Re: vhost-user protocol feature negotiation

[Alyssa Ross <hi@alyssa.is>]

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

> On Thu, Aug 06, 2020 at 08:59:09AM +0000, Alyssa Ross wrote:
>> "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
>
> Hmm I find this confusing. I think it's a good policy to ask qemu to
> acknowledge it. It's just that the client should not wait for
> VHOST_USER_SET_FEATURES before handling VHOST_USER_SET_PROTOCOL_FEATURES
> or VHOST_USER_GET_PROTOCOL_FEATURES.

To me, it's confusing that a frontend is expected to ack
VHOST_USER_F_PROTOCOL_FEATURES even though the ack can't have any effect
(because VHOST_USER_GET_PROTOCOL_FEATURES and
VHOST_USER_SET_PROTOCOL_FEATURES both have to work even if the ack
hasn't been received yet).

But, if the frontend is supposed to ack anyway, how about:

Signed-off-by: Alyssa Ross <hi@alysas.is>

---
diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst
index 10e3e3475e..bc78c9947f 100644
--- i/docs/interop/vhost-user.rst
+++ w/docs/interop/vhost-user.rst
@@ -854,9 +854,9 @@ 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.
+   While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a
+   backend 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
@@ -869,8 +869,12 @@ 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.
+   While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a
+   backend must allow ``VHOST_USER_SET_PROTOCOL_FEATURES`` even if
+   ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet.
+   The backend 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