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| author | Alex Bennée <alex.bennee@linaro.org> | 2022-03-21 15:30:31 +0000 |
|---|---|---|
| committer | Michael S. Tsirkin <mst@redhat.com> | 2022-05-16 04:38:40 -0400 |
| commit | fa9972662ce5b6d50fe7190efeb48a5a9fecca91 (patch) | |
| tree | 1e4c26418090f6b9bdef421fa8a4619eb373f288 /docs/interop/vhost-user.rst | |
| parent | bd59f2a1823a458ac27c09e1f27f30098745381e (diff) | |
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>
Diffstat (limited to 'docs/interop/vhost-user.rst')
| -rw-r--r-- | docs/interop/vhost-user.rst | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst index 9159476678..73e710fe32 100644 --- a/docs/interop/vhost-user.rst +++ b/docs/interop/vhost-user.rst @@ -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 |