QEMU tap数据接收流程

QEMU直接从tap/tun取数据

QEMU tap数据接收步骤：

1. qemu从tun取数据包
2. qemu将数据包放入virtio硬件网卡。
3. qemu触发中断。
4. 虚拟机收到中断，从virtio读取数据。

在qemu中步骤1（tap_read_packet）和步骤2（qemu_send_packet_async）都是在tap_send中完成的，其中步骤2是异步流程。

qemu/net/tap.c
static void tap_send(void *opaque)
 {
     TAPState *s = opaque;
     int size;
     int packets = 0;
     while (true) {
         uint8_t *buf = s->buf;
         uint8_t min_pkt[ETH_ZLEN];
         size_t min_pktsz = sizeof(min_pkt);
         size = tap_read_packet(s->fd, s->buf, sizeof(s->buf));
         if (size <= 0) {
             break;
         }                                                                       

         if (s->host_vnet_hdr_len && !s->using_vnet_hdr) {
             buf  += s->host_vnet_hdr_len;
             size -= s->host_vnet_hdr_len;
         }
         if (net_peer_needs_padding(&s->nc)) {
             if (eth_pad_short_frame(min_pkt, &min_pktsz, buf, size)) {
                 buf = min_pkt;
                 size = min_pktsz;
             }
         }                                                                       

         size = qemu_send_packet_async(&s->nc, buf, size, tap_send_completed);
         if (size == 0) {
             tap_read_poll(s, false);
             break;
         } else if (size < 0) {
             break;
         }
         /*
          * When the host keeps receiving more packets while tap_send() is
          * running we can hog the QEMU global mutex.  Limit the number of
          * packets that are processed per tap_send() callback to prevent
          * stalling the guest.
          */
         packets++;
         if (packets >= 50) {
             break;
         }
     }
 }

qemu通过qemu_net_queue_deliver将数据包发送到virtio_queue，在发送之前若delivering或!qemu_can_send_packet满足，则先将数据包加入packets队列，随后在qemu_net_queue_flush阶段将数据包发送到virtio_queue中，上图中virtio_net_receive就到达virtio虚拟硬件网卡了。

QEMU通过vhost-net从tap/tun取数据

vhost-net驱动加载时会生成/dev/vhost-net设备。

qemu-kvm启动时会open设备/dev/vhost-net，将调用vhost_net_open完成这个过程，vhost_net_open会进行handle_tx_net、handle_rx_net poll函数的初始化。

handle_tx_net、handle_rx_net最终会调用tun_recvmsg、tun_sendmsg进行数据收发。

/drivers/vhost/net.c:
static int vhost_net_open(struct inode *inode, struct file *f)
{
... ...
vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, EPOLLOUT, dev);
vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, EPOLLIN, dev);
... ...
}

static void handle_rx_net(struct vhost_work *work)
{
	struct vhost_net *net = container_of(work, struct vhost_net,
					     poll[VHOST_NET_VQ_RX].work);
	handle_rx(net);
}

handle_rx函数中recvmsg完成从tun取数据，通过copy_to_iter将msg放入virtio_queue，最后vhost_add_used_and_signal_n实现通知机制，qemu收到数据。

static void handle_rx(struct vhost_net *net)
{
... ...
		err = sock->ops->recvmsg(sock, &msg,
					 sock_len, MSG_DONTWAIT | MSG_TRUNC);
... ...
		num_buffers = cpu_to_vhost16(vq, headcount);
		if (likely(mergeable) &&
		    copy_to_iter(&num_buffers, sizeof num_buffers,
				 &fixup) != sizeof 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);
... ...
}

vhost_net通过vhost_worker内核线程进行工作队列的调度用于完成poll，vhost_worker内核线程是qemu通过vhost_dev_ioctl VHOST_SET_OWNER时创建的。

drivers/vhost/vhost.c:
static int vhost_poll_wakeup(wait_queue_entry_t *wait, unsigned mode, int sync,
			     void *key)
{
	struct vhost_poll *poll = container_of(wait, struct vhost_poll, wait);

	if (!(key_to_poll(key) & poll->mask))
		return 0;

	vhost_poll_queue(poll);
	return 0;
}

void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
{
	clear_bit(VHOST_WORK_QUEUED, &work->flags);
	work->fn = fn;
}
EXPORT_SYMBOL_GPL(vhost_work_init);

/* Init poll structure */
void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
		     __poll_t mask, struct vhost_dev *dev)
{
	init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
	init_poll_funcptr(&poll->table, vhost_poll_func);
	poll->mask = mask;
	poll->dev = dev;
	poll->wqh = NULL;

	vhost_work_init(&poll->work, fn);
}
EXPORT_SYMBOL_GPL(vhost_poll_init);

static int vhost_worker(void *data)
{
... ...
	for (;;) {
	... ...
		node = llist_del_all(&dev->work_list);
		if (!node)
			schedule();

		node = llist_reverse_order(node);
		/* make sure flag is seen after deletion */
		smp_wmb();
		llist_for_each_entry_safe(work, work_next, node, node) {
			clear_bit(VHOST_WORK_QUEUED, &work->flags);
			__set_current_state(TASK_RUNNING);
			work->fn(work);
			if (need_resched())
				schedule();
		}
	... ...
	}
... ...
}

long vhost_dev_set_owner(struct vhost_dev *dev)
{
... ...
	worker = kthread_create(vhost_worker, dev, "vhost-%d", current->pid);
... ...
}

long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
{
... ...
	if (ioctl == VHOST_SET_OWNER) {
		r = vhost_dev_set_owner(d);
		goto done;
	}
... ...
}

drivers/vhost/net.c:
static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
			    unsigned long arg)
{
... ...
	switch (ioctl) {
... ...
	case VHOST_RESET_OWNER:
		return vhost_net_reset_owner(n);
	case VHOST_SET_OWNER:
		return vhost_net_set_owner(n);
	default:
		mutex_lock(&n->dev.mutex);
		r = vhost_dev_ioctl(&n->dev, ioctl, argp);
		if (r == -ENOIOCTLCMD)
			r = vhost_vring_ioctl(&n->dev, ioctl, argp);
		else
			vhost_net_flush(n);
		mutex_unlock(&n->dev.mutex);
		return r;
	}
}

static long vhost_net_set_owner(struct vhost_net *n)
{
... ...
	r = vhost_dev_set_owner(&n->dev);
... ...
	return r;
}

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

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);
}

附

主机vhost驱动加载时调用vhost_net_init注册一个MISC驱动，生成/dev/vhost-net的设备文件。

主机qemu-kvm启动时调用open对应的vhost_net_open做主要创建队列和收发函数的挂载，接着调用ioctl启动内核线程vhost，做收发包的处理。

主机qemu通过ioctl配置kvm模块，主要设置通信方式，因为主机vhost和virtio只进行报文的传输，kvm进行提醒。

虚拟机virtio模块注册，生成虚拟机的网络设备，配置中断和NAPI。

虚拟机发包流程如下：

直接从应用层走协议栈最后调用发送接口ndo_start_xmit对应的start_xmit，将报文放入发送队列，vp_notify通知kvm。

kvm通过vmx_handle_exit一系列调用到wake_up_process唤醒vhost线程。

vhost模块的线程激活并且拿到报文，在通过之前绑定的发送接口handle_tx_kick进行发送，调用虚拟网卡的tun_sendmsg最终到netif_rx接口进入主机内核协议栈。

虚拟机收包流程如下：

tap设备的ndo_start_xmit对应的tun_net_xmit最终调用到wake_up_process激活vhost线程，调用handle_rx_kick，将报文放入接收队列。

通过一系列的调用到kvm模块的接口kvm_vcpu_kick，向qemu虚拟机注入中断。

虚拟机virtio模块中断调用接口vp_interrupt，调用virtnet_poll，再调用到netif_receive_skb进入虚拟机的协议栈。

资料来源：https://blog.csdn.net/qq_20817327/article/details/106838029