vhost-user question about VHOST_USER_F_PROTOCOL_FEATURES

[Vladimir Sementsov-Ogievskiy]

Hi all!

We now have a problem in downstream:

We have a vhost-user server, which doesn't support VHOST_USER_SET_VRING_ENABLE. 
As I understand, vhost-user specification allows it. So the server behaves as 
follows:

1. in GET_FEATURES, it sets VHOST_USER_F_PROTOCOL_FEATURES, to report support 
for protocol features.

2. if this flag VHOST_USER_F_PROTOCOL_FEATURES is set in SET_FEATURES, it 
reports an error

In my opinion, this doesn't violate the specification, maybe I'm wrong.

Newer QEMU, after commits

    02b61f38d357490 "hw/virtio: incorporate backend features in features"
    
    and
    
    4daa5054c599c8a
      "vhost: enable vrings in vhost_dev_start() for vhost-user devices"

actually doesn't support such behavior, as QEMU assumes that if we have the 
flag in GET_FEATURES, it is supported by SET_FEATURES.


I don't see any possibility to clearly support in QEMU both servers which 
supports vring enable/disable and older servers that doesn't.

=====

I also want to clarify,

1. is "feature negotiated" means set both in GET_FEATURES and SET_FEATURES, as 
this term is not directly defined.

2. What VHOST_USER_SET_VRING_ENABLE should do for vhost-user-blk? What meant by 
"side effects" in this case? All IO requests should just fail, not touching the 
actual storage?

3. Except for VHOST_USER_SET_VRING_ENABLE, there is another command require not just 
VHOST_USER_F_PROTOCOL_FEATURES being present in GET_FEATURES, but "negotiated" 
i.e., as I understand being then set in SET_OPTIONS by client. That's 
VHOST_USER_SET_SLAVE_REQ_FD. Why? Is it a mistake?

4. Also, in VHOST_USER_SET_FEATURES definition:

         Feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` signals
  back-end support for ``VHOST_USER_GET_PROTOCOL_FEATURES`` and
  ``VHOST_USER_SET_PROTOCOL_FEATURES``.


That seems wrong, as the flag in SET command is about vring enable/disable 
related behavior.

--
Best regards,
Vladimir