Re: vhost-user protocol feature negotiation

[Alyssa Ross <hi@alyssa.is>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> 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.

Thanks for the explanation.  That matches what I had in mind with (1).

> 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?

Yes.  That was my motivation for asking what the correct behaviour was,
so that I could fix the incorrect one. :)  I suspect that up to this point,
the cloud-hypervisor vhost-user-net backend has only been used with
cloud-hypervisor, and so this incompatibilty with QEMU was not noticed.

> How would you suggest clarifying the wording?

Do you think this communicates everything required?

---
diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst
index 10e3e3475e..72724d292a 100644
--- i/docs/interop/vhost-user.rst
+++ w/docs/interop/vhost-user.rst
@@ -854,9 +854,8 @@ Master message types
   ``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.
+   ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged
+   with ``VHOST_USER_SET_FEATURES``.
 
 ``VHOST_USER_SET_PROTOCOL_FEATURES``
   :id: 16
@@ -869,8 +868,8 @@ Master message types
   ``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.
+   ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged
+   with ``VHOST_USER_SET_FEATURES``.
 
 ``VHOST_USER_SET_OWNER``
   :id: 3