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Diffstat (limited to 'doc/guides/prog_guide/vhost_lib.rst')
| -rw-r--r-- | doc/guides/prog_guide/vhost_lib.rst | 223 |
1 files changed, 156 insertions, 67 deletions
diff --git a/doc/guides/prog_guide/vhost_lib.rst b/doc/guides/prog_guide/vhost_lib.rst index 48e1fffb..14d5e675 100644 --- a/doc/guides/prog_guide/vhost_lib.rst +++ b/doc/guides/prog_guide/vhost_lib.rst @@ -1,5 +1,5 @@ .. BSD LICENSE - Copyright(c) 2010-2014 Intel Corporation. All rights reserved. + Copyright(c) 2010-2016 Intel Corporation. All rights reserved. All rights reserved. Redistribution and use in source and binary forms, with or without @@ -31,105 +31,194 @@ Vhost Library ============= -The vhost library implements a user space vhost driver. It supports both vhost-cuse -(cuse: user space character device) and vhost-user(user space socket server). -It also creates, manages and destroys vhost devices for corresponding virtio -devices in the guest. Vhost supported vSwitch could register callbacks to this -library, which will be called when a vhost device is activated or deactivated -by guest virtual machine. +The vhost library implements a user space virtio net server allowing the user +to manipulate the virtio ring directly. In another words, it allows the user +to fetch/put packets from/to the VM virtio net device. To achieve this, a +vhost library should be able to: + +* Access the guest memory: + + For QEMU, this is done by using the ``-object memory-backend-file,share=on,...`` + option. Which means QEMU will create a file to serve as the guest RAM. + The ``share=on`` option allows another process to map that file, which + means it can access the guest RAM. + +* Know all the necessary information about the vring: + + Information such as where the available ring is stored. Vhost defines some + messages to tell the backend all the information it needs to know how to + manipulate the vring. + +Currently, there are two ways to pass these messages and as a result there are +two Vhost implementations in DPDK: *vhost-cuse* (where the character devices +are in user space) and *vhost-user*. + +Vhost-cuse creates a user space character device and hook to a function ioctl, +so that all ioctl commands that are sent from the frontend (QEMU) will be +captured and handled. + +Vhost-user creates a Unix domain socket file through which messages are +passed. + +.. Note:: + + Since DPDK v2.2, the majority of the development effort has gone into + enhancing vhost-user, such as multiple queue, live migration, and + reconnect. Thus, it is strongly advised to use vhost-user instead of + vhost-cuse. + Vhost API Overview ------------------ -* Vhost driver registration +The following is an overview of the Vhost API functions: + +* ``rte_vhost_driver_register(path, flags)`` + + This function registers a vhost driver into the system. For vhost-cuse, a + ``/dev/path`` character device file will be created. For vhost-user server + mode, a Unix domain socket file ``path`` will be created. + + Currently two flags are supported (these are valid for vhost-user only): + + - ``RTE_VHOST_USER_CLIENT`` + + DPDK vhost-user will act as the client when this flag is given. See below + for an explanation. + + - ``RTE_VHOST_USER_NO_RECONNECT`` + + When DPDK vhost-user acts as the client it will keep trying to reconnect + to the server (QEMU) until it succeeds. This is useful in two cases: + + * When QEMU is not started yet. + * When QEMU restarts (for example due to a guest OS reboot). + + This reconnect option is enabled by default. However, it can be turned off + by setting this flag. - rte_vhost_driver_register registers the vhost driver into the system. - For vhost-cuse, character device file will be created under the /dev directory. - Character device name is specified as the parameter. - For vhost-user, a Unix domain socket server will be created with the parameter as - the local socket path. +* ``rte_vhost_driver_session_start()`` -* Vhost session start + This function starts the vhost session loop to handle vhost messages. It + starts an infinite loop, therefore it should be called in a dedicated + thread. - rte_vhost_driver_session_start starts the vhost session loop. - Vhost session is an infinite blocking loop. - Put the session in a dedicate DPDK thread. +* ``rte_vhost_driver_callback_register(virtio_net_device_ops)`` -* Callback register + This function registers a set of callbacks, to let DPDK applications take + the appropriate action when some events happen. The following events are + currently supported: - Vhost supported vSwitch could call rte_vhost_driver_callback_register to - register two callbacks, new_destory and destroy_device. - When virtio device is activated or deactivated by guest virtual machine, - the callback will be called, then vSwitch could put the device onto data - core or remove the device from data core by setting or unsetting - VIRTIO_DEV_RUNNING on the device flags. + * ``new_device(int vid)`` -* Read/write packets from/to guest virtual machine + This callback is invoked when a virtio net device becomes ready. ``vid`` + is the virtio net device ID. - rte_vhost_enqueue_burst transmit host packets to guest. - rte_vhost_dequeue_burst receives packets from guest. + * ``destroy_device(int vid)`` -* Feature enable/disable + This callback is invoked when a virtio net device shuts down (or when the + vhost connection is broken). - Now one negotiate-able feature in vhost is merge-able. - vSwitch could enable/disable this feature for performance consideration. + * ``vring_state_changed(int vid, uint16_t queue_id, int enable)`` -Vhost Implementation --------------------- + This callback is invoked when a specific queue's state is changed, for + example to enabled or disabled. -Vhost cuse implementation +* ``rte_vhost_enqueue_burst(vid, queue_id, pkts, count)`` + + Transmits (enqueues) ``count`` packets from host to guest. + +* ``rte_vhost_dequeue_burst(vid, queue_id, mbuf_pool, pkts, count)`` + + Receives (dequeues) ``count`` packets from guest, and stored them at ``pkts``. + +* ``rte_vhost_feature_disable/rte_vhost_feature_enable(feature_mask)`` + + This function disables/enables some features. For example, it can be used to + disable mergeable buffers and TSO features, which both are enabled by + default. + + +Vhost Implementations +--------------------- + +Vhost-cuse implementation ~~~~~~~~~~~~~~~~~~~~~~~~~ + When vSwitch registers the vhost driver, it will register a cuse device driver into the system and creates a character device file. This cuse driver will -receive vhost open/release/IOCTL message from QEMU simulator. +receive vhost open/release/IOCTL messages from the QEMU simulator. -When the open call is received, vhost driver will create a vhost device for the -virtio device in the guest. +When the open call is received, the vhost driver will create a vhost device +for the virtio device in the guest. -When VHOST_SET_MEM_TABLE IOCTL is received, vhost searches the memory region -to find the starting user space virtual address that maps the memory of guest -virtual machine. Through this virtual address and the QEMU pid, vhost could -find the file QEMU uses to map the guest memory. Vhost maps this file into its -address space, in this way vhost could fully access the guest physical memory, -which means vhost could access the shared virtio ring and the guest physical -address specified in the entry of the ring. +When the ``VHOST_SET_MEM_TABLE`` ioctl is received, vhost searches the memory +region to find the starting user space virtual address that maps the memory of +the guest virtual machine. Through this virtual address and the QEMU pid, +vhost can find the file QEMU uses to map the guest memory. Vhost maps this +file into its address space, in this way vhost can fully access the guest +physical memory, which means vhost could access the shared virtio ring and the +guest physical address specified in the entry of the ring. The guest virtual machine tells the vhost whether the virtio device is ready -for processing or is de-activated through VHOST_NET_SET_BACKEND message. -The registered callback from vSwitch will be called. +for processing or is de-activated through the ``VHOST_NET_SET_BACKEND`` +message. The registered callback from vSwitch will be called. -When the release call is released, vhost will destroy the device. +When the release call is made, vhost will destroy the device. -Vhost user implementation +Vhost-user implementation ~~~~~~~~~~~~~~~~~~~~~~~~~ -When vSwitch registers a vhost driver, it will create a Unix domain socket server -into the system. This server will listen for a connection and process the vhost message from -QEMU simulator. -When there is a new socket connection, it means a new virtio device has been created in -the guest virtual machine, and the vhost driver will create a vhost device for this virtio device. +Vhost-user uses Unix domain sockets for passing messages. This means the DPDK +vhost-user implementation has two options: + +* DPDK vhost-user acts as the server. + + DPDK will create a Unix domain socket server file and listen for + connections from the frontend. + + Note, this is the default mode, and the only mode before DPDK v16.07. + + +* DPDK vhost-user acts as the client. + + Unlike the server mode, this mode doesn't create the socket file; + it just tries to connect to the server (which responses to create the + file instead). + + When the DPDK vhost-user application restarts, DPDK vhost-user will try to + connect to the server again. This is how the "reconnect" feature works. + + Note: the "reconnect" feature requires **QEMU v2.7** (or above). + +No matter which mode is used, once a connection is established, DPDK +vhost-user will start receiving and processing vhost messages from QEMU. + +For messages with a file descriptor, the file descriptor can be used directly +in the vhost process as it is already installed by the Unix domain socket. -For messages with a file descriptor, the file descriptor could be directly used in the vhost -process as it is already installed by Unix domain socket. +The supported vhost messages are: - * VHOST_SET_MEM_TABLE - * VHOST_SET_VRING_KICK - * VHOST_SET_VRING_CALL - * VHOST_SET_LOG_FD - * VHOST_SET_VRING_ERR +* ``VHOST_SET_MEM_TABLE`` +* ``VHOST_SET_VRING_KICK`` +* ``VHOST_SET_VRING_CALL`` +* ``VHOST_SET_LOG_FD`` +* ``VHOST_SET_VRING_ERR`` -For VHOST_SET_MEM_TABLE message, QEMU will send us information for each memory region and its -file descriptor in the ancillary data of the message. The fd is used to map that region. +For ``VHOST_SET_MEM_TABLE`` message, QEMU will send information for each +memory region and its file descriptor in the ancillary data of the message. +The file descriptor is used to map that region. -There is no VHOST_NET_SET_BACKEND message as in vhost cuse to signal us whether virtio device -is ready or should be stopped. -VHOST_SET_VRING_KICK is used as the signal to put the vhost device onto data plane. -VHOST_GET_VRING_BASE is used as the signal to remove vhost device from data plane. +There is no ``VHOST_NET_SET_BACKEND`` message as in vhost-cuse to signal +whether the virtio device is ready or stopped. Instead, +``VHOST_SET_VRING_KICK`` is used as the signal to put the vhost device into +the data plane, and ``VHOST_GET_VRING_BASE`` is used as the signal to remove +the vhost device from the data plane. When the socket connection is closed, vhost will destroy the device. Vhost supported vSwitch reference --------------------------------- -For more vhost details and how to support vhost in vSwitch, please refer to vhost example in the -DPDK Sample Applications Guide. +For more vhost details and how to support vhost in vSwitch, please refer to +the vhost example in the DPDK Sample Applications Guide. |