KVM Vhost-net 和 Virtio-net代碼詳解（十八）

場景

Host上運行qemu kvm虛擬機，其中虛擬機的網卡類型爲virtio-net，而Host上virtio-net backend使用vhost-net

數據包進入虛擬機代碼分析

首先看vhost-net模塊註冊，主要使用linux內核提供的內存註冊機制，這部分開發過linux kernel的人都應該很瞭解啦

static struct miscdevice vhost_net_misc = {
    .minor = VHOST_NET_MINOR,
    .name = "vhost-net",
    .fops = &vhost_net_fops,
};
 
static int vhost_net_init(void)
{
    if (experimental_zcopytx)
        vhost_net_enable_zcopy(VHOST_NET_VQ_TX);
    return misc_register(&vhost_net_misc);
}
module_init(vhost_net_init);
 
static void vhost_net_exit(void)
{
    misc_deregister(&vhost_net_misc);
}
module_exit(vhost_net_exit);
 
MODULE_VERSION("0.0.1");
MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("Michael S. Tsirkin");
MODULE_DESCRIPTION("Host kernel accelerator for virtio net");
MODULE_ALIAS_MISCDEV(VHOST_NET_MINOR);
MODULE_ALIAS("devname:vhost-net");

其中vhost_net_fops代表字符設備支持的用戶態接口。字符設備爲/dev/vhost-net

static const struct file_operations vhost_net_fops = {
    .owner          = THIS_MODULE,
    .release        = vhost_net_release,
    .unlocked_ioctl = vhost_net_ioctl,
#ifdef CONFIG_COMPAT
    .compat_ioctl   = vhost_net_compat_ioctl,
#endif
    .open           = vhost_net_open,
    .llseek     = noop_llseek,
};

當用戶態進行使用open系統調用的使用，則執行vhost_net_open函數，該函數主要對該字符設備進行初始化

static int vhost_net_open(struct inode *inode, struct file *f)
{
    struct vhost_net *n = kmalloc(sizeof *n, GFP_KERNEL);
    struct vhost_dev *dev;
    struct vhost_virtqueue **vqs;
    int r, i;
 
    if (!n)
        return -ENOMEM;
    vqs = kmalloc(VHOST_NET_VQ_MAX * sizeof(*vqs), GFP_KERNEL);
    if (!vqs) {
        kfree(n);
        return -ENOMEM;
    }
 
    dev = &n->dev;
    vqs[VHOST_NET_VQ_TX] = &n->vqs[VHOST_NET_VQ_TX].vq;
    vqs[VHOST_NET_VQ_RX] = &n->vqs[VHOST_NET_VQ_RX].vq;
    n->vqs[VHOST_NET_VQ_TX].vq.handle_kick = handle_tx_kick;
    n->vqs[VHOST_NET_VQ_RX].vq.handle_kick = handle_rx_kick;
    for (i = 0; i < VHOST_NET_VQ_MAX; i++) {
        n->vqs[i].ubufs = NULL;
        n->vqs[i].ubuf_info = NULL;
        n->vqs[i].upend_idx = 0;
        n->vqs[i].done_idx = 0;
        n->vqs[i].vhost_hlen = 0;
        n->vqs[i].sock_hlen = 0;
    }
    r = vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
    if (r < 0) {
        kfree(n);
        kfree(vqs);
        return r;
    }
 
    vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
    vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);
 
    f->private_data = n;
 
    return 0;
}

從上述代碼中可以看出vhost-net模塊的核心數據結果關係圖如下

爲了獲取tap設備的數據包，vhost-net模塊註冊了該設備的tun scoket

static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
{
	struct vhost_virtqueue *vq;
	...
    sock = get_socket(fd);
    if (IS_ERR(sock)) {
        r = PTR_ERR(sock);
        goto err_vq;
    }
 	...
    vq->private_data = sock;
 	...
}

tun socket的收發包函數爲

static const struct proto_ops tun_socket_ops = {
    .sendmsg = tun_sendmsg,
    .recvmsg = tun_recvmsg,
    .release = tun_release,
};

當tap獲取到數據包的時候，vhost-net會調用

static void handle_rx(struct vhost_net *net)
{
    struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_RX];
    struct vhost_virtqueue *vq = &nvq->vq;
    unsigned uninitialized_var(in), log;
    struct vhost_log *vq_log;
    struct msghdr msg = {
        .msg_name = NULL,
        .msg_namelen = 0,
        .msg_control = NULL, /* FIXME: get and handle RX aux data. */
        .msg_controllen = 0,
        .msg_iov = vq->iov,
        .msg_flags = MSG_DONTWAIT,
    };
    struct virtio_net_hdr_mrg_rxbuf hdr = {
        .hdr.flags = 0,
        .hdr.gso_type = VIRTIO_NET_HDR_GSO_NONE
    };
    size_t total_len = 0;
    int err, mergeable;
    s16 headcount;
    size_t vhost_hlen, sock_hlen;
    size_t vhost_len, sock_len;
    struct socket *sock;
 
    mutex_lock(&vq->mutex);
    sock = vq->private_data;
    if (!sock)
        goto out;
    vhost_disable_notify(&net->dev, vq);
 
    vhost_hlen = nvq->vhost_hlen;
    sock_hlen = nvq->sock_hlen;
 
    vq_log = unlikely(vhost_has_feature(vq, VHOST_F_LOG_ALL)) ?
        vq->log : NULL;
    mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
 
    while ((sock_len = peek_head_len(sock->sk))) {
        sock_len += sock_hlen;
        vhost_len = sock_len + vhost_hlen;
        headcount = get_rx_bufs(vq, vq->heads, vhost_len,
                    &in, vq_log, &log,
                    likely(mergeable) ? UIO_MAXIOV : 1);
        /* On error, stop handling until the next kick. */
        if (unlikely(headcount < 0))
            break;
        /* On overrun, truncate and discard */
        if (unlikely(headcount > UIO_MAXIOV)) {
            msg.msg_iovlen = 1;
            err = sock->ops->recvmsg(NULL, sock, &msg,
                         1, MSG_DONTWAIT | MSG_TRUNC);
            pr_debug("Discarded rx packet: len %zd\n", sock_len);
            continue;
        }
        /* OK, now we need to know about added descriptors. */
        if (!headcount) {
            if (unlikely(vhost_enable_notify(&net->dev, vq))) {
                /* They have slipped one in as we were
                 * doing that: check again. */
                vhost_disable_notify(&net->dev, vq);
                continue;
            }
            /* Nothing new?  Wait for eventfd to tell us
             * they refilled. */
            break;
        }
        /* We don't need to be notified again. */
        if (unlikely((vhost_hlen)))
            /* Skip header. TODO: support TSO. */
            move_iovec_hdr(vq->iov, nvq->hdr, vhost_hlen, in);
        else
            /* Copy the header for use in VIRTIO_NET_F_MRG_RXBUF:
             * needed because recvmsg can modify msg_iov. */
            copy_iovec_hdr(vq->iov, nvq->hdr, sock_hlen, in);
        msg.msg_iovlen = in;
        err = sock->ops->recvmsg(NULL, sock, &msg,
                     sock_len, MSG_DONTWAIT | MSG_TRUNC);
        /* Userspace might have consumed the packet meanwhile:
         * it's not supposed to do this usually, but might be hard
         * to prevent. Discard data we got (if any) and keep going. */
        if (unlikely(err != sock_len)) {
            pr_debug("Discarded rx packet: "
                 " len %d, expected %zd\n", err, sock_len);
            vhost_discard_vq_desc(vq, headcount);
            continue;
        }
        if (unlikely(vhost_hlen) &&
            memcpy_toiovecend(nvq->hdr, (unsigned char *)&hdr, 0,
                      vhost_hlen)) {
            vq_err(vq, "Unable to write vnet_hdr at addr %p\n",
                   vq->iov->iov_base);
            break;
        }
        /* TODO: Should check and handle checksum. */
 
        hdr.num_buffers = cpu_to_vhost16(vq, headcount);
        if (likely(mergeable) &&
            memcpy_toiovecend(nvq->hdr, (void *)&hdr.num_buffers,
                      offsetof(typeof(hdr), num_buffers),
                      sizeof hdr.num_buffers)) {
            vq_err(vq, "Failed num_buffers write");
            vhost_discard_vq_desc(vq, headcount);
            break;
        }
        vhost_add_used_and_signal_n(&net->dev, vq, vq->heads,
                        headcount);
        if (unlikely(vq_log))
            vhost_log_write(vq, vq_log, log, vhost_len);
        total_len += vhost_len;
        if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
            vhost_poll_queue(&vq->poll);
            break;
        }
    }
out:
    mutex_unlock(&vq->mutex);
}

從上述代碼中可以看出sock->ops->recvmsg會執行tun socket ops的tun_recvmsg函數，把tap收到的skb，放到virt_queue結構體中然後通過qemu kvm，以中斷的形式喚醒virtio-net驅動的收報函數，注意vhost-net的收發包隊列與virtio-net的收發包隊列是共享的

static int virtnet_poll(struct napi_struct *napi, int budget)
{
    struct receive_queue *rq =
        container_of(napi, struct receive_queue, napi);
    struct virtnet_info *vi = rq->vq->vdev->priv;
    void *buf;
    unsigned int r, len, received = 0;
 
again:
    while (received < budget &&
           (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
        receive_buf(vi, rq, buf, len);
        --rq->num;
        received++;
    }
 
    if (rq->num < rq->max / 2) {
        if (!try_fill_recv(vi, rq, GFP_ATOMIC))
            schedule_delayed_work(&vi->refill, 0);
    }
 
    /* Out of packets? */
    if (received < budget) {
        r = virtqueue_enable_cb_prepare(rq->vq);
        napi_complete(napi);
        if (unlikely(virtqueue_poll(rq->vq, r)) &&
            napi_schedule_prep(napi)) {
            virtqueue_disable_cb(rq->vq);
            __napi_schedule(napi);
            goto again;
        }
    }
 
    return received;
}

該函數receive_buf會調用linux kernel標準的協議棧收報函數netif_receive_skb，至此數據包就通過tap到vhost-net最終送到了虛擬機中

虛擬機向外發包

虛擬機向外發送數據包，首先會走linux 協議棧，協議棧發包最終都會調用網卡的xmit函數，對於virtio-net網卡其xmit函數爲

static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
{
    struct virtnet_info *vi = netdev_priv(dev);
    int qnum = skb_get_queue_mapping(skb);
    struct send_queue *sq = &vi->sq[qnum];
    int err;
    struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
    bool kick = !skb->xmit_more;
 
    /* Free up any pending old buffers before queueing new ones. */
    free_old_xmit_skbs(sq);
 
    /* Try to transmit */
    err = xmit_skb(sq, skb);
 
    /* This should not happen! */
    if (unlikely(err)) {
        dev->stats.tx_fifo_errors++;
        if (net_ratelimit())
            dev_warn(&dev->dev,
                 "Unexpected TXQ (%d) queue failure: %d\n", qnum, err);
        dev->stats.tx_dropped++;
        kfree_skb(skb);
        return NETDEV_TX_OK;
    }
 
    /* Don't wait up for transmitted skbs to be freed. */
    skb_orphan(skb);
    nf_reset(skb);
 
    /* Apparently nice girls don't return TX_BUSY; stop the queue
     * before it gets out of hand.  Naturally, this wastes entries. */
    if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
        netif_stop_subqueue(dev, qnum);
        if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
            /* More just got used, free them then recheck. */
            free_old_xmit_skbs(sq);
            if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
                netif_start_subqueue(dev, qnum);
                virtqueue_disable_cb(sq->vq);
            }
        }
    }
 
    if (kick || netif_xmit_stopped(txq))
        virtqueue_kick(sq->vq);
 
    return NETDEV_TX_OK;
}

從代碼中看就是把skb發到virtqueue中，然後調用virtqueue_kick通知qemu kvm，kvm 會把該數據包送給vhost-net，vhost-net會調用

static void handle_tx(struct vhost_net *net)
{
    struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
    struct vhost_virtqueue *vq = &nvq->vq;
    unsigned out, in, s;
    int head;
    struct msghdr msg = {
        .msg_name = NULL,
        .msg_namelen = 0,
        .msg_control = NULL,
        .msg_controllen = 0,
        .msg_iov = vq->iov,
        .msg_flags = MSG_DONTWAIT,
    };
    size_t len, total_len = 0;
    int err;
    size_t hdr_size;
    struct socket *sock;
    struct vhost_net_ubuf_ref *uninitialized_var(ubufs);
    bool zcopy, zcopy_used;
 
    mutex_lock(&vq->mutex);
    sock = vq->private_data;
    if (!sock)
        goto out;
 
    vhost_disable_notify(&net->dev, vq);
 
    hdr_size = nvq->vhost_hlen;
    zcopy = nvq->ubufs;
 
    for (;;) {
        /* Release DMAs done buffers first */
        if (zcopy)
            vhost_zerocopy_signal_used(net, vq);
 
        /* If more outstanding DMAs, queue the work.
         * Handle upend_idx wrap around
         */
        if (unlikely((nvq->upend_idx + vq->num - VHOST_MAX_PEND)
                  % UIO_MAXIOV == nvq->done_idx))
            break;
 
        head = vhost_get_vq_desc(vq, vq->iov,
                     ARRAY_SIZE(vq->iov),
                     &out, &in,
                     NULL, NULL);
        /* On error, stop handling until the next kick. */
        if (unlikely(head < 0))
            break;
        /* Nothing new?  Wait for eventfd to tell us they refilled. */
        if (head == vq->num) {
            if (unlikely(vhost_enable_notify(&net->dev, vq))) {
                vhost_disable_notify(&net->dev, vq);
                continue;
            }
            break;
        }
        if (in) {
            vq_err(vq, "Unexpected descriptor format for TX: "
                   "out %d, int %d\n", out, in);
            break;
        }
        /* Skip header. TODO: support TSO. */
        s = move_iovec_hdr(vq->iov, nvq->hdr, hdr_size, out);
        msg.msg_iovlen = out;
        len = iov_length(vq->iov, out);
        /* Sanity check */
        if (!len) {
            vq_err(vq, "Unexpected header len for TX: "
                   "%zd expected %zd\n",
                   iov_length(nvq->hdr, s), hdr_size);
            break;
        }
        zcopy_used = zcopy && (len >= VHOST_GOODCOPY_LEN ||
                       nvq->upend_idx != nvq->done_idx);
 
        /* use msg_control to pass vhost zerocopy ubuf info to skb */
        if (zcopy_used) {
            vq->heads[nvq->upend_idx].id = cpu_to_vhost32(vq, head);
            if (!vhost_net_tx_select_zcopy(net) ||
                len < VHOST_GOODCOPY_LEN) {
                /* copy don't need to wait for DMA done */
                vq->heads[nvq->upend_idx].len =
                            VHOST_DMA_DONE_LEN;
                msg.msg_control = NULL;
                msg.msg_controllen = 0;
                ubufs = NULL;
            } else {
                struct ubuf_info *ubuf;
                ubuf = nvq->ubuf_info + nvq->upend_idx;
 
                vq->heads[nvq->upend_idx].len =
                    VHOST_DMA_IN_PROGRESS;
                ubuf->callback = vhost_zerocopy_callback;
                ubuf->ctx = nvq->ubufs;
                ubuf->desc = nvq->upend_idx;
                msg.msg_control = ubuf;
                msg.msg_controllen = sizeof(ubuf);
                ubufs = nvq->ubufs;
                atomic_inc(&ubufs->refcount);
            }
            nvq->upend_idx = (nvq->upend_idx + 1) % UIO_MAXIOV;
        } else
            msg.msg_control = NULL;
        /* TODO: Check specific error and bomb out unless ENOBUFS? */
        err = sock->ops->sendmsg(NULL, sock, &msg, len);
        if (unlikely(err < 0)) {
            if (zcopy_used) {
                if (ubufs)
                    vhost_net_ubuf_put(ubufs);
                nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
                    % UIO_MAXIOV;
            }
            vhost_discard_vq_desc(vq, 1);
            break;
        }
        if (err != len)
            pr_debug("Truncated TX packet: "
                 " len %d != %zd\n", err, len);
        if (!zcopy_used)
            vhost_add_used_and_signal(&net->dev, vq, head, 0);
        else
            vhost_zerocopy_signal_used(net, vq);
        total_len += len;
        vhost_net_tx_packet(net);
        if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
            vhost_poll_queue(&vq->poll);
            break;
        }
    }
out:
    mutex_unlock(&vq->mutex);
}

在該函數中會調用sock->ops->sendmsg，也就是tun_sendmsg,在該函數中最終會調用netif_rx,該函數就是協議棧網卡的收報函數，代表tap網卡已經收到數據包了，然後就可以通過linux協議的briage把數據包發送出去啦

原文鏈接：https://www.cnblogs.com/allcloud/p/7716613.html