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

["Alex Bennée" <alex.bennee@linaro.org>]

Alyssa Ross <hi@alyssa.is> writes:

> 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.

I think this is just restating the same logic from above introduced in:

  fa9972662c (vhost-user.rst: add clarifying language about protocol negotiation)

that we don't need VHOST_USER_F_PROTOCOL_FEATURES to be acked by
VHOST_USER_SET_FEATURES? I agree the wording is hard to follow though. I
suspect what would really help is some clear message sequence diagrams
in the document showing the negotiation steps.


>  
>  ``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


-- 
Alex Bennée