vhost-user protocol feature negotiation

vhost-user protocol feature negotiation

[Alyssa Ross]

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.

Re: vhost-user protocol feature negotiation

[Michael S. Tsirkin]

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

Re: vhost-user protocol feature negotiation

[Alyssa Ross]

"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

Re: vhost-user protocol feature negotiation

[Michael S. Tsirkin]

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.


> @@ -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

Re: vhost-user protocol feature negotiation

[Alyssa Ross]

"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

Re: vhost-user protocol feature negotiation

[Michael S. Tsirkin]

On Thu, Aug 06, 2020 at 11:24:59AM +0000, Alyssa Ross wrote:
> "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



That looks good to me.
Pls post a patch on list, preferably after qemu is released.