[PATCH] docs: clarify absence of set_features in vhost-user

[Alyssa Ross]

The previous wording was (at least to me) ambiguous about whether a
backend should enable features immediately after they were set using
VHOST_USER_SET_PROTOCOL_FEATURES, or wait for support for protocol
features to be acknowledged if it hasn't been yet before enabling
those features.

This patch attempts to make it clearer that
VHOST_USER_SET_PROTOCOL_FEATURES should immediately enable features,
even if support for protocol features has not yet been acknowledged,
while still also making clear that the frontend SHOULD acknowledge
support for protocol features.

Previous discussion begins here:
<https://lore.kernel.org/qemu-devel/87sgd1ktx9.fsf@alyssa.is/>

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Alyssa Ross <hi@alyssa.is>
---
 docs/interop/vhost-user.rst | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 3f18ab424e..c8b9771a16 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -906,9 +906,9 @@ Front-end message types
   ``VHOST_USER_SET_FEATURES``.
 
 .. Note::
-   Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must
-   support this message even before ``VHOST_USER_SET_FEATURES`` was
-   called.
+   While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a
+   back-end must allow ``VHOST_USER_GET_PROTOCOL_FEATURES`` even if
+   ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet.
 
 ``VHOST_USER_SET_PROTOCOL_FEATURES``
   :id: 16
@@ -923,8 +923,12 @@ Front-end message types
   ``VHOST_USER_SET_FEATURES``.
 
 .. Note::
-   Back-ends that report ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
-   this message even before ``VHOST_USER_SET_FEATURES`` was called.
+   While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a
+   back-end must allow ``VHOST_USER_SET_PROTOCOL_FEATURES`` even if
+   ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet.
+   The back-end must not wait for ``VHOST_USER_SET_FEATURES`` before
+   enabling protocol features requested with
+   ``VHOST_USER_SET_PROTOCOL_FEATURES``.
 
 ``VHOST_USER_SET_OWNER``
   :id: 3

base-commit: e93ded1bf6c94ab95015b33e188bc8b0b0c32670
-- 
2.37.1