887d577586
This adds basic documentation for vhost-user-input. Signed-off-by: Leo Yan <leo.yan@linaro.org> Message-Id: <20231120043721.50555-3-leo.yan@linaro.org> Signed-off-by: Alex Bennée <alex.bennee@linaro.org> Message-Id: <20240104210945.1223134-10-alex.bennee@linaro.org> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
130 lines
3.7 KiB
ReStructuredText
130 lines
3.7 KiB
ReStructuredText
.. _vhost_user:
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vhost-user back ends
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--------------------
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vhost-user back ends are way to service the request of VirtIO devices
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outside of QEMU itself. To do this there are a number of things
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required.
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vhost-user device
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=================
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These are simple stub devices that ensure the VirtIO device is visible
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to the guest. The code is mostly boilerplate although each device has
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a ``chardev`` option which specifies the ID of the ``--chardev``
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device that connects via a socket to the vhost-user *daemon*.
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Each device will have an virtio-mmio and virtio-pci variant. See your
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platform details for what sort of virtio bus to use.
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.. list-table:: vhost-user devices
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:widths: 20 20 60
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:header-rows: 1
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* - Device
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- Type
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- Notes
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* - vhost-user-blk
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- Block storage
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- See contrib/vhost-user-blk
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* - vhost-user-fs
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- File based storage driver
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- See https://gitlab.com/virtio-fs/virtiofsd
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* - vhost-user-gpio
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- Proxy gpio pins to host
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- See https://github.com/rust-vmm/vhost-device
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* - vhost-user-gpu
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- GPU driver
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- See contrib/vhost-user-gpu
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* - vhost-user-i2c
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- Proxy i2c devices to host
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- See https://github.com/rust-vmm/vhost-device
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* - vhost-user-input
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- Generic input driver
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- :ref:`vhost_user_input`
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* - vhost-user-rng
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- Entropy driver
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- :ref:`vhost_user_rng`
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* - vhost-user-scmi
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- System Control and Management Interface
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- See https://github.com/rust-vmm/vhost-device
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* - vhost-user-snd
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- Audio device
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- See https://github.com/rust-vmm/vhost-device/staging
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* - vhost-user-scsi
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- SCSI based storage
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- See contrib/vhost-user-scsi
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* - vhost-user-vsock
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- Socket based communication
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- See https://github.com/rust-vmm/vhost-device
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The referenced *daemons* are not exhaustive, any conforming backend
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implementing the device and using the vhost-user protocol should work.
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vhost-user-device
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^^^^^^^^^^^^^^^^^
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The vhost-user-device is a generic development device intended for
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expert use while developing new backends. The user needs to specify
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all the required parameters including:
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- Device ``virtio-id``
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- The ``num_vqs`` it needs and their ``vq_size``
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- The ``config_size`` if needed
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.. note::
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To prevent user confusion you cannot currently instantiate
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vhost-user-device without first patching out::
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/* Reason: stop inexperienced users confusing themselves */
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dc->user_creatable = false;
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in ``vhost-user-device.c`` and ``vhost-user-device-pci.c`` file and
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rebuilding.
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vhost-user daemon
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=================
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This is a separate process that is connected to by QEMU via a socket
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following the :ref:`vhost_user_proto`. There are a number of daemons
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that can be built when enabled by the project although any daemon that
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meets the specification for a given device can be used.
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.. _shared_memory_object:
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Shared memory object
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====================
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In order for the daemon to access the VirtIO queues to process the
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requests it needs access to the guest's address space. This is
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achieved via the ``memory-backend-file`` or ``memory-backend-memfd``
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objects. A reference to a file-descriptor which can access this object
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will be passed via the socket as part of the protocol negotiation.
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Currently the shared memory object needs to match the size of the main
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system memory as defined by the ``-m`` argument.
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Example
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=======
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First start your daemon.
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.. parsed-literal::
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$ virtio-foo --socket-path=/var/run/foo.sock $OTHER_ARGS
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Then you start your QEMU instance specifying the device, chardev and
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memory objects.
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.. parsed-literal::
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$ |qemu_system| \\
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-m 4096 \\
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-chardev socket,id=ba1,path=/var/run/foo.sock \\
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-device vhost-user-foo,chardev=ba1,$OTHER_ARGS \\
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-object memory-backend-memfd,id=mem,size=4G,share=on \\
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-numa node,memdev=mem \\
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...
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