场景

Host上运行qemu kvm虚拟机，其中虚拟机的网卡类型为virtio-net，而Host上virtio-net backend使用vhost-net

数据包进入虚拟机代码分析

首先看vhost-net模块注册，主要使用linux内核提供的内存注册机制，这部分开发过linux kernel的人都应该很了解啦

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代表字符设备支持的用户态接口。字符设备为/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,
};

当用户态进行使用open系统调用的使用，则执行vhost_net_open函数，该函数主要对该字符设备进行初始化

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;
}

从上述代码中可以看出vhost-net模块的核心数据结果关系图如下

为了获取tap设备的数据包，vhost-net模块注册了该设备的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的收发包函数为

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

当tap获取到数据包的时候，vhost-net会调用

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);
}

从上述代码中可以看出sock->ops->recvmsg会执行tun socket ops的tun_recvmsg函数，把tap收到的skb，放到virt_queue结构体中然后通过qemu kvm，以中断的形式唤醒virtio-net驱动的收报函数，注意vhost-net的收发包队列与virtio-net的收发包队列是共享的

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;
}

该函数receive_buf会调用linux kernel标准的协议栈收报函数netif_receive_skb，至此数据包就通过tap到vhost-net最终送到了虚拟机中

虚拟机向外发包

虚拟机向外发送数据包，首先会走linux 协议栈，协议栈发包最终都会调用网卡的xmit函数，对于virtio-net网卡其xmit函数为

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;
}

从代码中看就是把skb发到virtqueue中，然后调用virtqueue_kick通知qemu kvm，kvm 会把该数据包送给vhost-net，vhost-net会调用

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;} elsemsg.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);elsevhost_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);
}

在该函数中会调用sock->ops->sendmsg，也就是tun_sendmsg,在该函数中最终会调用netif_rx,该函数就是协议栈网卡的收报函数，代表tap网卡已经收到数据包了，然后就可以通过linux协议的briage把数据包发送出去啦

原文链接：https://www.cnblogs.com/allcloud/p/7716613.html

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KVM Vhost-net 和 Virtio-net代码详解（十八）

场景

数据包进入虚拟机代码分析

虚拟机向外发包

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