[Rust-VMM] [VHOST USER SPEC PATCH] vhost-user.rst: add clarifying language about protocol negotiation

Alex Bennée alex.bennee at linaro.org
Fri Feb 26 11:16:19 UTC 2021

In practice the protocol negotiation between vhost master and slave
occurs before the final feature negotiation between backend and
frontend. This has lead to an inconsistency between the rust-vmm vhost
implementation and the libvhost-user library in their approaches to
checking if all the requirements for REPLY_ACK processing were met.
As this is purely a function of the protocol negotiation and not of
interest to the frontend lets make the language clearer about the
requirements for a successfully negotiated protocol feature.

Signed-off-by: Alex Bennée <alex.bennee at linaro.org>
Cc: Jiang Liu <gerry at linux.alibaba.com>
 docs/interop/vhost-user.rst | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index d6085f7045..3ac221a8c7 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -301,12 +301,22 @@ If *slave* detects some error such as incompatible features, it may also
 close the connection. This should only happen in exceptional circumstances.
 Any protocol extensions are gated by protocol feature bits, which
-allows full backwards compatibility on both master and slave.  As
-older slaves don't support negotiating protocol features, a feature
+allows full backwards compatibility on both master and slave. As older
+slaves don't support negotiating protocol features, a device feature
 bit was dedicated for this purpose::
+However as the protocol negotiation something that only occurs between
+parts of the backend implementation it is permissible to for the master
+to mask the feature bit from the guest. As noted for the
+``VHOST_USER_SET_PROTOCOL_FEATURES`` messages this occurs before a
+final ``VHOST_USER_SET_FEATURES`` comes from the guest. So the
+enabling of protocol features need only require the advertising of the
+feature by the slave and the successful get/set protocol features
 Starting and stopping rings

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