=====
vHost
=====

QEMU实现的virtio网络后端带来的网络性能并不如意，究其原因是因为频繁的上下文切换，
低效的数据拷贝、线程间同步等。于是，内核实现了一个新的virtio网络后端驱动，名为
vhost-net。与之而来的是一套新的vhost协议。


vhost-user
----------

基于vhost协议，DPDK设计了一套新的用户态协议，名为vhost-user协议.

.. code-block:: text

          +-------------------------------------------+
          | Qemu process                              |
          |                                           |
          |    +---------------------------------+    |
          |    | Guest OS                        |    |
          |    |                        User     |    |
          |    | +---------------------------+   |    |
          |    |                       Kernel    |    |
          |    |     +-------------+             |    |
          |    |     |  virtio-net |             |    |
          |    |     +-----+-^-----+             |    |
          |    |           | |                   |    |
          |    +---------------------------------+    |
          |                | |                        |
          |          +-----v-+-----+                  |
          |          | vhost-user  |                  |
          |          +-------------+                  |
          +-------------------------------------------+

     Host User Space
    +-----------------------------------------------------+
     Host Kernel Space


vhost-net
---------

- better than vhost-user

vhost协议可以将允许VMM将virtio的数据面offload到另一个组件上，而这个组件正是
vhost-net。

QEMU会打开并初始化字符设备`/dev/vhost-net`，并调用ioctl来与vhost-net进行控制面
通信。

.. code-block:: text

          +-------------------------------------------+
          | Qemu process                              |
          |                                           |
          |    +---------------------------------+    |
          |    | Guest OS                        |    |
          |    |                                 |    |
          |    |                        User     |    |
          |    | +---------------------------+   |    |
          |    |                       Kernel    |    |
          |    |     +-------------+             |    |
          |    |     |  virtio-net |             |    |
          |    |     +-----+-^-----+             |    |
          |    |           | |                   |    |
          |    +---------------------------------+    |
          |                | |                        |
          +-------------------------------------------+
                           | |
     Host User Space       | |
    +-----------------------------------------------------+
     Host Kernel Space     | |
                           | |
                     +-----v-+-----+
                     | vhost-net   |
                     +-------------+