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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
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
-----------
@ -890,7 +902,8 @@ Front-end message types
Get the protocol feature bitmask from the underlying vhost
implementation. 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::
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.
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::
Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must support