgoff << PAGE_SHIFT;    if (off + (vma->vm_end - vma->vm_start) > memsize)        return -EINVAL;    if (memflags & NETMAP_MEM_IO) {        vm_ooffset_t pa;        pa = netmap_mem_ofstophys(na->nm_mem, 0);        if (pa == 0)            return -EINVAL;        // remap_pfn_range 函数的作用就是映射内核态内存到用户态        return remap_pfn_range(vma, vma->vm_start,                pa >> PAGE_SHIFT,  // vm_start 为映射用户态空间起始位置                vma->vm_end - vma->vm_start,  // vm_end 是结束位置                vma->vm_page_prot);    }    return 0;}

全局的内存分配器nm_mem

struct netmap_mem_d nm_mem = {    .pools = {        [NETMAP_IF_POOL] = {  // netmap_if            .name   = "netmap_if",            .objminsize = sizeof(struct netmap_if),            .objmaxsize = 4096,            .nummin     = 10,            .nummax     = 10000,        },        [NETMAP_RING_POOL] = {  // netmap_ring            .name   = "netmap_ring",            .objminsize = sizeof(struct netmap_ring),            .objmaxsize = 32*PAGE_SIZE,            .nummin     = 2,            .nummax     = 1024,        },        [NETMAP_BUF_POOL] = {  // netmap_buf            .name   = "netmap_buf",            .objminsize = 64,            .objmaxsize = 65536,            .nummin     = 4,            .nummax     = 1000000,        },    },    .nm_id = 1,    .nm_grp = -1,    .prev = &nm_mem,    .next = &nm_mem,    .ops = &netmap_mem_global_ops};

用户态如何得到这些东东呢？

六、用户态映射过程

简单来看是这样的

fd = open("/dev/netmap", 0);strcpy(req.nr_name, "ethX");ioctl(fd, NIOCREGIF, &req);mem = mmap(NULL, req.nr_memsize, PROT_WRITE | PROT_READ, MAP_SHARED, fd, 0);nifp = NETMAP_IF(mem, req.nr_offset);  // 用户态通过 offset 得到 struct netmap_if