Re: vhost-user protocol feature negotiation

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 05, 2020 at 03:13:06PM +0000, Alyssa Ross wrote:
> Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in
> vhost-user.rst:
> 
> >   Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in
> >   ``VHOST_USER_GET_FEATURES``.
> > 
> > .. Note::
> >    Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support
> >    this message even before ``VHOST_USER_SET_FEATURES`` was called.
> 
> To me, this could mean either of two things:
> 
> (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving
>     VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the
>     protocol features immediately.
> 
> (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving
>     VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those
>     feature bits, but not actually consider them to be enabled until
>     after VHOST_USER_SET_FEATURES has been received (presumably
>     containing VHOST_USER_F_PROTOCOL_FEATURES).
> 
> The reason I bring this up is that QEMU appears to interpret it as (1),
> while the vhost-user-net backend in Intel's cloud-hypervisor[1]
> interprets it as (2).  So I'm looking for a clarification.
> 
> [1]: https://github.com/cloud-hypervisor/cloud-hypervisor
> 
> Thanks in advance.


IMHO the intent was this: VHOST_USER_F_PROTOCOL_FEATURES bit in
VHOST_USER_GET_FEATURES means that qemu can send
VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.

With most feature bits in VHOST_USER_GET_FEATURES, the
specific functionality needs to only be enabled after
VHOST_USER_SET_FEATURES.

However, this is for functionality dealing with guest activity.
VHOST_USER_SET_PROTOCOL_FEATURES has nothing to do with guest directly,
it's about negotiation between qemu and backend: it is only in
VHOST_USER_GET_FEATURES for the reason that this is the only message
(very) old backends reported.  Thus, the backend should not check
whether VHOST_USER_SET_FEATURES sets VHOST_USER_F_PROTOCOL_FEATURES,
instead it should simply always be ready to receive
VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.

Backend that isn't always ready to handle
VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES
should not set VHOST_USER_F_PROTOCOL_FEATURES in
VHOST_USER_GET_FEATURES.

This appears to be closer to (1), but if qemu can't distinguish
then we don't care, right? For example, VHOST_USER_PROTOCOL_F_REPLY_ACK
enables acks on arbitrary messages. Does the backend in question
ignore the affected bit until SET_FEATURES? If yes won't this
make qemu hang?

How would you suggest clarifying the wording?

Thanks,

-- 
MST