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[PATCH v2] vhost-user.rst: add clarifying language about protocol negoti
From: |
Alex Bennée |
Subject: |
[PATCH v2] vhost-user.rst: add clarifying language about protocol negotiation |
Date: |
Wed, 3 Mar 2021 14:50:11 +0000 |
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>
---
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
- [PATCH v2] vhost-user.rst: add clarifying language about protocol negotiation,
Alex Bennée <=