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vhost-user.rst: add clarifying language about protocol negotiation 

Make the language about feature negotiation explicitly clear about the
handling of the VHOST_USER_F_PROTOCOL_FEATURES feature bit. Try and
avoid the sort of bug introduced in vhost.rs REPLY_ACK processing:

  https://github.com/rust-vmm/vhost/pull/24

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Jiang Liu <gerry@linux.alibaba.com>
Message-Id: <20210226111619.21178-1-alex.bennee@linaro.org>

Message-Id: <20220321153037.3622127-8-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Alex Bennée 2022-03-21 15:30:31 +00:00 committed by Michael S. Tsirkin
parent bd59f2a182
commit fa9972662c
1 changed files with 16 additions and 2 deletions
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18
docs/interop/vhost-user.rst
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@ -332,6 +332,18 @@ bit was dedicated for this purpose::
#define VHOST_USER_F_PROTOCOL_FEATURES 30 #define VHOST_USER_F_PROTOCOL_FEATURES 30
Note that VHOST_USER_F_PROTOCOL_FEATURES is the UNUSED (30) feature
bit defined in `VIRTIO 1.1 6.3 Legacy Interface: Reserved Feature Bits
<https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-4130003>`_.
VIRTIO devices do not advertise this feature bit and therefore VIRTIO
drivers cannot negotiate it.
This reserved feature bit was reused by the vhost-user protocol to add
vhost-user protocol feature negotiation in a backwards compatible
fashion. Old vhost-user master and slave implementations continue to
work even though they are not aware of vhost-user protocol feature
negotiation.
Ring states Ring states
----------- -----------
@ -890,7 +902,8 @@ Front-end message types
Get the protocol feature bitmask from the underlying vhost Get the protocol feature bitmask from the underlying vhost
implementation. Only legal if feature bit implementation. Only legal if feature bit
``VHOST_USER_F_PROTOCOL_FEATURES`` is present in ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
``VHOST_USER_GET_FEATURES``. ``VHOST_USER_GET_FEATURES``. It does not need to be acknowledged by
``VHOST_USER_SET_FEATURES``.
.. Note:: .. Note::
Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must
@ -906,7 +919,8 @@ Front-end message types
Enable protocol features in the underlying vhost implementation. Enable protocol features in the underlying vhost implementation.
Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
``VHOST_USER_GET_FEATURES``. ``VHOST_USER_GET_FEATURES``. It does not need to be acknowledged by
``VHOST_USER_SET_FEATURES``.
.. Note:: .. Note::
Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must support Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must support