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.ben...@linaro.org> Cc: Jiang Liu <ge...@linux.alibaba.com> Message-Id: <20210226111619.21178-1-alex.ben...@linaro.org> --- v2 - use Stefan's suggested wording - Be super explicit in the message descriptions --- docs/interop/vhost-user.rst | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst index 2918d7c757..7c1fb8c209 100644 --- a/docs/interop/vhost-user.rst +++ b/docs/interop/vhost-user.rst @@ -307,6 +307,18 @@ 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 ----------- @@ -865,7 +877,8 @@ 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 @@ -881,7 +894,8 @@ 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 -- 2.20.1
