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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>
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@ -332,6 +332,18 @@ bit was dedicated for this purpose::
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#define VHOST_USER_F_PROTOCOL_FEATURES 30
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Note that VHOST_USER_F_PROTOCOL_FEATURES is the UNUSED (30) feature
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bit defined in `VIRTIO 1.1 6.3 Legacy Interface: Reserved Feature Bits
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<https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-4130003>`_.
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VIRTIO devices do not advertise this feature bit and therefore VIRTIO
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drivers cannot negotiate it.
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This reserved feature bit was reused by the vhost-user protocol to add
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vhost-user protocol feature negotiation in a backwards compatible
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fashion. Old vhost-user master and slave implementations continue to
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work even though they are not aware of vhost-user protocol feature
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negotiation.
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Ring states
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-----------
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@ -890,7 +902,8 @@ Front-end message types
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Get the protocol feature bitmask from the underlying vhost
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implementation. Only legal if feature bit
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``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
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``VHOST_USER_GET_FEATURES``.
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``VHOST_USER_GET_FEATURES``. It does not need to be acknowledged by
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``VHOST_USER_SET_FEATURES``.
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.. Note::
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Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must
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@ -906,7 +919,8 @@ Front-end message types
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Enable protocol features in the underlying vhost implementation.
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Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
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``VHOST_USER_GET_FEATURES``.
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``VHOST_USER_GET_FEATURES``. It does not need to be acknowledged by
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``VHOST_USER_SET_FEATURES``.
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.. Note::
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Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
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